--- title: "Catch" language: "en" type: "Symbol" summary: "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]." keywords: - computed goto - control - divert - errors - handling of errors - jump - longjmp - non-local returns - scope - stop - CATCH - ON_ERROR - catch - finally - try - catch - lasterror - traperror - catch - try canonical_url: "https://reference.wolfram.com/language/ref/Catch.html" source: "Wolfram Language Documentation" related_guides: - title: "Flow Control" link: "https://reference.wolfram.com/language/guide/FlowControl.en.md" - title: "Looping Constructs" link: "https://reference.wolfram.com/language/guide/LoopingConstructs.en.md" - title: "Scoping Constructs" link: "https://reference.wolfram.com/language/guide/ScopingConstructs.en.md" - title: "Procedural Programming" link: "https://reference.wolfram.com/language/guide/ProceduralProgramming.en.md" - title: "Language Overview" link: "https://reference.wolfram.com/language/guide/LanguageOverview.en.md" - title: "Robustness & Error Handling" link: "https://reference.wolfram.com/language/guide/RobustnessAndErrorHandling.en.md" related_functions: - title: "Throw" link: "https://reference.wolfram.com/language/ref/Throw.en.md" - title: "Enclose" link: "https://reference.wolfram.com/language/ref/Enclose.en.md" - title: "Check" link: "https://reference.wolfram.com/language/ref/Check.en.md" - title: "CheckAbort" link: "https://reference.wolfram.com/language/ref/CheckAbort.en.md" - title: "Reap" link: "https://reference.wolfram.com/language/ref/Reap.en.md" related_tutorials: - title: "Loops and Control Structures" link: "https://reference.wolfram.com/language/tutorial/EvaluationOfExpressions.en.md#11091" --- # 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 (15) ### Basic Examples (3) Exit to the enclosing ``Catch`` as soon as the ``Throw`` is evaluated: ```wl In[1]:= Catch[a;b;Throw[c];d;e] Out[1]= c ``` --- Define a function that can "throw an exception": ```wl In[1]:= f[x_] := If[x > 10, Throw[overflow], x!] ``` The result of the ``Catch`` is just what is thrown by ``Throw`` : ```wl In[2]:= Catch[f[2] + f[11]] Out[2]= overflow In[3]:= Catch[f[2] + f[3]] Out[3]= 8 ``` --- Use ``Throw`` to exit a loop when a criterion is satisfied: ```wl In[1]:= Catch[Do[If[i! > 10 ^ 10, Throw[i]], {i, 100}]] Out[1]= 14 ``` ### Scope (5) ``Catch`` can catch a ``Throw`` from inside essentially any function: ```wl In[1]:= Catch[If[# < 0, Throw[#]]& /@ {1, 2, 0, -1, 5, 6}] Out[1]= -1 In[2]:= Catch[{a, Throw[b], c}] Out[2]= b In[3]:= Catch[a ^ 2 + b ^ 2 + c ^ 2 /. b :> Throw[bbb]] Out[3]= bbb ``` --- The nearest enclosing ``Catch`` catches the ``Throw`` : ```wl In[1]:= Catch[{Catch[{a, Throw[b], c}], d, e}] Out[1]= {b, d, e} ``` --- ``Catch`` picks up the first ``Throw`` that is evaluated: ```wl In[1]:= Catch[{Throw[a], Throw[b], Throw[c]}] Out[1]= a In[2]:= Catch[Throw /@ {a, b, c}] Out[2]= a ``` --- ``Throw`` need not occur lexically inside ``Catch`` : ```wl In[1]:= f[x_] := (If[x < 0, Throw["negative"]];Sqrt[x]) In[2]:= Catch[Sum[f[i], {i, 5, -5, -1}]] Out[2]= "negative" ``` --- A function that can