expr(n) Tcl (8.0) 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 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 + 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 interpretation is possible, 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 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 operand.
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expr(n) Tcl (8.0) expr(n)
[3] 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
[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
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 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
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
divisor.
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expr(n) Tcl (8.0) expr(n)
+ - 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.
& 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 operators 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
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expr(n) Tcl (8.0) expr(n)
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:
acos cos hypot sinh
asin cosh log sqrt
atan exp log10 tan
atan2 floor pow tanh
ceil fmod sin
Each of these functions invokes the math library function of
the same name; see the manual entries for the library
functions for details on what they do. Tcl also implements
the following functions for conversion between integers and
floating-point numbers and the generation of random numbers:
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.
double(arg)
If arg is a floating value, returns arg, otherwise
converts arg to floating and returns the converted
value.
int(arg)
If arg is an integer value, returns arg, otherwise
converts arg to integer by truncation and returns the
converted value.
rand()
Returns a floating point number from zero to just less
than one or, in mathematical terms, the range [0,1).
The seed comes from the internal clock of the machine
or may be set manual 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.
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expr(n) Tcl (8.0) expr(n)srand(arg)
The arg, which must be an integer, is used to reset the
seed for the random number generator. Returns the
first random number from that seed. Each interpreter
has it's own seed.
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 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 / 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. 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
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expr(n) Tcl (8.0) expr(n)
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 string compare 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.
KEYWORDS
arithmetic, boolean, compare, expression, fuzzy comparison
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