expr

expr(3tcl)                   Tcl Built-In Commands                  expr(3tcl)



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NAME
       expr - Evaluate an expression

SYNOPSIS
       expr arg ?arg arg ...?
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DESCRIPTION
       Concatenates args (adding separator spaces between them), evaluates the
       result as a Tcl expression, and returns the value.  The operators
       permitted in Tcl expressions include a subset of the operators
       permitted in C expressions.  For those operators common to both Tcl and
       C, Tcl applies the same meaning and precedence as the corresponding C
       operators.  Expressions almost always yield numeric results (integer or
       floating-point values).  For example, the expression

              expr 8.2 + 6

       evaluates to 14.2.  Tcl expressions differ from C expressions in the
       way that operands are specified.  Also, Tcl expressions support non-
       numeric operands and string comparisons, as well as some additional
       operators not found in C.

   OPERANDS
       A Tcl expression consists of a combination of operands, operators,
       parentheses and commas.  White space may be used between the operands
       and operators and parentheses (or commas); it is ignored by the
       expression's instructions.  Where possible, operands are interpreted as
       integer values.  Integer values may be specified in decimal (the normal
       case), in binary (if the first two characters of the operand are 0b),
       in octal (if the first two characters of the operand are 0o), or in
       hexadecimal (if the first two characters of the operand are 0x).  For
       compatibility with older Tcl releases, an octal integer value is also
       indicated simply when the first character of the operand is 0, whether
       or not the second character is also o.  If an operand does not have one
       of the integer formats given above, then it is treated as a floating-
       point number if that is possible.  Floating-point numbers may be
       specified in any of several common formats making use of the decimal
       digits, the decimal point ., the characters e or E indicating
       scientific notation, and the sign characters + or -.  For example, all
       of the following are valid floating-point numbers:  2.1, 3., 6e4,
       7.91e+16.  Also recognized as floating point values are the strings Inf
       and NaN making use of any case for each character.  If no numeric
       interpretation is possible (note that all literal operands that are not
       numeric or boolean must be quoted with either braces or with double
       quotes), then an operand is left as a string (and only a limited set of
       operators may be applied to it).

       Operands may be specified in any of the following ways:

       [1]    As a numeric value, either integer or floating-point.

       [2]    As a boolean value, using any form understood by string is
              boolean.

       [3]    As a Tcl variable, using standard $ notation.  The variable's
              value will be used as the operand.

       [4]    As a string enclosed in double-quotes.  The expression parser
              will perform backslash, variable, and command substitutions on
              the information between the quotes, and use the resulting value
              as the operand

       [5]    As a string enclosed in braces.  The characters between the open
              brace and matching close brace will be used as the operand
              without any substitutions.

       [6]    As a Tcl command enclosed in brackets.  The command will be
              executed and its result will be used as the operand.

       [7]    As a mathematical function whose arguments have any of the above
              forms for operands, such as sin($x).  See MATH FUNCTIONS below
              for a discussion of how mathematical functions are handled.

       Where the above substitutions occur (e.g. inside quoted strings), they
       are performed by the expression's instructions.  However, the command
       parser may already have performed one round of substitution before the
       expression processor was called.  As discussed below, it is usually
       best to enclose expressions in braces to prevent the command parser
       from performing substitutions on the contents.

       For some examples of simple expressions, suppose the variable a has the
       value 3 and the variable b has the value 6.  Then the command on the
       left side of each of the lines below will produce the value on the
       right side of the line:

              expr 3.1 + $a           6.1
              expr 2 + "$a.$b"        5.6
              expr 4*[llength "6 2"]  8
              expr {{word one} < "word $a"}0

   OPERATORS
       The valid operators (most of which are also available as commands in
       the tcl::mathop namespace; see the mathop(3tcl) manual page for
       details) are listed below, grouped in decreasing order of precedence:

       -  +  ~  !          Unary minus, unary plus, bit-wise NOT, logical NOT.
                           None of these operators may be applied to string
                           operands, and bit-wise NOT may be applied only to
                           integers.

       **                  Exponentiation.  Valid for any numeric operands.

       *  /  %             Multiply, divide, remainder.  None of these
                           operators may be applied to string operands, and
                           remainder may be applied only to integers.  The
                           remainder will always have the same sign as the
                           divisor and an absolute value smaller than the
                           absolute value of the divisor.

