gvpr

GVPR(1)                     General Commands Manual                    GVPR(1)



NAME
       gvpr - graph pattern scanning and processing language
       ( previously known as gpr )

SYNOPSIS
       gvpr [-icqV?]  [ -o outfile ] [ -a args ] [ 'prog' | -f progfile ] [
       files ]

DESCRIPTION
       gvpr is a graph stream editor inspired by awk.  It copies input graphs
       to its output, possibly transforming their structure and attributes,
       creating new graphs, or printing arbitrary information.  The graph
       model is that provided by libcgraph(3).  In particular, gvpr reads and
       writes graphs using the dot language.

       Basically, gvpr traverses each input graph, denoted by $G, visiting
       each node and edge, matching it with the predicate‐action rules
       supplied in the input program.  The rules are evaluated in order.  For
       each predicate evaluating to true, the corresponding action is
       performed.  During the traversal, the current node or edge being
       visited is denoted by $.

       For each input graph, there is a target subgraph, denoted by $T,
       initially empty and used to accumulate chosen entities, and an output
       graph, $O, used for final processing and then written to output.  By
       default, the output graph is the target graph.  The output graph can be
       set in the program or, in a limited sense, on the command line.

OPTIONS
       The following options are supported:

       -a args
              The string args is split into whitespace‐separated tokens, with
              the individual tokens available as strings in the gvpr program
              as ARGV[0],...,ARGV[ARGC-1].  Whitespace characters within
              single or double quoted substrings, or preceded by a backslash,
              are ignored as separators.  In general, a backslash character
              turns off any special meaning of the following character.  Note
              that the tokens derived from multiple -a flags are concatenated.

       -c     Use the source graph as the output graph.

       -i     Derive the node‐induced subgraph extension of the output graph
              in the context of its root graph.

       -o outfile
              Causes the output stream to be written to the specified file; by
              default, output is written to stdout.

       -f progfile
              Use the contents of the specified file as the program to execute
              on the input. If progfile contains a slash character, the name
              is taken as the pathname of the file. Otherwise, gvpr will use
              the directories specified in the environment variable GPRPATH to
              look for the file. If -f is not given, gvpr will use the first
              non‐option argument as the program.

       -q     Turns off warning messages.

       -V     Causes the program to print version information and exit.

       -?     Causes the program to print usage information and exit.

OPERANDS
       The following operand is supported:

       files   Names of files containing 1 or more graphs in the dot language.
               If no -f option is given, the first name is removed from the
               list and used as the input program. If the list of files is
               empty, stdin will be used.

PROGRAMS
       A gvpr program consists of a list of predicate‐action clauses, having
       one of the forms:

              BEGIN { action }

              BEG_G { action }

              N [ predicate ] { action }

              E [ predicate ] { action }

              END_G { action }

              END { action }

       A program can contain at most one of each of the BEGIN, END_G and END
       clauses.  There can be any number of BEG_G, N and E statements, the
       first applied to graphs, the second to nodes, the third to edges.
       These are separated into blocks, a block consisting of an optional
       BEG_G statement and all N and E statements up to the next BEG_G
       statement, if any.  The top‐level semantics of a gvpr program are:

              Evaluate the BEGIN clause, if any.
              For each input graph G {
                  For each block {
                      Set G as the current graph and current object.
                      Evaluate the BEG_G clause, if any.
                      For each node and edge in G {
                          Set the node or edge as the current object.
                          Evaluate the N or E clauses, as appropriate.
                      }
                  }
                  Set G as the current object.
                  Evaluate the END_G clause, if any.
              }
              Evaluate the END clause, if any.

       The actions of the BEGIN, BEG_G, END_G and END clauses are performed
       when the clauses are evaluated.  For N or E clauses, either the
       predicate or action may be omitted.  If there is no predicate with an
       action, the action is performed on every node or edge, as appropriate.
       If there is no action and the predicate evaluates to true, the
       associated node or edge is added to the target graph.

       The blocks are evaluated in the order in which they occur.  Within a
       block, the N clauses (E clauses, respectively) are evaluated in the
       order in which the occur. Note, though, that within a block, N or E
       clauses may be interlaced, depending on the traversal order.

