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Concatenates args (adding separator spaces between
them), evaluates the result as a Tcl expression, and returns the value.
The operators permitted in Tcl expressions are a subset of the operators
permitted in C expressions, and they have 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 + 6evaluates 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.
A Tcl expression consists of a combination of operands, operators, and parentheses. White space may be used between the operands and operators and parentheses; 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 octal (if the first character of the operand is 0), or in hexadecimal (if the first two characters of the operand are 0x). 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 the ways accepted by an ANSI-compliant C compiler (except that the f, F, l, and L suffixes will not be permitted in most installations). For example, all of the following are valid floating-point numbers: 2.1, 3., 6e4, 7.91e+16. 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).
On 32-bit systems, integer values MAX_INT (0x7FFFFFFF) and MIN_INT (-0x80000000) will be represented as 32-bit values, and integer values outside that range will be represented as 64-bit values (if that is possible at all.)
Operands may be specified in any of the following ways:
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 $a"} 0
The valid operators are listed below, grouped in decreasing order of precedence:
See the C manual for more details on the results produced by each
operator. All of the binary operators group left-to-right within the same
precedence level. For example, the command
expr 4*2 < 7returns 0.
The &&, ||, and ?: operators have ‘‘lazy evaluation’’, just as in C,
which means that operands are not evaluated if they are not needed to
determine the outcome. For example, in the command
expr {$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.
Tcl supports the following
mathematical functions in expressions, all of which work solely with floating-point
numbers unless otherwise noted:
abs cosh log sqrt
acos double log10 srand
asin exp pow tan
atan floor rand tanh
atan2 fmod round wide
ceil hypot sin
cos int sinh
In addition to these predefined functions, applications may define additional functions using Tcl_CreateMathFunc().
All internal computations involving integers are done with the C type long, and 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 a Tcl error. For conversion to integer from string, detection of overflow depends on the behavior of some routines in the local C library, so it should be regarded as unreliable. In any case, integer overflow and underflow are generally not detected reliably for intermediate results. 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 / 4returns 1, while
expr 5 / 4.0both 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 5 / ( [string length "abcd"] + 0.0 )
expr 20.0/5.0returns 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, 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, the numeric operand is converted
back to a string using the C sprintf format specifier %d for integers and
%g for floating-point values. For example, the commands
expr {"0x03" > "2"}both return 1. The first comparison is done using integer comparison, and
the second is done using string comparison after the second operand is
converted to the string 18. Because of Tcl’s tendency to treat values as
numbers whenever possible, it isn’t generally a good idea to use operators
like == when you really want string comparison and the values of the operands
could be arbitrary; it’s better in these cases to use the eq or ne operators,
or the string command instead.
expr {"0y" < "0x12"}
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 3return 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 $a + 2*4.
set b {$a + 2}
expr $b*4
Most expressions do not require a second round of substitutions. Either they are enclosed in braces or, if not, their variable and command substitutions yield numbers or strings that don’t themselves require substitutions. However, because a few unbraced expressions need two rounds of substitutions, the bytecode compiler must emit additional instructions to handle this situation. The most expensive code is required for unbraced expressions that contain command substitutions. These expressions must be implemented by generating new code each time the expression is executed.
proc calc {x y} {
expr { ($x*$x - $y*$y) / exp($x*$x + $y*$y) }
}
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()) }]