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


       expr - Evaluate an expression

       expr arg ?arg arg ...?

       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.

       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

       [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

       [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

       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

       **                  Exponentiation.  Valid for any numeric operands.  The
                           maximum exponent value that Tcl can handle if the
                           first number is an integer > 1 is 268435455.

       *  /  %             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

                           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

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

       ^                   Bit-wise exclusive OR.  Valid for integer operands

       |                   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.
                           This operator evaluates lazily; it only evaluates its
                           second operand if it must in order to determine its

       ||                  Logical OR.  Produces a 0 result if both operands are
                           zero, 1 otherwise.  Valid for boolean and numeric
                           (integers or floating-point) operands only.  This
                           operator evaluates lazily; it only evaluates its
                           second operand if it must in order to determine its

       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.  This operator evaluates
                           lazily; it evaluates only one of y or z.

       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

              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.

       When the expression parser encounters a mathematical function such as
       sin($x), it replaces it with a call to an ordinary Tcl command 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.

       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 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.

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

       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.

       If it is necessary to include a non-constant expression string within the
       wider context of an otherwise-constant expression, the most efficient
       technique is to put the varying part inside a recursive expr, as this at
       least allows for the compilation of the outer part, though it does mean
       that the varying part must itself be evaluated as a separate expression.
       Thus, in this example the result is 20 and the outer expression benefits
       from fully cached bytecode compilation.

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

       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.

       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

              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()) }]

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

       arithmetic, boolean, compare, expression, fuzzy comparison

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

Tcl                                    8.5                            expr(3tcl)