       Predicates and actions are sequences of statements in the C dialect
       supported by the libexpr(3) library.  The only difference between
       predicates and actions is that the former must have a type that may
       interpreted as either true or false.  Here the usual C convention is
       followed, in which a non‐zero value is considered true. This would
       include non‐empty strings and non‐empty references to nodes, edges,
       etc. However, if a string can be converted to an integer, this value is
       used.

       In addition to the usual C base types (void, int, char, float, long,
       unsigned and double), gvpr provides string as a synonym for char*, and
       the graph‐based types node_t, edge_t, graph_t and obj_t.  The obj_t
       type can be viewed as a supertype of the other 3 concrete types; the
       correct base type is maintained dynamically.  Besides these base types,
       the only other supported type expressions are (associative) arrays.

       Constants follow C syntax, but strings may be quoted with either "..."
       or '...'. In certain contexts, string values are interpreted as
       patterns for the purpose of regular expression matching.  Patterns use
       ksh(1) file match pattern syntax.  gvpr accepts C++ comments as well as
       cpp‐type comments.  For the latter, if a line begins with a '#'
       character, the rest of the line is ignored.

       A statement can be a declaration of a function, a variable or an array,
       or an executable statement. For declarations, there is a single scope.
       Array declarations have the form:

               type array [ type0 ]

       where  type0  is optional. If it is supplied, the parser will enforce
       that all array subscripts have the specified type. If it is not
       supplied, objects of all types can be used as subscripts.  As in C,
       variables and arrays must be declared. In particular, an undeclared
       variable will be interpreted as the name of an attribute of a node,
       edge or graph, depending on the context.

       Executable statements can be one of the following:
              { [ statement ... ] }
              expression              // commonly var = expression
              if( expression ) statement [ else statement ]
              for( expression ; expression ; expression ) statement
              for( array [ var ]) statement
              forr( array [ var ]) statement
              while( expression ) statement
              switch( expression ) case statements
              break [ expression ]
              continue [ expression ]
              return [ expression ]
       Items in brackets are optional.

       In the second form of the for statement and the forr statement, the
       variable var is set to each value used as an index in the specified
       array and then the associated statement is evaluated. For numeric and
       string indices, the indices are returned in increasing (decreasing)
       numeric or lexicographic order for for (forr, respectively). This can
       be used for sorting.

       Function definitions can only appear in the BEGIN clause.

       Expressions include the usual C expressions.  String comparisons using
       == and != treat the right hand operand as a pattern.  gvpr will attempt
       to use an expression as a string or numeric value as appropriate.

       Expressions of graphical type (i.e., graph_t, node_t, edge_t, obj_t)
       may be followed by a field reference in the form of .name. The
       resulting value is the value of the attribute named name of the given
       object.  In addition, in certain contexts an undeclared, unmodified
       identifier is taken to be an attribute name. Specifically, such
       identifiers denote attributes of the current node or edge,
       respectively, in N and E clauses, and the current graph in BEG_G and
       END_G clauses.

       As usual in the libcgraph(3) model, attributes are string‐valued.  In
       addition, gvpr supports certain pseudo‐attributes of graph objects, not
       necessarily string‐valued. These reflect intrinsic properties of the
       graph objects and cannot be set by the user.

       head : node_t
              the head of an edge.

       tail : node_t
              the tail of an edge.

       name : string
              the name of an edge, node or graph. The name of an edge has the
              form "<tail‐name><edge‐op><head‐name>[<key>]", where <edge‐op>
              is "->" or "--" depending on whether the graph is directed or
              not. The bracket part [<key>] only appears if the edge has a
              non‐trivial key.

       indegree : int
              the indegree of a node.

       outdegree : int
              the outdegree of a node.

       degree : int
              the degree of a node.

       root : graph_t
              the root graph of an object. The root of a root graph is itself.

       parent : graph_t
              the parent graph of a subgraph. The parent of a root graph is
              NULL

       n_edges : int
              the number of edges in the graph

       n_nodes : int
              the number of nodes in the graph

       directed : int
              true (non‐zero) if the graph is directed

       strict : int
              true (non‐zero) if the graph is strict

BUILT‐IN FUNCTIONS
       The following functions are built into gvpr. Those functions returning
       references to graph objects return NULL in case of failure.

