expr(3Tcl) expr(3Tcl)
expr  Evaluate an expression
expr arg ?arg arg ...?
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 floatingpoint 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 nonnumeric
operands and string comparisons.
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 floatingpoint
number if that is possible. Floatingpoint numbers may be specified in
any of the ways accepted by an ANSIcompliant 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 floatingpoint
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 floatingpoint.
[2] As a Tcl variable, using standard $ notation. The variable's value
will be used as the operand.
[3] As a string enclosed in doublequotes. The expression parser will
perform backslash, variable, and command substitutions on the
information between the quotes, and use the resulting value as the
operand
Page 1
expr(3Tcl) expr(3Tcl)
[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
The valid operators are listed below, grouped in decreasing order of
precedence:
 + ~ ! Unary minus, unary plus, bitwise NOT, logical NOT. 
None of these operands may be applied to string
operands, and bitwise 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.
+  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
Page 2
expr(3Tcl) expr(3Tcl)
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.
& Bitwise AND. Valid for integer operands only.
^ Bitwise exclusive OR. Valid for integer operands
only.
 Bitwise OR. Valid for integer operands only.
&& Logical AND. Produces a 1 result if both operands
are nonzero, 0 otherwise. Valid for numeric
operands only (integers or floatingpoint).
 Logical OR. Produces a 0 result if both operands are
zero, 1 otherwise. Valid for numeric operands only
(integers or floatingpoint).
x?y:z Ifthenelse, as in C. If x evaluates to nonzero,
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 lefttoright 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.
Tcl supports the following mathematical functions in expressions: 
acos cos hypot sinh 
asin cosh log sqrt 
atan exp log10 tan 
atan2 floor pow tanh 
Page 3
expr(3Tcl) expr(3Tcl)
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 floatingpoint numbers:
abs(arg)
Returns the absolute value of arg. Arg may be either integer or 
floatingpoint, 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 the C type
long, and all internal computations involving floatingpoint are done
with the C type double. When converting a string to floatingpoint,
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. Floatingpoint overflow
and underflow are detected to the degree supported by the hardware, which
is generally pretty reliable.
Conversion among internal representations for integer, floatingpoint,
and string operands is done automatically as needed. For arithmetic
computations, integers are used until some floatingpoint number is
introduced, after which floatingpoint 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. Floatingpoint values are always returned with a ``.''
or an ``e'' so that they will not look like integer values. For example,
Page 4
expr(3Tcl) expr(3Tcl)
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 floatingpoint 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 values may be used as operands of the comparison operators,
although the expression evaluator tries to do comparisons as integer or
floatingpoint 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 floatingpoint 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 better in these cases to
use the string compare command instead.
arithmetic, boolean, compare, expression
PPPPaaaaggggeeee 5555 [ Back ]