                           When applied to integers, the division and
                           remainder operators can be considered to partition
                           the number line into a sequence of equal-sized
                           adjacent non-overlapping pieces where each piece is
                           the size of the divisor; the division result
                           identifies which piece the divisor lay within, and
                           the remainder result identifies where within that
                           piece the divisor lay. A consequence of this is
                           that the result of “-57 / 10” is always -6, and the
                           result of “-57 % 10” is always 3.

       +  -                Add and subtract.  Valid for any numeric operands.

       <<  >>              Left and right shift.  Valid for integer operands
                           only.  A right shift always propagates the sign
                           bit.

       <  >  <=  >=        Boolean less, greater, less than or equal, and
                           greater than or equal.  Each operator produces 1 if
                           the condition is true, 0 otherwise.  These
                           operators may be applied to strings as well as
                           numeric operands, in which case string comparison
                           is used.

       ==  !=              Boolean equal and not equal.  Each operator
                           produces a zero/one result.  Valid for all operand
                           types.

       eq  ne              Boolean string equal and string not equal.  Each
                           operator produces a zero/one result.  The operand
                           types are interpreted only as strings.

       in  ni              List containment and negated list containment.
                           Each operator produces a zero/one result and treats
                           its first argument as a string and its second
                           argument as a Tcl list.  The in operator indicates
                           whether the first argument is a member of the
                           second argument list; the ni operator inverts the
                           sense of the result.

       &                   Bit-wise AND.  Valid for integer operands only.

       ^                   Bit-wise exclusive OR.  Valid for integer operands
                           only.

       |                   Bit-wise OR.  Valid for integer operands only.

       &&                  Logical AND.  Produces a 1 result if both operands
                           are non-zero, 0 otherwise.  Valid for boolean and
                           numeric (integers or floating-point) operands only.

       ||                  Logical OR.  Produces a 0 result if both operands
                           are zero, 1 otherwise.  Valid for boolean and
                           numeric (integers or floating-point) operands only.

       x?y:z               If-then-else, as in C.  If x evaluates to non-zero,
                           then the result is the value of y.  Otherwise the
                           result is the value of z.  The x operand must have
                           a boolean or numeric value.

       See the C manual for more details on the results produced by each
       operator.  The exponentiation operator promotes types like the multiply
       and divide operators, and produces a result that is the same as the
       output of the pow function (after any type conversions.)  All of the
       binary operators but exponentiation group left-to-right within the same
       precedence level; exponentiation groups right-to-left.  For example,
       the command

              expr {4*2 < 7}

       returns 0, while

              expr {2**3**2}

       returns 512.

       The &&, ||, and ?: operators have “lazy evaluation”, just as in C,
       which means that operands are not evaluated if they are not needed to
       determine the outcome.  For example, in the command

              expr {$v ? [a] : [b]}

       only one of “[a]” or “[b]” will actually be evaluated, depending on the
       value of $v.  Note, however, that this is only true if the entire
       expression is enclosed in braces;  otherwise the Tcl parser will
       evaluate both “[a]” and “[b]” before invoking the expr command.

   MATH FUNCTIONS
       When the expression parser encounters a mathematical function such as
       sin($x), it replaces it with a call to an ordinary Tcl function in the
       tcl::mathfunc namespace.  The processing of an expression such as:

              expr {sin($x+$y)}

       is the same in every way as the processing of:

              expr {[tcl::mathfunc::sin [expr {$x+$y}]]}

       which in turn is the same as the processing of:

              tcl::mathfunc::sin [expr {$x+$y}]

       The executor will search for tcl::mathfunc::sin using the usual rules
       for resolving functions in namespaces. Either ::tcl::mathfunc::sin or
       [namespace current]::tcl::mathfunc::sin will satisfy the request, and
       others may as well (depending on the current namespace path setting).

       Some mathematical functions have several arguments, separated by commas
       like in C. Thus:

              expr {hypot($x,$y)}

       ends up as

              tcl::mathfunc::hypot $x $y

       See the mathfunc(3tcl) manual page for the math functions that are
       available by default.

   TYPES, OVERFLOW, AND PRECISION
       All internal computations involving integers are done calling on the
       LibTomMath multiple precision integer library as required so that all
       integer calculations are performed exactly.  Note that in Tcl releases
       prior to 8.5, integer calculations were performed with one of the C
       types long int or Tcl_WideInt, causing implicit range truncation in
       those calculations where values overflowed the range of those types.
       Any code that relied on these implicit truncations will need to
       explicitly add int() or wide() function calls to expressions at the
       points where such truncation is required to take place.

       All internal computations involving floating-point are done with the C
       type double.  When converting a string to floating-point, exponent
       overflow is detected and results in the double value of Inf or -Inf as
       appropriate.  Floating-point overflow and underflow are detected to the
       degree supported by the hardware, which is generally pretty reliable.