   Graphs and subgraph
       graph(s : string, t : string) : graph_t
              creates a graph whose name is s and whose type is specified by
              the string t. Ignoring case, the characters U, D, S, N have the
              interpretation undirected, directed, strict, and non‐strict,
              respectively. If t is empty, a directed, non‐strict graph is
              generated.

       subg(g : graph_t, s : string) : graph_t
              creates a subgraph in graph g with name s. If the subgraph
              already exists, it is returned.

       isSubg(g : graph_t, s : string) : graph_t
              returns the subgraph in graph g with name s, if it exists, or
              NULL otherwise.

       fstsubg(g : graph_t) : graph_t
              returns the first subgraph in graph g, or NULL if none exists.

       nxtsubg(sg : graph_t) : graph_t
              returns the next subgraph after sg, or NULL.

       isDirect(g : graph_t) : int
              returns true if and only if g is directed.

       isStrict(g : graph_t) : int
              returns true if and only if g is strict.

       nNodes(g : graph_t) : int
              returns the number of nodes in g.

       nEdges(g : graph_t) : int
              returns the number of edges in g.

   Nodes
       node(sg : graph_t, s : string) : node_t
              creates a node in graph g of name s. If such a node already
              exists, it is returned.

       subnode(sg : graph_t, n : node_t) : node_t
              inserts the node n into the subgraph g. Returns the node.

       fstnode(g : graph_t) : node_t
              returns the first node in graph g, or NULL if none exists.

       nxtnode(n : node_t) : node_t
              returns the next node after n in the root graph, or NULL.

       nxtnode_sg(sg : graph_t, n : node_t) : node_t
              returns the next node after n in sg, or NULL.

       isNode(sg : graph_t, s : string) : node_t
              looks for a node in (sub)graph sg of name s. If such a node
              exists, it is returned. Otherwise, NULL is returned.

       isSubnode(sg : graph_t, n : node_t) : int
              returns non-zero if node n is in (sub)graph sg, or zero
              otherwise.

       indegreeOf(sg : graph_t, n : node_t) : int
              returns the indegree of node n in (sub)graph sg.

       outdegreeOf(sg : graph_t, n : node_t) : int
              returns the outdegree of node n in (sub)graph sg.

       degreeOf(sg : graph_t, n : node_t) : int
              returns the degree of node n in (sub)graph sg.

   Edges
       edge(t : node_t, h : node_t, s : string) : edge_t
              creates an edge with tail node t, head node h and name s in the
              root graph. If the graph is undirected, the distinction between
              head and tail nodes is unimportant.  If such an edge already
              exists, it is returned.

       edge_sg(sg : graph_t, t : node_t, h : node_t, s : string) : edge_t
              creates an edge with tail node t, head node h and name s in
              (sub)graph sg (and all parent graphs). If the graph is
              undirected, the distinction between head and tail nodes is
              unimportant.  If such an edge already exists, it is returned.

       subedge(g : graph_t, e : edge_t) : edge_t
              inserts the edge e into the subgraph g. Returns the edge.

       isEdge(t : node_t, h : node_t, s : string) : edge_t
              looks for an edge with tail node t, head node h and name s. If
              the graph is undirected, the distinction between head and tail
              nodes is unimportant.  If such an edge exists, it is returned.
              Otherwise, NULL is returned.

       isEdge_sg(sg : graph_t, t : node_t, h : node_t, s : string) : edge_t
              looks for an edge with tail node t, head node h and name s in
              (sub)graph sg. If the graph is undirected, the distinction
              between head and tail nodes is unimportant.  If such an edge
              exists, it is returned. Otherwise, NULL is returned.

       isSubedge(g : graph_t, e : edge_t) : int
              returns non-zero if edge e is in (sub)graph sg, or zero
              otherwise.

       fstout(n : node_t) : edge_t
              returns the first outedge of node n in the root graph.

       fstout_sg(sg : graph_t, n : node_t) : edge_t
              returns the first outedge of node n in (sub)graph sg.

       nxtout(e : edge_t) : edge_t
              returns the next outedge after e in the root graph.

