BUILT-IN MATHEMATICA SYMBOL

Catch

Catch[expr]
returns the argument of the first Throw generated in the evaluation of expr.

Catch

returns value from the first Throw

for which form matches tag.

Catch

returns

.

MORE INFORMATION

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.

EXAMPLES

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Basic Examples (3)

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:

In[1]:=

Out[1]=

Define a function that can "throw an exception":

In[1]:=

The result of the Catch is just what is thrown by Throw:

In[2]:=

Out[2]=

In[3]:=

Out[3]=

Use Throw to exit a loop when a criterion is satisfied:

In[1]:=

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:

Generalizations & Extensions (3)

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:

Properties & Relations (1)

Use Check to throw an exception if a message is generated: