expr(n) Tcl (7.0) expr(n)
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NAME
expr - Evalue 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 processor. 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 (7.0) 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 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 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, 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 (7.0) expr(n)
+ - Add and subtract. Valid for any numeric
operands.
<< >> Left and right shift. Valid for integer
operands only.
< > <= >= 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 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 (integers 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.
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expr(n) Tcl (7.0) expr(n)
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: |
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: |
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 predifined functions, applications may |
define additional functions using Tcl_CreateMathFunc().
TYPES, OVERFLOW, AND PRECISION
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expr(n) Tcl (7.0) expr(n)
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''. The global variable |
tcl_precision determines the 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 converted
back to a string using the C sprintf format specifier %d for
integers and %g for floating-point values. For example, the
commands
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expr(n) Tcl (7.0) expr(n)
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''.
KEYWORDS
arithmetic, boolean, compare, expression
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