       nxtout_sg(sg : graph_t, e : edge_t) : edge_t
              returns the next outedge after e in graph sg.

       fstin(n : node_t) : edge_t
              returns the first inedge of node n in the root graph.

       fstin_sg(sg : graph_t, n : node_t) : edge_t
              returns the first inedge of node n in graph sg.

       nxtin(e : edge_t) : edge_t
              returns the next inedge after e in the root graph.

       nxtin_sg(sg : graph_t, e : edge_t) : edge_t
              returns the next inedge after e in graph sg.

       fstedge(n : node_t) : edge_t
              returns the first edge of node n in the root graph.

       fstedge_sg(sg : graph_t, n : node_t) : edge_t
              returns the first edge of node n in graph sg.

       nxtedge(e : edge_t, node_t) : edge_t
              returns the next edge after e in the root graph.

       nxtedge_sg(sg : graph_t, e : edge_t, node_t) : edge_t
              returns the next edge after e in the graph sg.

   Graph I/O
       write(g : graph_t) : void
              prints g in dot format onto the output stream.

       writeG(g : graph_t, fname : string) : void
              prints g in dot format into the file fname.

       fwriteG(g : graph_t, fd : int) : void
              prints g in dot format onto the open stream denoted by the
              integer fd.

       readG(fname : string) : graph_t
              returns a graph read from the file fname. The graph should be in
              dot format. If no graph can be read, NULL is returned.

       freadG(fd : int) : graph_t
              returns the next graph read from the open stream fd.  Returns
              NULL at end of file.

   Graph miscellany
       delete(g : graph_t, x : obj_t) : void
              deletes object x from graph g.  If g is NULL, the function uses
              the root graph of x.  If x is a graph or subgraph, it is closed
              unless x is locked.

       isIn(g : graph_t, x : obj_t) : int
              returns true if x is in subgraph g.

       clone(g : graph_t, x : obj_t) : obj_t
              creates a clone of object x in graph g.  In particular, the new
              object has the same name/value attributes and structure as the
              original object.  If an object with the same key as x already
              exists, its attributes are overlaid by those of x and the object
              is returned.  If an edge is cloned, both endpoints are
              implicitly cloned.  If a graph is cloned, all nodes, edges and
              subgraphs are implicitly cloned.  If x is a graph, g may be
              NULL, in which case the cloned object will be a new root graph.

       copy(g : graph_t, x : obj_t) : obj_t
              creates a copy of object x in graph g, where the new object has
              the same name/value attributes as the original object.  If an
              object with the same key as x already exists, its attributes are
              overlaid by those of x and the object is returned.  Note that
              this is a shallow copy. If x is a graph, none of its nodes,
              edges or subgraphs are copied into the new graph. If x is an
              edge, the endpoints are created if necessary, but they are not
              cloned.  If x is a graph, g may be NULL, in which case the
              cloned object will be a new root graph.

       copyA(src : obj_t, tgt : obj_t) : int
              copies the attributes of object src to object tgt, overwriting
              any attribute values tgt may initially have.

       induce(g : graph_t) : void
              extends g to its node‐induced subgraph extension in its root
              graph.

       hasAttr(src : obj_t, name : string) : int
              returns non-zero if object src has an attribute whose name is
              name. It returns 0 otherwise.

       isAttr(g : graph_t, kind : string, name : string) : int
              returns non-zero if an attribute name has been defined in g for
              objects of the given kind. For nodes, edges, and graphs, kind
              should be "N", "E", and "G", respectively.  It returns 0
              otherwise.

       aget(src : obj_t, name : string) : string
              returns the value of attribute name in object src. This is
              useful for those cases when name conflicts with one of the
              keywords such as "head" or "root".  If the attribute has not
              been declared in the graph, the function will initialize it with
              a default value of "". To avoid this, one should use the hasAttr
              or isAttr function to check that the attribute exists.

       aset(src : obj_t, name : string, value : string) : int
              sets the value of attribute name in object src to value.
              Returns 0 on success, non‐zero on failure. See aget above.

