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 per-
mitted 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 opera-
tors. 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-nu-
meric operands and string comparisons, as well as some additional oper-
ators 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 expres-
sion's instructions. Where possible, operands are interpreted as inte-
ger 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 speci-
fied in any of several common formats making use of the decimal digits,
the decimal point ., the characters e or E indicating scientific nota-
tion, and the sign characters + or -. For example, all of the follow-
ing 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 bool-
ean 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 bool-
ean.
[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 with-
out any substitutions.
[6] As a Tcl command enclosed in brackets. The command will be exe-
cuted 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 de-
tails) 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.
The maximum exponent value that Tcl can handle if
the first number is an integer > 1 is 268435455.
* / % Multiply, divide, remainder. None of these opera-
tors may be applied to string operands, and remain-
der 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 remain-
der 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 remain-
der result identifies where within that piece the
divisor lay. A consequence of this is that the re-
sult 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 opera-
tors may be applied to strings as well as numeric
operands, in which case string comparison is used.
== != Boolean equal and not equal. Each operator pro-
duces 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 argu-
ment as a Tcl list. The in operator indicates
whether the first argument is a member of the sec-
ond 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.
This operator evaluates lazily; it only evaluates
its second operand if it must in order to determine
its result.
|| Logical OR. Produces a 0 result if both operands
are zero, 1 otherwise. Valid for boolean and nu-
meric (integers or floating-point) operands only.
This operator evaluates lazily; it only evaluates
its second operand if it must in order to determine
its result.
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 evalu-
ates lazily; it evaluates only one of y or z.
See the C manual for more details on the results produced by each oper-
ator. 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 prece-
dence 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 $v. Note, however, that this is only true if the entire ex-
pression is enclosed in braces; otherwise the Tcl parser will evaluate
both “[a]” and “[b]” before invoking the expr command.
MATH FUNCTIONS
When the expression parser encounters a mathematical function such as
sin($x), it replaces it with a call to an ordinary Tcl 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.
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 explic-
itly 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 in-
troduced, 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, al-
though 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 op-
erators. If one of the operands of a comparison is a string and the
other has a numeric value, a canonical string representation of the nu-
meric 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 ten-
dency to treat values as numbers whenever possible, it is not generally
a good idea to use operators like == when you really want string com-
parison and the values of the operands could be arbitrary; it is bet-
ter 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 stor-
age 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 “$a + 2*4”.
Most expressions do not require a second round of substitutions. Ei-
ther they are enclosed in braces or, if not, their variable and command
substitutions yield numbers or strings that do not themselves require
substitutions. However, because a few unbraced expressions need two
rounds of substitutions, the bytecode compiler must emit additional in-
structions to handle this situation. The most expensive code is re-
quired for unbraced expressions that contain command substitutions.
These expressions must be implemented by generating new code each time
the expression is executed.
If it is necessary to include a non-constant expression string within
the wider context of an otherwise-constant expression, the most effi-
cient technique is to put the varying part inside a recursive expr, as
this at least allows for the compilation of the outer part, though it
does mean that the varying part must itself be evaluated as a separate
expression. Thus, in this example the result is 20 and the outer ex-
pression benefits from fully cached bytecode compilation.
set a 3
set b {$a + 2}
expr {[expr $b] * 4}
When the expression is unbraced to allow the substitution of a function
or operator, consider using the commands documented in the math-
func(3tcl) or mathop(3tcl) manual pages directly instead.
EXAMPLES
Define a procedure that computes an “interesting” mathematical func-
tion:
proc tcl::mathfunc::calc {x y} {
expr { ($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), name-
space(3tcl), proc(3tcl), string(3tcl), Tcl(3tcl), while(3tcl)
KEYWORDS
arithmetic, boolean, compare, expression, fuzzy comparison
COPYRIGHT
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)
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