diff options
Diffstat (limited to 'doc')
| -rw-r--r-- | doc/expr.n | 28 |
1 files changed, 23 insertions, 5 deletions
@@ -106,7 +106,7 @@ 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 8c +.ta 9c \fBexpr\fR 3.1 + $a \fI6.1\fR \fBexpr\fR 2 + "$a.$b" \fI5.6\fR \fBexpr\fR 4*[llength "6 2"] \fI8\fR @@ -227,6 +227,7 @@ of the \fBpow\fR function (after any type conversions.) All of the binary operators but exponentiation group left-to-right within the same precedence level; exponentiation groups right-to-left. For example, the command .PP +.PP .CS \fBexpr\fR {4*2 < 7} .CE @@ -264,7 +265,7 @@ before invoking the \fBexpr\fR command. .PP When the expression parser encounters a mathematical function such as \fBsin($x)\fR, it replaces it with a call to an ordinary -Tcl function in the \fBtcl::mathfunc\fR namespace. The processing +Tcl command in the \fBtcl::mathfunc\fR namespace. The processing of an expression such as: .PP .CS @@ -396,9 +397,11 @@ set b {$a + 2} .CE .PP return 11, not a multiple of 4. -This is because the Tcl parser will first substitute \fB$a + 2\fR for -the variable \fBb\fR, -then the \fBexpr\fR command will evaluate the expression \fB$a + 2*4\fR. +This is because the Tcl parser will first substitute +.QW "\fB$a + 2\fR" +for the variable \fBb\fR, +then the \fBexpr\fR command will evaluate the expression +.QW "\fB$a + 2*4\fR" . .PP Most expressions do not require a second round of substitutions. Either they are enclosed in braces or, if not, @@ -412,6 +415,21 @@ The most expensive code is required for unbraced expressions that contain command substitutions. These expressions must be implemented by generating new code each time the expression is executed. +.PP +If it is necessary to include a non-constant expression string within the +wider context of an otherwise-constant expression, the most efficient +technique is to put the varying part inside a recursive \fBexpr\fR, as this at +least allows for the compilation of the outer part, though it does mean that +the varying part must itself be evaluated as a separate expression. Thus, in +this example the result is 20 and the outer expression benefits from fully +cached bytecode compilation. +.PP +.CS +set a 3 +set b {$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. |