       getDflt(g : graph_t, kind : string, name : string) : string
              returns the default value of attribute name in objects in g of
              the given kind. For nodes, edges, and graphs, kind should be
              "N", "E", and "G", respectively.  If the attribute has not been
              declared in the graph, the function will initialize it with a
              default value of "". To avoid this, one should use the isAttr
              function to check that the attribute exists.

       setDflt(g : graph_t, kind : string, name : string, value : string) :
       int
              sets the default value of attribute name to value in objects in
              g of the given kind. For nodes, edges, and graphs, kind should
              be "N", "E", and "G", respectively.  Returns 0 on success, non‐
              zero on failure. See getDflt above.

       fstAttr(g : graph_t, kind : string) : string
              returns the name of the first attribute of objects in g of the
              given kind. For nodes, edges, and graphs, kind should be "N",
              "E", and "G", respectively.  If there are no attributes, the
              string "" is returned.

       nxtAttr(g : graph_t, kind : string, name : string) : string
              returns the name of the next attribute of objects in g of the
              given kind after the attribute name.  The argument name must be
              the name of an existing attribute; it will typically be the
              return value of an previous call to fstAttr or nxtAttr.  For
              nodes, edges, and graphs, kind should be "N", "E", and "G",
              respectively.  If there are no attributes left, the string "" is
              returned.

       compOf(g : graph_t, n : node_t) : graph_t
              returns the connected component of the graph g containing node
              n, as a subgraph of g. The subgraph only contains the nodes. One
              can use induce to add the edges. The function fails and returns
              NULL if n is not in g. Connectivity is based on the underlying
              undirected graph of g.

       kindOf(obj : obj_t) : string
              returns an indication of what kind of graph object is the
              argument.  For nodes, edges, and graphs, it returns should be
              "N", "E", and "G", respectively.

       lock(g : graph_t, v : int) : int
              implements graph locking on root graphs. If the integer v is
              positive, the graph is set so that future calls to delete have
              no immediate effect.  If v is zero, the graph is unlocked. If
              there has been a call to delete the graph while it was locked,
              the graph is closed.  If v is negative, nothing is done.  In all
              cases, the previous lock value is returned.

   Strings
       sprintf(fmt : string, ...) : string
              returns the string resulting from formatting the values of the
              expressions occurring after fmt according to the printf(3)
              format fmt

       gsub(str : string, pat : string) : string

       gsub(str : string, pat : string, repl : string) : string
              returns str with all substrings matching pat deleted or replaced
              by repl, respectively.

       sub(str : string, pat : string) : string

       sub(str : string, pat : string, repl : string) : string
              returns str with the leftmost substring matching pat deleted or
              replaced by repl, respectively. The characters '^' and '$' may
              be used at the beginning and end, respectively, of pat to anchor
              the pattern to the beginning or end of str.

       substr(str : string, idx : int) : string

       substr(str : string, idx : int, len : int) : string
              returns the substring of str starting at position idx to the end
              of the string or of length len, respectively.  Indexing starts
              at 0. If idx is negative or idx is greater than the length of
              str, a fatal error occurs. Similarly, in the second case, if len
              is negative or idx + len is greater than the length of str, a
              fatal error occurs.

       length(s : string) : int
              returns the length of the string s.

       index(s : string, t : string) : int

       rindex(s : string, t : string) : int
              returns the index of the character in string s where the
              leftmost (rightmost) copy of string t can be found, or -1 if t
              is not a substring of s.

       match(s : string, p : string) : int
              returns the index of the character in string s where the
              leftmost match of pattern p can be found, or -1 if no substring
              of s matches p.

       toupper(s : string) : string
              returns a version of s with the alphabetic characters converted
              to upper-case.

       tolower(s : string) : string
              returns a version of s with the alphabetic characters converted
              to lower-case.

       canon(s : string) : string
              returns a version of s appropriate to be used as an identifier
              in a dot file.

       xOf(s : string) : string
              returns the string "x" if s has the form "x,y", where both x and
              y are numeric.

       yOf(s : string) : string
              returns the string "y" if s has the form "x,y", where both x and
              y are numeric.

       llOf(s : string) : string
              returns the string "llx,lly" if s has the form
              "llx,lly,urx,ury", where all of llx, lly, urx, and ury are
              numeric.