       Conversion among internal representations for integer, floating-point,
       and string operands is done automatically as needed.  For arithmetic
       computations, integers are used until some floating-point number is
       introduced, after which floating-point is used.  For example,

              expr {5 / 4}

       returns 1, while

              expr {5 / 4.0}
              expr {5 / ( [string length "abcd"] + 0.0 )}

       both return 1.25.  Floating-point values are always returned with a “.”
       or an “e” so that they will not look like integer values.  For example,

              expr {20.0/5.0}

       returns 4.0, not 4.

   STRING OPERATIONS
       String values may be used as operands of the comparison operators,
       although the expression evaluator tries to do comparisons as integer or
       floating-point when it can, i.e., when all arguments to the operator
       allow numeric interpretations, except in the case of the eq and ne
       operators.  If one of the operands of a comparison is a string and the
       other has a numeric value, a canonical string representation of the
       numeric operand value is generated to compare with the string operand.
       Canonical string representation for integer values is a decimal string
       format.  Canonical string representation for floating-point values is
       that produced by the %g format specifier of Tcl's format command.  For
       example, the commands

              expr {"0x03" > "2"}
              expr {"0y" > "0x12"}

       both return 1.  The first comparison is done using integer comparison,
       and the second is done using string comparison.  Because of Tcl's
       tendency to treat values as numbers whenever possible, it is not
       generally a good idea to use operators like == when you really want
       string comparison and the values of the operands could be arbitrary;
       it is better in these cases to use the eq or ne operators, or the
       string command instead.

PERFORMANCE CONSIDERATIONS
       Enclose expressions in braces for the best speed and the smallest
       storage requirements.  This allows the Tcl bytecode compiler to
       generate the best code.

       As mentioned above, expressions are substituted twice: once by the Tcl
       parser and once by the expr command.  For example, the commands

              set a 3
              set b {$a + 2}
              expr $b*4

       return 11, not a multiple of 4.  This is because the Tcl parser will
       first substitute $a + 2 for the variable b, then the expr command will
       evaluate the expression $a + 2*4.

       Most expressions do not require a second round of substitutions.
       Either they are enclosed in braces or, if not, their variable and
       command substitutions yield numbers or strings that do not themselves
       require substitutions.  However, because a few unbraced expressions
       need two rounds of substitutions, the bytecode compiler must emit
       additional instructions to handle this situation.  The most expensive
       code is required for unbraced expressions that contain command
       substitutions.  These expressions must be implemented by generating new
       code each time the expression is executed.  When the expression is
       unbraced to allow the substitution of a function or operator, consider
       using the commands documented in the mathfunc(3tcl) or mathop(3tcl)
       manual pages directly instead.

EXAMPLES
       Define a procedure that computes an “interesting” mathematical
       function:

              proc tcl::mathfunc::calc {x y} {
                  expr { ($x**2 - $y**2) / exp($x**2 + $y**2) }
              }

       Convert polar coordinates into cartesian coordinates:

              # convert from ($radius,$angle)
              set x [expr { $radius * cos($angle) }]
              set y [expr { $radius * sin($angle) }]

       Convert cartesian coordinates into polar coordinates:

              # convert from ($x,$y)
              set radius [expr { hypot($y, $x) }]
              set angle  [expr { atan2($y, $x) }]

       Print a message describing the relationship of two string values to
       each other:

              puts "a and b are [expr {$a eq $b ? {equal} : {different}}]"

       Set a variable to whether an environment variable is both defined at
       all and also set to a true boolean value:

              set isTrue [expr {
                  [info exists ::env(SOME_ENV_VAR)] &&
                  [string is true -strict $::env(SOME_ENV_VAR)]
              }]

       Generate a random integer in the range 0..99 inclusive:

              set randNum [expr { int(100 * rand()) }]

SEE ALSO
       array(3tcl), for(3tcl), if(3tcl), mathfunc(3tcl), mathop(3tcl),
       namespace(3tcl), proc(3tcl), string(3tcl), Tcl(3tcl), while(3tcl)

KEYWORDS
       arithmetic, boolean, compare, expression, fuzzy comparison

COPYRIGHT
       Copyright (c) 1993 The Regents of the University of California.
       Copyright (c) 1994-2000 Sun Microsystems Incorporated.
       Copyright (c) 2005 by Kevin B. Kenny <kennykb@acm.org>. All rights reserved.



Tcl                                   8.5                           expr(3tcl)