Programmer Guide/Command Reference/COND: Difference between revisions
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Generally, the conditional assignment takes the following form: | Generally, the conditional assignment takes the following form: | ||
target := COND [[Programmer_Guide/ | target := COND [[Programmer_Guide/Introduction#Conditional_Expressions|condition]] ? expr1 : expr2 | ||
;<code>target</code>: is a normal assignment target, usually the name of an {{STX}} variable, e.g. <code>#var</code>. | ;<code>target</code>: is a normal assignment target, usually the name of an {{STX}} variable, e.g. <code>#var</code>. | ||
;<code>condition</code>: is a [[Programmer_Guide/ | ;<code>condition</code>: is a [[Programmer_Guide/Introduction#Conditional_Expressions|conditional expression]] like used with the <code>[[User Guide/Workspace/Parameter Processing|IF]]</code> statement and the miscellaneous loop commands, e.g. the string <code>$#a == 7 || $#a == 42</code> (for more examples, see the <code>[[User Guide/Workspace/Parameter Processing|IF]]</code> statement). | ||
:Note that with conditional {{STX}} expressions there must always be intervening whitespaces between operators and their arguments (unless the argument is quoted). So, both <code>$#a == 7</code> and <code>'$#a'=='7'</code> are valid expression, whereas <code>$#a==7</code> is not. | :Note that with conditional {{STX}} expressions there must always be intervening whitespaces between operators and their arguments (unless the argument is quoted). So, both <code>$#a == 7</code> and <code>'$#a'=='7'</code> are valid expression, whereas <code>$#a==7</code> is not. | ||
;<code>expr1</code> and <code>expr2</code>: Both <code>expr1</code> and <code>expr2</code> may be any expressions that may be normally used in an {{STX}} <code>:=</code> assignment, with the only exception of a <code>COND</code> expression (meaning that conditional assignments must not be nested for the time being). If, at runtime, the conditional expression <code>condition</code> evaluates to truth, the value determined by the first expression, <code>expr1</code>, will be assigned to <code>target</code>. If <code>condition</code> evaluates to falsehood, it will be the second expression, <code>expr2</code>, that gets assigned to target. | ;<code>expr1</code> and <code>expr2</code>: Both <code>expr1</code> and <code>expr2</code> may be any expressions that may be normally used in an {{STX}} <code>:=</code> assignment, with the only exception of a <code>COND</code> expression (meaning that conditional assignments must not be nested for the time being). If, at runtime, the conditional expression <code>condition</code> evaluates to truth, the value determined by the first expression, <code>expr1</code>, will be assigned to <code>target</code>. If <code>condition</code> evaluates to falsehood, it will be the second expression, <code>expr2</code>, that gets assigned to target. | ||
You may use any {{STX}} [[Programmer_Guide/ | You may use any {{STX}} [[Programmer_Guide/Introduction#Conditional_Expressions|conditional expression]] you like for <code>condition</code> with the exception of conditions themselves containing the <code>COND</code> keyword. Equally, there is no restriction on <code>expr1</code> and <code>expr2</code>, except that they must not be built up from <code>COND</code> expressions themselves, either. It is even (and also) possible to use command substitutions (<code>$(...)</code>), nesting them as deeply as one feels inclined to. | ||
=== Notes === | === Notes === | ||
Revision as of 12:08, 18 April 2011
The COND command performs a conditional assignment.
Generally, the conditional assignment takes the following form:
target := COND condition ? expr1 : expr2
target- is a normal assignment target, usually the name of an STx variable, e.g.
#var. condition- is a conditional expression like used with the
IFstatement and the miscellaneous loop commands, e.g. the string$#a == 7 || $#a == 42(for more examples, see theIFstatement). - Note that with conditional STx expressions there must always be intervening whitespaces between operators and their arguments (unless the argument is quoted). So, both
$#a == 7and'$#a'=='7'are valid expression, whereas$#a==7is not. expr1andexpr2- Both
expr1andexpr2may be any expressions that may be normally used in an STx:=assignment, with the only exception of aCONDexpression (meaning that conditional assignments must not be nested for the time being). If, at runtime, the conditional expressionconditionevaluates to truth, the value determined by the first expression,expr1, will be assigned totarget. Ifconditionevaluates to falsehood, it will be the second expression,expr2, that gets assigned to target.
You may use any STx conditional expression you like for condition with the exception of conditions themselves containing the COND keyword. Equally, there is no restriction on expr1 and expr2, except that they must not be built up from COND expressions themselves, either. It is even (and also) possible to use command substitutions ($(...)), nesting them as deeply as one feels inclined to.
Notes
- The
CONDcommand is processed by the loader, and is therefore not available in the command line interface. - The
CONDcommand may not contain a nestedCONDcommand.
Examples
#min := cond $#a < $#b ? $#a : $#b // calculate minimum of #a and #b #abs := cond $#a < 0 ? num -$#a : $#a // calculate absolute value of #a #abs := cond $#a < 0 ? eval $#a*(-1) : $#a // an alternative to the above #absdiff := cond $#a > $#b ? eval $#a-$#b : eval $#b-$#a // absolute difference #len := cond $(length $#a) > 0 ? length $#a : length $#b // length of #a or, if empty, of #b
See the file conditional_assignment.sts for further working examples.