       urOf(s)
              urOf(s : string) : string returns the string "urx,ury" if s has
              the form "llx,lly,urx,ury", where all of llx, lly, urx, and ury
              are numeric.

       sscanf(s : string, fmt : string, ...) : int
              scans the string s, extracting values according to the sscanf(3)
              format fmt.  The values are stored in the addresses following
              fmt, addresses having the form &v, where v is some declared
              variable of the correct type.  Returns the number of items
              successfully scanned.

       split(s : string, arr : array, seps : string) : int

       split(s : string, arr : array) : int

       tokens(s : string, arr : array, seps : string) : int

       tokens(s : string, arr : array) : int
              The split function breaks the string s into fields, while the
              tokens function breaks the string into tokens.  A field consists
              of all non-separator characters between two separator characters
              or the beginning or end of the string. Thus, a field may be the
              empty string. A token is a maximal, non-empty substring not
              containing a separator character.  The separator characters are
              those given in the seps argument.  If seps is not provided, the
              default value is " \t\n".  The functions return the number of
              fields or tokens.

              The fields and tokens are stored in the argument array. The
              array must be string-valued and, if an index type is specified,
              it must be int. The entries are indexed by consecutive integers,
              starting at 0. Any values already stored in the array will be
              either overwritten, or still be present after the function
              returns.

   I/O
       print(...) : void
              print( expr, ... ) prints a string representation of each
              argument in turn onto stdout, followed by a newline.

       printf(fmt : string, ...) : int

       printf(fd : int, fmt : string, ...) : int
              prints the string resulting from formatting the values of the
              expressions following fmt according to the printf(3) format fmt.
              Returns 0 on success.  By default, it prints on stdout.  If the
              optional integer fd is given, output is written on the open
              stream associated with fd.

       scanf(fmt : string, ...) : int

       scanf(fd : int, fmt : string, ...) : int
              scans in values from an input stream according to the scanf(3)
              format fmt.  The values are stored in the addresses following
              fmt, addresses having the form &v, where v is some declared
              variable of the correct type.  By default, it reads from stdin.
              If the optional integer fd is given, input is read from the open
              stream associated with fd.  Returns the number of items
              successfully scanned.

       openF(s : string, t : string) : int
              opens the file s as an I/O stream. The string argument t
              specifies how the file is opened. The arguments are the same as
              for the C function fopen(3).  It returns an integer denoting the
              stream, or -1 on error.

              As usual, streams 0, 1 and 2 are already open as stdin, stdout,
              and stderr, respectively. Since gvpr may use stdin to read the
              input graphs, the user should avoid using this stream.

       closeF(fd : int) : int
              closes the open stream denoted by the integer fd.  Streams  0, 1
              and 2 cannot be closed.  Returns 0 on success.

       readL(fd : int) : string
              returns the next line read from the input stream fd. It returns
              the empty string "" on end of file. Note that the newline
              character is left in the returned string.

   Math
       exp(d : double) : double
              returns e to the dth power.

       log(d : double) : double
              returns the natural log of d.

       sqrt(d : double) : double
              returns the square root of the double d.

       pow(d : double, x : double) : double
              returns d raised to the xth power.

       cos(d : double) : double
              returns the cosine of d.

       sin(d : double) : double
              returns the sine of d.

       atan2(y : double, x : double) : double
              returns the arctangent of y/x in the range -pi to pi.

       MIN(y : double, x : double) : double
              returns the minimum of y and x.

       MAX(y : double, x : double) : double
              returns the maximum of y and x.

   Associative Arrays
       # arr : int
              returns the number of elements in the array arr.

       idx in arr : int
              returns 1 if a value has been set for index idx in the array
              arr.  It returns 0 otherwise.

       unset(v : array, IidxP) : int
              removes the item indexed by idx. It returns 1 if the item
              existed, 0 otherwise.

       unset(v : array) : void
              re-initializes the array.

