This is documentation for Mathematica 8, which was
based on an earlier version of the Wolfram Language.

# Catch

 Catch[expr]returns the argument of the first Throw generated in the evaluation of expr. Catchreturns value from the first Throw for which form matches tag. Catchreturns .
• Catch always returns the value of expr if no Throw was generated during the evaluation.
• form can be any expression, and is often a pattern.
• tag in Throw is re-evaluated every time it is compared to form.
Exit to the enclosing Catch as soon as the Throw is evaluated:
Define a function that can "throw an exception":
The result of the Catch is just what is thrown by Throw:
Use Throw to exit a loop when a criterion is satisfied:
Exit to the enclosing Catch as soon as the Throw is evaluated:
 Out[1]=

Define a function that can "throw an exception":
The result of the Catch is just what is thrown by Throw:
 Out[2]=
 Out[3]=

Use Throw to exit a loop when a criterion is satisfied:
 Out[1]=
 Scope   (5)
Catch can catch a Throw from inside essentially any function:
The nearest enclosing Catch catches the Throw:
Catch picks up the first Throw that is evaluated:
Throw need not occur lexically inside Catch:
A function that can throw a number of different exceptions:
A handler for the possible exceptions:
Catch the Throw with tag :
The inner Catch catches the Throw:
The outer Catch catches the Throw:
Keep the tag local:
 Applications   (3)
Find the next prime after :
Find the first power of 17 equal to 1 mod 19:
Stop if an iteration gets too large:
Use Check to throw an exception if a message is generated: