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Condition (/;)

patt/;test
is a pattern which matches only if the evaluation of test yields True.
lhs:>rhs/;test
represents a rule which applies only if the evaluation of test yields True.
lhs:=rhs/;test
is a definition to be used only if test yields True.
  • All pattern variables used in test must also appear in patt.
  • lhs:=Module[{vars}, rhs/;test] allows local variables to be shared between test and rhs. You can use the same construction with Block and With.  »
Make a definition with the condition that x should be positive:
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Replace all elements which satisfy the condition of being negative:
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