   Miscellaneous
       exit(v : int) : void
              causes gvpr to exit with the exit code v.

       system(cmd : string) : int
              provides the standard C function system(3).  It executes cmd if
              the user's shell environment, and returns the exit status of the
              shell.

       rand() : double
              returns a pseudo‐random double between 0 and 1.

       srand() : int

       srand(v : int) : int
              sets a seed for the random number generator. The optional
              argument gives the seed; if it is omitted, the current time is
              used. The previous seed value is returned. srand should be
              called before any calls to rand.

       colorx(color : string, fmt : string) : string
              translates a color from one format to another. The color
              argument should be a color in one of the recognized string
              representations. The fmt value should be one of "RGB", "RGBA",
              "HSV", "HSVA", or "CMYK".  An empty string is returned on error.

BUILT‐IN VARIABLES
       gvpr provides certain special, built‐in variables, whose values are set
       automatically by gvpr depending on the context. Except as noted, the
       user cannot modify their values.

       $ : obj_t
              denotes the current object (node, edge, graph) depending on the
              context.  It is not available in BEGIN or END clauses.

       $F : string
              is the name of the current input file.

       $G : graph_t
              denotes the current graph being processed. It is not available
              in BEGIN or END clauses.

       $O : graph_t
              denotes the output graph. Before graph traversal, it is
              initialized to the target graph. After traversal and any END_G
              actions, if it refers to a non‐empty graph, that graph is
              printed onto the output stream.  It is only valid in N, E and
              END_G clauses.  The output graph may be set by the user.

       $T : graph_t
              denotes the current target graph. It is a subgraph of $G and is
              available only in N, E and END_G clauses.

       $tgtname : string
              denotes the name of the target graph.  By default, it is set to
              "gvpr_result".  If used multiple times during the execution of
              gvpr, the name will be appended with an integer.  This variable
              may be set by the user.

       $tvroot : node_t
              indicates the starting node for a (directed or undirected)
              depth‐first traversal of the graph (cf. $tvtype below).  The
              default value is NULL for each input graph.

       $tvedge : edge_t
              For BFS and DFS traversals, this is set to the edge used to
              arrive at the current node or edge. At the beginning of a
              traversal, or for other traversal types, the value is NULL.

       $tvtype : tvtype_t
              indicates how gvpr traverses a graph. It can only take one of
              the constant values with the previx "TV_" described below.
              TV_flat is the default.

              In the underlying graph library cgraph(3), edges in undirected
              graphs are given an arbitrary direction. This is used for
              traversals, such as TV_fwd, requiring directed edges.

       ARGC : int
              denotes the number of arguments specified by the -a args
              command‐line argument.

       ARGV : string array
              denotes the array of arguments specified by the -a args command‐
              line argument. The ith argument is given by ARGV[i].

BUILT‐IN CONSTANTS
       There are several symbolic constants defined by gvpr.

       NULL : obj_t
              a null object reference, equivalent to 0.

       TV_flat : tvtype_t
              a simple, flat traversal, with graph objects visited in
              seemingly arbitrary order.

       TV_ne : tvtype_t
              a traversal which first visits all of the nodes, then all of the
              edges.

       TV_en : tvtype_t
              a traversal which first visits all of the edges, then all of the
              nodes.

       TV_dfs : tvtype_t
       TV_postdfs : tvtype_t
       TV_prepostdfs : tvtype_t
              a traversal of the graph using a depth‐first search on the
              underlying undirected graph.  To do the traversal, gvpr will
              check the value of $tvroot. If this has the same value that it
              had previously (at the start, the previous value is initialized
              to NULL.), gvpr will simply look for some unvisited node and
              traverse its connected component. On the other hand, if $tvroot
              has changed, its connected component will be toured, assuming it
              has not been previously visited or, if $tvroot is NULL, the
              traversal will stop. Note that using TV_dfs and $tvroot, it is
              possible to create an infinite loop.

              By default, the traversal is done in pre-order. That is, a node
              is visited before all of its unvisited edges. For TV_postdfs,
              all of a node's unvisited edges are visited before the node. For
              TV_prepostdfs, a node is visited twice, before and after all of
              its unvisited edges.

       TV_fwd : tvtype_t
       TV_postfwd : tvtype_t
       TV_prepostfwd : tvtype_t
              A traversal of the graph using a depth‐first search on the graph
              following only forward arcs.  The choice of roots for the
              traversal is the same as described for TV_dfs above.  The
              different order of visitation specified by TV_fwd, TV_postfwd
              and TV_prepostfwd are the same as those specified by the
              analogous traversals TV_dfs, TV_postdfs and TV_prepostdfs.

       TV_rev : tvtype_t
       TV_postrev : tvtype_t
       TV_prepostrev : tvtype_t
              A traversal of the graph using a depth‐first search on the graph
              following only reverse arcs.  The choice of roots for the
              traversal is the same as described for TV_dfs above.  The
              different order of visitation specified by TV_rev, TV_postrev
              and TV_prepostrev are the same as those specified by the
              analogous traversals TV_dfs, TV_postdfs and TV_prepostdfs.

       TV_bfs : tvtype_t
              A traversal of the graph using a bread‐first search on the graph
              ignoring edge directions. See the item on TV_dfs above for the
              role of $tvroot.

EXAMPLES
              gvpr -i 'N[color=="blue"]' file.dot

       Generate the node‐induced subgraph of all nodes with color blue.

              gvpr -c 'N[color=="blue"]{color = "red"}' file.dot

       Make all blue nodes red.

              BEGIN { int n, e; int tot_n = 0; int tot_e = 0; }
              BEG_G {
                n = nNodes($G);
                e = nEdges($G);
                printf ("%d nodes %d edges %s0, n, e, $G.name);
                tot_n += n;
                tot_e += e;
              }
              END { printf ("%d nodes %d edges total0, tot_n, tot_e) }

       Version of the program gc.

              gvpr -c ""

       Equivalent to nop.

              BEG_G { graph_t g = graph ("merge", "S"); }
              E {
                node_t h = clone(g,$.head);
                node_t t = clone(g,$.tail);
                edge_t e = edge(t,h,"");
                e.weight = e.weight + 1;
              }
              END_G { $O = g; }

       Produces a strict version of the input graph, where the weight
       attribute of an edge indicates how many edges from the input graph the
       edge represents.

              BEGIN {node_t n; int deg[]}
              E{deg[head]++; deg[tail]++; }
              END_G {
                for (deg[n]) {
                  printf ("deg[%s] = %d0, n.name, deg[n]);
                }
              }

       Computes the degrees of nodes with edges.

ENVIRONMENT
       GPRPATH
              Colon‐separated list of directories to be searched to find the
              file specified by the -f option.

BUGS AND WARNINGS
       When the program is given as a command line argument, the usual shell
       interpretation takes place, which may affect some of the special names
       in gvpr. To avoid this, it is best to wrap the program in single
       quotes.

       As of 24 April 2008, gvpr switched to using a new, underlying graph
       library, which uses the simpler model that there is only one copy of a
       node, not one copy for each subgraph logically containing it. This
       means that iterators such as InxtnodeP cannot traverse a subgraph using
       just a node argument. For this reason, subgraph traversal requires new
       functions ending in "_sg", which also take a subgraph argument. The
       versions without that suffix will always traverse the root graph.

       There is a single global scope, except for formal function parameters,
       and even these can interfere with the type system. Also, the extent of
       all variables is the entire life of the program.  It might be
       preferable for scope to reflect the natural nesting of the clauses, or
       for the program to at least reset locally declared variables.  For now,
       it is advisable to use distinct names for all variables.

       If a function ends with a complex statement, such as an IF statement,
       with each branch doing a return, type checking may fail.  Functions
       should use a return at the end.

       The expr library does not support string values of (char*)0.  This
       means we can't distinguish between "" and (char*)0 edge keys.  For the
       purposes of looking up and creating edges, we translate "" to be
       (char*)0, since this latter value is necessary in order to look up any
       edge with a matching head and tail.

       Related to this, strings converted to integers act like char pointers,
       getting the value 0 or 1 depending on whether the string consists
       solely of zeroes or not. Thus, the ((int)"2") evaluates to 1.

       The language inherits the usual C problems such as dangling references
       and the confusion between '=' and '=='.

AUTHOR
       Emden R. Gansner <erg@research.att.com>

SEE ALSO
       awk(1), gc(1), dot(1), nop(1), libexpr(3), libcgraph(3)



                                  3 July 2009                          GVPR(1)