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/Introduction#Conditional_Expressions|condition]] ? expr1 : expr2
  '#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/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 || $#a == 42</code> (for more examples, see the <code>[[User Guide/Workspace/Parameter Processing|IF]]</code> statement).
;<code>''condition''</code>: is a [[Programmer_Guide/Introduction#Conditional_Expressions|conditional expression]] like used with the <code>[[Programmer_Guide/Command_Reference|IF]]</code> statement and the miscellaneous conditional [[Programmer_Guide/Introduction#Control_Commands|control commands, e.g. the string <code>$#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.

Revision as of 12:11, 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 IF statement and the miscellaneous conditional [[Programmer_Guide/Introduction#Control_Commands|control commands, e.g. the string $#a == 7 || $#a == 42 (for more examples, see the IF 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 $#a == 7 and '$#a'=='7' are valid expression, whereas $#a==7 is not.
expr1 and expr2
Both expr1 and expr2 may be any expressions that may be normally used in an STx := assignment, with the only exception of a COND expression (meaning that conditional assignments must not be nested for the time being). If, at runtime, the conditional expression condition evaluates to truth, the value determined by the first expression, expr1, will be assigned to target. If condition evaluates 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 COND command is processed by the loader, and is therefore not available in the command line interface.
  • The COND command may not contain a nested COND command.

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.

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