test is a pattern which matches only if the evaluation of test yields True.
test represents a rule which applies only if the evaluation of test yields True.
test is a definition to be used only if test yields True.
Example: The pattern x_
0 represents an expression which must be positive.
All pattern variables used in test must also appear in patt.
1 defines a function in the case when .
] allows local variables to be shared between test and rhs. You can use the same construction with Block and With.
See the Mathematica book: Section 2.3.5, Section 2.5.8.
See also: If, Switch, Which, PatternTest.
This gives a definition for a function fac that applies only when its argument n is positive.
Here is a polynomial that is defined differently for positive and negative argument.
Evaluate the cell to see the graphic.