expr(n) Tcl Built-In Commands expr(n)
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NAME
expr - Evaluate an expression
SYNOPSIS
expr arg ?arg arg ...?
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DESCRIPTION
Concatenates arg's (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 per-mitted permitted
mitted in C expressions, and they have the same meaning and precedence as the corresponding C opera-tors. operators.
tors. Expressions almost always yield numeric results (integer or floating-point values). For exam-ple, example,
ple, 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.
OPERANDS
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 inter-pretation interpretation
pretation is possible, 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:
[1] As an numeric value, either integer or floating-point.
[2] As a Tcl variable, using standard $ notation. The variable's value will be used as the oper-and. operand.
and.
[3] As a string enclosed in double-quotes. The expression parser will perform backslash, vari-able, variable,
able, and command substitutions on the information between the quotes, and use the resulting
value as the operand
[4] 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.
[5] As a Tcl command enclosed in brackets. The command will be executed and its result will be
used as the operand.
[6] As a mathematical function whose arguments have any of the above forms for operands, such as
sin($x). See below for a list of defined functions.
Where substitutions occur above (e.g. inside quoted strings), they are performed by the expression's
instructions. However, an additional layer of substitution may already have been performed by the
command parser 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 con-tents. contents.
tents.
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 are listed below, grouped in decreasing order of precedence:
- + ~ ! Unary minus, unary plus, bit-wise NOT, logical NOT. None of these operands may
be applied to string operands, and bit-wise NOT may be applied only to integers.
* / % Multiply, divide, remainder. None of these operands may be applied to string op-
erands, 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 divisor.
+ - 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 oper-ator operator
ator 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.
& 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 numeric
value.
See the C manual for more details on the results produced by each operator. All of the binary opera-tors operators
tors group left-to-right within the same precedence level. For example, the command
expr 4*2 < 7
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
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
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
abs(arg)
Returns the absolute value of arg. Arg may be either integer or floating-point, and the
result is returned in the same form.
acos(arg)
Returns the arc cosine of arg, in the range [0,pi] radians. Arg should be in the range [-1,1].
asin(arg)
Returns the arc sine of arg, in the range [-pi/2,pi/2] radians. Arg should be in the range
[-1,1].
atan(arg)
Returns the arc tangent of arg, in the range [-pi/2,pi/2] radians.
atan2(y, x)
Returns the arc tangent of y/x, in the range [-pi,pi] radians. x and y cannot both be 0. If
x is greater than 0, this is equivalent to atan(y/x).
ceil(arg)
Returns the smallest integral floating-point value (i.e. with a zero fractional part) not less
than arg.
cos(arg)
Returns the cosine of arg, measured in radians.
cosh(arg)
Returns the hyperbolic cosine of arg. If the result would cause an overflow, an error is
returned.
double(arg)
If arg is a floating-point value, returns arg, otherwise converts arg to floating-point and
returns the converted value.
exp(arg)
Returns the exponential of arg, defined as e**arg. If the result would cause an overflow, an
error is returned.
floor(arg)
Returns the largest integral floating-point value (i.e. with a zero fractional part) not
greater than arg.
fmod(x, y)
Returns the floating-point remainder of the division of x by y. If y is 0, an error is
returned.
hypot(x, y)
Computes the length of the hypotenuse of a right-angled triangle sqrt(x*x+y*y).
int(arg)
If arg is an integer value of the same width as the machine word, returns arg, otherwise con- |
verts arg to an integer (of the same size as a machine word, i.e. 32-bits on 32-bit systems, |
and 64-bits on 64-bit systems) by truncation and returns the converted value.
log(arg)
Returns the natural logarithm of arg. Arg must be a positive value.
log10(arg)
Returns the base 10 logarithm of arg. Arg must be a positive value.
pow(x, y)
Computes the value of x raised to the power y. If x is negative, y must be an integer value.
rand() Returns a pseudo-random floating-point value in the range (0,1). The generator algorithm is a
simple linear congruential generator that is not cryptographically secure. Each result from
rand completely determines all future results from subsequent calls to rand, so rand should
not be used to generate a sequence of secrets, such as one-time passwords. The seed of the
generator is initialized from the internal clock of the machine or may be set with the srand
function.
round(arg)
If arg is an integer value, returns arg, otherwise converts arg to integer by rounding and
returns the converted value.
sin(arg)
Returns the sine of arg, measured in radians.
sinh(arg)
Returns the hyperbolic sine of arg. If the result would cause an overflow, an error is
returned.
sqrt(arg)
Returns the square root of arg. Arg must be non-negative.
srand(arg)
The arg, which must be an integer, is used to reset the seed for the random number generator
of rand. Returns the first random number (see rand()) from that seed. Each interpreter has
its own seed.
tan(arg)
Returns the tangent of arg, measured in radians.
tanh(arg)
Returns the hyperbolic tangent of arg.
wide(arg)
Converts arg to an integer value at least 64-bits wide (by sign-extension if arg is a 32-bit |
number) if it is not one already.
In addition to these predefined functions, applications may define additional functions using
Tcl_CreateMathFunc().
TYPES, OVERFLOW, AND PRECISION
All internal computations involving integers are done with the C type long, and all internal computa-
tions involving floating-point are done with the C type double. When converting a string to float-ing-point, floating-point,
ing-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 / 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, 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"}
expr {"0y" < "0x12"}
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 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 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 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.
SEE ALSO
array(n), string(n), Tcl(n)
KEYWORDS
arithmetic, boolean, compare, expression, fuzzy comparison
Tcl 8.4 expr(n)
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