Catch

✖
`Catch`

✖

Catch[expr]

returns the argument of the first Throw generated in the evaluation of expr.

✖

Catch[expr,form]

returns value from the first Throw[value,tag] for which form matches tag.

✖

Catch[expr,form,f]

returns f[value,tag].

Details

Catch[expr,…] 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[value,tag] is re‐evaluated every time it is compared to form.

Examples

open allclose all

Basic Examples (3)Summary of the most common use cases

Exit to the enclosing Catch as soon as the Throw is evaluated:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-kp4

Out[1]=1

Define a function that can "throw an exception":

In[1]:=1

✖

https://wolfram.com/xid/0y76co-gq0

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

In[2]:=2

✖

https://wolfram.com/xid/0y76co-f8o

Out[2]=2

In[3]:=3

✖

https://wolfram.com/xid/0y76co-rfa

Out[3]=3

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

In[1]:=1

✖

https://wolfram.com/xid/0y76co-c34

Out[1]=1

Scope (5)Survey of the scope of standard use cases

Catch can catch a Throw from inside essentially any function:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-yal

Out[1]=1

In[2]:=2

✖

https://wolfram.com/xid/0y76co-yfa

Out[2]=2

In[3]:=3

✖

https://wolfram.com/xid/0y76co-f8w

Out[3]=3

The nearest enclosing Catch catches the Throw:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-y40

Out[1]=1

Catch picks up the first Throw that is evaluated:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-u

Out[1]=1

In[2]:=2

✖

https://wolfram.com/xid/0y76co-x87

Out[2]=2

Throw need not occur lexically inside Catch:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-bdikhw

In[2]:=2

✖

https://wolfram.com/xid/0y76co-npb522

Out[2]=2

A function that can throw a number of different exceptions:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-fmolww

A handler for the possible exceptions:

In[2]:=2

✖

https://wolfram.com/xid/0y76co-laohle

In[3]:=3

✖

https://wolfram.com/xid/0y76co-32ihy

Out[3]=3

Generalizations & Extensions (3)Generalized and extended use cases

Catch the Throw with tag u:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-tax

Out[1]=1

The inner Catch catches the Throw:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-wu9

Out[1]=1

The outer Catch catches the Throw:

In[2]:=2

✖

https://wolfram.com/xid/0y76co-iog

Out[2]=2

Keep the tag local:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-uxg

Out[1]=1

Applications (3)Sample problems that can be solved with this function

Find the next prime after 10¹⁰:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-vr3

Out[1]=1

Find the first power of 17 equal to 1 mod 19:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-bcn

Out[1]=1

Stop if an iteration gets too large:

In[1]:=1

✖

https://wolfram.com/xid/0y76co-vx3

Out[1]=1

In[2]:=2

✖

https://wolfram.com/xid/0y76co-is4

Out[2]=2

Properties & Relations (1)Properties of the function, and connections to other functions

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

In[1]:=1

✖

https://wolfram.com/xid/0y76co-frl

Out[1]=1

In[2]:=2

✖

https://wolfram.com/xid/0y76co-j0n

Out[2]=2

Top