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
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, 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 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
The valid operators (most of which are also available as commands in the
tcl::mathop namespace; see the
mathop(n) 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 group
left-to-right within the same precedence level. For example, the command
returns 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
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 function in the
tcl::mathfunc namespace. The processing of an expression such as:
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).
See the
mathfunc(n) 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,
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,
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 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
$a + 2*4.
Most expressions do not require a second round of substitutions. Either they are
enclosed in braces or, if not, their variable and command substitutions yield
numbers or strings that do not themselves require substitutions. However,
because a few unbraced expressions need two rounds of substitutions, the
bytecode compiler must emit additional instructions to handle this situation.
The most expensive code is required for unbraced expressions that contain
command substitutions. These expressions must be implemented by generating new
code each time the expression is executed. When the expression is unbraced to
allow the substitution of a function or operator, consider using the commands
documented in the
mathfunc(n) or
mathop(n) manual pages directly
instead.
Define a procedure that computes an “interesting” mathematical
function:
proc tcl::mathfunc::calc {x y} {
expr { ($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, $x) }]
Print a message describing the relationship of two string values to each other:
puts "a and b are [ expr {$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()) }]
array(n), for(n), if(n), mathfunc(n), mathop(n), namespace(n), proc(n),
string(n), Tcl(n), while(n)
arithmetic, boolean, compare, expression, fuzzy comparison
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.