Tcl Source Code

For some examples of simple expressions, suppose the variable
\fBa\fR has the value 3 and
the variable \fBb\fR 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:
.PP
.CS
.ta 9c
\fBexpr\fR 3.1 + $a	\fI6.1\fR
\fBexpr\fR 2 + "$a.$b"	\fI5.6\fR
\fBexpr\fR 4*[llength "6 2"]	\fI8\fR
\fBexpr\fR {{word one} < "word $a"}	\fI0\fR
.CE
.SS OPERATORS
.PP

produced by each operator.
The exponentiation operator promotes types like the multiply and
divide operators, and produces a result that is the same as the output
of the \fBpow\fR function (after any type conversions.)
All of the binary operators but exponentiation group left-to-right
within the same precedence level; exponentiation groups right-to-left.  For example, the command
.PP
.PP
.CS
\fBexpr\fR {4*2 < 7}
.CE
.PP
returns 0, while
.PP
.CS

and
.QW \fB[b]\fR
before invoking the \fBexpr\fR command.
.SS "MATH FUNCTIONS"
.PP
When the expression parser encounters a mathematical function
such as \fBsin($x)\fR, it replaces it with a call to an ordinary
Tcl command in the \fBtcl::mathfunc\fR namespace.  The processing
of an expression such as:
.PP
.CS
\fBexpr\fR {sin($x+$y)}
.CE
.PP
is the same in every way as the processing of:

.CS
set a 3
set b {$a + 2}
\fBexpr\fR $b*4
.CE
.PP
return 11, not a multiple of 4.
This is because the Tcl parser will first substitute
.QW "\fB$a + 2\fR"
for the variable \fBb\fR,
then the \fBexpr\fR command will evaluate the expression
.QW "\fB$a + 2*4\fR" .
.PP
Most expressions do not require a second round of substitutions.
Either they are enclosed in braces or, if not,
their variable and command substitutions yield numbers or strings
that do not themselves require substitutions.
However, because a few unbraced expressions
need two rounds of substitutions,
the bytecode compiler must emit
additional instructions to handle this situation.
The most expensive code is required for
unbraced expressions that contain command substitutions.
These expressions must be implemented by generating new code
each time the expression is executed.
.PP
If it is necessary to include a non-constant expression string within the
wider context of an otherwise-constant expression, the most efficient
technique is to put the varying part inside a recursive \fBexpr\fR, as this at
least allows for the compilation of the outer part, though it does mean that
the varying part must itself be evaluated as a separate expression. Thus, in
this example the result is 20 and the outer expression benefits from fully
cached bytecode compilation.
.PP
.CS
set a 3
set b {$a + 2}
\fBexpr\fR {[\fBexpr\fR $b] * 4}
.CE
.PP
When the expression is unbraced to allow the substitution of a function or
operator, consider using the commands documented in the \fBmathfunc\fR(n) or
\fBmathop\fR(n) manual pages directly instead.
.SH EXAMPLES
.PP
Define a procedure that computes an
.QW interesting