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 corre-
sponding 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 expres-
sions in the way that operands are specified. Also, Tcl
expressions support non-numeric operands and string com-
parisons.
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 processor. Where possible,
operands are interpreted as integer values. Integer val-
ues 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 installa-
tions). 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.
sion 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 substitu-
tions.
[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 func-
tions.
Where substitutions occur above (e.g. inside quoted
strings), they are performed by the expression processor.
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 pre-
vent the command parser from performing substitutions on
the contents.
For some examples of simple expressions, suppose the vari-
able 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 $a"}0
OPERATORS
The valid operators are listed below, grouped in decreas-
ing order of precedence:
- + ~ ! Unary minus, unary plus, bit-wise NOT,
logical NOT. None of these operands
may be applied to string operands, and
bit-wise NOT may be applied only to
integers.
* / % Multiply, divide, remainder. None of
these operands may be applied to
string operands, and remainder may be
applied only to integers. The remain-
smaller than the divisor.
+ - Add and subtract. Valid for any
numeric operands.
<< >> Left and right shift. Valid for inte-
ger operands only. A right shift
always propagates the sign bit.
< > <= >= Boolean less, greater, less than or
equal, and greater than or equal.
Each operator produces 1 if the condi-
tion is true, 0 otherwise. These
operators may be applied to strings as
well as numeric operands, in which
case string comparison is used.
== != Boolean equal and not equal. Each
operator produces a zero/one result.
Valid for all operand types.
& Bit-wise AND. Valid for integer
operands only.
^ Bit-wise exclusive OR. Valid for
integer operands only.
| Bit-wise OR. Valid for integer
operands only.
&& Logical AND. Produces a 1 result if
both operands are non-zero, 0 other-
wise. Valid for numeric operands only
(integers or floating-point).
|| Logical OR. Produces a 0 result if
both operands are zero, 1 otherwise.
Valid for numeric operands only (inte-
gers or floating-point).
x?y:z If-then-else, as in C. If x evaluates
to non-zero, then the result is the
value of y. Otherwise the result is
the value of z. The x operand must
have a numeric value.
See the C manual for more details on the results produced
by each operator. All of the binary operators group left-
to-right within the same precedence level. For example,
the command
expr 4*2 < 7
returns 0.
just as in C, which means that operands are not evaluated
if they are not needed to determine the outcome. For
example, in the command
expr {$v ? [a] : [b]}
only one of [a] or [b] will actually be evaluated, depend-
ing on the value of $v. Note, however, that this is only
true if the entire expression is enclosed in braces; oth-
erwise 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 imple-
ments the following functions for conversion between inte-
gers and floating-point 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.
round(arg))
If arg is an integer value, returns arg, otherwise
converts arg to integer by rounding and returns the
converted value.
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
verting 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''. The global variable
tcl_precision determines the number of significant digits
that are retained when floating values are converted to
strings (except that trailing zeroes are omitted). If
tcl_precision is unset then 6 digits of precision are
used. To retain all of the significant bits of an IEEE
floating-point number set tcl_precision to 17; if a value
is converted to string with 17 digits of precision and
then converted back to binary for some later calculation,
the resulting binary value is guaranteed to be identical
to the original one.
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 con-
verted back to a string using the C sprintf format speci-
fier %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 num-
bers whenever possible, it isn't generally a good idea to
trary; it's better in these cases to use the string com-
pare command instead.
KEYWORDS
arithmetic, boolean, compare, expression