throw a number of different exceptions: ```wl In[1]:= f[x_] := Which[ x < 0, Throw[x, error[negative]], x == 0, Throw[x, error[zero]], True, 1 / Sqrt[x]] ``` A handler for the possible exceptions: ```wl In[2]:= ff[x_] := Catch[f[x], error[_], Function[{value, tag}, tag /. {error[negative] :> Indeterminate, error[zero] :> Infinity, _ :> Throw[value, tag]}]] In[3]:= ff /@ {-1, 0, 1} Out[3]= {Indeterminate, ∞, 1} ``` ### Generalizations & Extensions (3) Catch the ``Throw`` with tag ``u`` : ```wl In[1]:= Catch[Throw[a, u], u] Out[1]= a ``` --- The inner ``Catch`` catches the ``Throw`` : ```wl In[1]:= Catch[f[Catch[Throw[a, u], u]], v] Out[1]= f[a] ``` The outer ``Catch`` catches the ``Throw`` : ```wl In[2]:= Catch[f[Catch[Throw[a, u], v]], u] Out[2]= a ``` --- Keep the tag local: ```wl In[1]:= Module[{u}, Catch[Throw[a, u], u]] Out[1]= a ``` ### Applications (3) Find the next prime after ``10^10`` : ```wl In[1]:= Catch[Do[If[PrimeQ[i], Throw[i]], {i, 10 ^ 10, 10 ^ 10 + 1000}]] Out[1]= 10000000019 ``` --- Find the first power of 17 equal to 1 mod 19: ```wl In[1]:= Catch[Do[If[Mod[17 ^ i, 19] == 1, Throw[i]], {i, 19}]] Out[1]= 9 ``` --- Stop if an iteration gets too large: ```wl In[1]:= NestList[# ^ 2 + 1&, 2, 6] Out[1]= {2, 5, 26, 677, 458330, 210066388901, 44127887745906175987802} In[2]:= Catch[NestList[If[# > 1000, Throw[#], # ^ 2 + 1]&, 2, 6]] Out[2]= 458330 ``` ### Properties & Relations (1) Use ``Check`` to throw an exception if a message is generated: ```wl In[1]:= Catch[2 + Check[1 / (1 + 1 / 0), Throw[error]]] ``` Power::infy: Infinite expression 1/0 encountered. ```wl Out[1]= error In[2]:= Catch[2 + Check[1 / (1 + 1 / 2), Throw[error]]] Out[2]= (8/3) ``` ## See Also * [`Throw`](https://reference.wolfram.com/language/ref/Throw.en.md) * [`Enclose`](https://reference.wolfram.com/language/ref/Enclose.en.md) * [`Check`](https://reference.wolfram.com/language/ref/Check.en.md) * [`CheckAbort`](https://reference.wolfram.com/language/ref/CheckAbort.en.md) * [`Reap`](https://reference.wolfram.com/language/ref/Reap.en.md) ## Tech Notes * [Loops and Control Structures](https://reference.wolfram.com/language/tutorial/EvaluationOfExpressions.en.md#11091) ## Related Guides * [Flow Control](https://reference.wolfram.com/language/guide/FlowControl.en.md) * [Looping Constructs](https://reference.wolfram.com/language/guide/LoopingConstructs.en.md) * [Scoping Constructs](https://reference.wolfram.com/language/guide/ScopingConstructs.en.md) * [Procedural Programming](https://reference.wolfram.com/language/guide/ProceduralProgramming.en.md) * [Language Overview](https://reference.wolfram.com/language/guide/LanguageOverview.en.md) * [Robustness & Error Handling](https://reference.wolfram.com/language/guide/RobustnessAndErrorHandling.en.md) ## Related Links * [Fast Introduction for Programmers: Procedures](http://www.wolfram.com/language/fast-introduction-for-programmers/procedures/) * [NKS\|Online](http://www.wolframscience.com/nks/search/?q=Catch) * [A New Kind of Science](http://www.wolframscience.com/nks/) ## History * Introduced in 1988 (1.0) \| Updated in 1996 (3.0)