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Difference between revisions of "UM:NetXMS Scripting Language (NXSL)"
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UM:NetXMS Scripting Language (NXSL) (view source)
Revision as of 12:17, 27 December 2012
, 12:17, 27 December 2012no edit summary
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The last example of expressions is combined operator-assignment expressions. You already know that if you want to increment ''a'' by 1, you can simply write '''a''++' or '++''a'''. But what if you want to add more than one to it, for instance 3? In NXSL, adding 3 to the current value of ''a'' can be written '''a'' += 3'. This means exactly "take the value of ''a'', add 3 to it, and assign it back into ''a''". In addition to being shorter and clearer, this also results in faster execution. The value of '''a'' += 3', like the value of a regular assignment, is the assigned value. Notice that it is NOT 3, but the combined value of ''a'' plus 3 (this is the value that's assigned into ''a''). Any two-place operator can be used in this operator-assignment mode. | The last example of expressions is combined operator-assignment expressions. You already know that if you want to increment ''a'' by 1, you can simply write '''a''++' or '++''a'''. But what if you want to add more than one to it, for instance 3? In NXSL, adding 3 to the current value of ''a'' can be written '''a'' += 3'. This means exactly "take the value of ''a'', add 3 to it, and assign it back into ''a''". In addition to being shorter and clearer, this also results in faster execution. The value of '''a'' += 3', like the value of a regular assignment, is the assigned value. Notice that it is NOT 3, but the combined value of ''a'' plus 3 (this is the value that's assigned into ''a''). Any two-place operator can be used in this operator-assignment mode. | ||
== Short-circuit evaluation == | |||
[http://en.wikipedia.org/wiki/Short-circuit_evaluation Short-circuit evaluation] denotes the semantics of some Boolean operators in which the second argument is only executed or evaluated if the first argument does not suffice to determine the value of the expression: when the first argument of the AND function evaluates to false, the overall value must be false; and when the first argument of the OR function evaluates to true, the overall value must be true. NXSL uses short-circuit evaluation for && and || boolean operators. This feature permits two useful programming constructs. Firstly, if the first sub-expression checks whether an expensive computation is needed and the check evaluates to false, one can eliminate expensive computation in the second argument. Secondly, it permits a construct where the first expression guarantees a condition without which the second expression may cause a run-time error. Both are illustrated in the following example: | |||
<syntaxhighlight lang="c"> | |||
if ((x != null) && ((trim(x) == "abc") || (long_running_test(x))) | |||
do_something(); | |||
</syntaxhighlight> | |||
Without short-circuit evaluation, trim(x) would cause run-time error if x is null. Also, long running function will only be called if condition (trim(x) == "abc") will be false. | |||