--- title: "Check" language: "en" type: "Symbol" summary: "Check[expr, failexpr] evaluates expr, and returns the result, unless messages were generated, in which case it evaluates and returns failexpr. Check[expr, failexpr, {s1::t1, s2::t2, ...}] checks only for the specified messages. Check[expr, failexpr, name] checks only for messages in the named message group." keywords: - errors - numerical overflow - trapping errors - iserr - iserror - ON_ERROR - assert - lasterror - traperror - try canonical_url: "https://reference.wolfram.com/language/ref/Check.html" source: "Wolfram Language Documentation" related_guides: - title: "Messages" link: "https://reference.wolfram.com/language/guide/Messages.en.md" - title: "Conditionals" link: "https://reference.wolfram.com/language/guide/Conditionals.en.md" - title: "Package Development" link: "https://reference.wolfram.com/language/guide/PackageDevelopment.en.md" - title: "Scoping Constructs" link: "https://reference.wolfram.com/language/guide/ScopingConstructs.en.md" - title: "Robustness & Error Handling" link: "https://reference.wolfram.com/language/guide/RobustnessAndErrorHandling.en.md" - title: "Tuning & Debugging" link: "https://reference.wolfram.com/language/guide/TuningAndDebugging.en.md" related_workflows: - title: "Suppress Error Messages" link: "https://reference.wolfram.com/language/workflow/SuppressErrorMessages.en.md" - title: "Set Up Error Checking and Messages in a Function" link: "https://reference.wolfram.com/language/workflow/SetUpErrorCheckingAndMessagesInAFunction.en.md" - title: "Understand Error Messages" link: "https://reference.wolfram.com/language/workflow/UnderstandErrorMessages.en.md" related_functions: - title: "Catch" link: "https://reference.wolfram.com/language/ref/Catch.en.md" - title: "ConfirmQuiet" link: "https://reference.wolfram.com/language/ref/ConfirmQuiet.en.md" - title: "Failsafe" link: "https://reference.wolfram.com/language/ref/Failsafe.en.md" - title: "MessageList" link: "https://reference.wolfram.com/language/ref/MessageList.en.md" - title: "$MessageList" link: "https://reference.wolfram.com/language/ref/$MessageList.en.md" - title: "Message" link: "https://reference.wolfram.com/language/ref/Message.en.md" - title: "Quiet" link: "https://reference.wolfram.com/language/ref/Quiet.en.md" - title: "$MessageGroups" link: "https://reference.wolfram.com/language/ref/$MessageGroups.en.md" - title: "Indeterminate" link: "https://reference.wolfram.com/language/ref/Indeterminate.en.md" - title: "TimeConstrained" link: "https://reference.wolfram.com/language/ref/TimeConstrained.en.md" - title: "CheckAbort" link: "https://reference.wolfram.com/language/ref/CheckAbort.en.md" - title: "Assert" link: "https://reference.wolfram.com/language/ref/Assert.en.md" related_tutorials: - title: "Messages" link: "https://reference.wolfram.com/language/tutorial/TextualInputAndOutput.en.md#12413" - title: "Loops and Control Structures" link: "https://reference.wolfram.com/language/tutorial/EvaluationOfExpressions.en.md#11091" --- # Check Check[expr, failexpr] evaluates expr, and returns the result, unless messages were generated, in which case it evaluates and returns failexpr. Check[expr, failexpr, {s1::t1, s2::t2, …}] checks only for the specified messages. Check[expr, failexpr, "name"] checks only for messages in the named message group. ## Details * ``Check`` has attribute ``HoldAll``. * ``Check`` does not test for messages that have been switched off using ``Off``, or by uses of ``Quiet`` that occur inside it. * The operation of ``Check`` is not affected by being enclosed inside ``Quiet``. * In ``Check[expr, failexpr, "name"]``, possible named message groups are given by ``\$MessageGroups``. * Explicit message names of the form ``s::t`` can be mixed with named message groups. --- ## Examples (10) ### Basic Examples (2) Return ``err`` when any message is issued: ```wl In[1]:= Check[1 ^ 0, err] Out[1]= 1 In[2]:= Check[0 ^ 0, err] ``` Power::indet: Indeterminate expression 0^0 encountered. ```wl Out[2]= err ``` --- Check only for specific messages: ```wl In[1]:= Check[Sin[0 ^ 0], err, Sin::argx] ``` Power::indet: Indeterminate expression 0^0 encountered. ```wl Out[1]= Indeterminate In[2]:= Check[Sin[x, y], err, Sin::argx] ``` Sin::argx: Sin called with 2 arguments; 1 argument is expected. ```wl Out[2]= err ``` ### Scope (3) Check for a message from a specific function: ```wl In[1]:= Check[0 ^ 0, "message issued", Power::indet] ``` Power::indet: Indeterminate expression 0^0 encountered. ```wl Out[1]= "message issued" ``` The ``ArcTan::indet`` is not caught by ``Check`` : ```wl In[2]:= Check[ArcTan[0, 0], "message issued", Power::indet] ``` ArcTan::indet: Indeterminate expression ArcTan[0,0] encountered. ```wl Out[2]= Indeterminate ``` Use the head ``General`` to catch the message from any function: ```wl In[3]:= Check[ArcTan[0, 0], "message issued", General::indet] ``` ArcTan::indet: Indeterminate expression ArcTan[0,0] encountered. ```wl Out[3]= "message issued" ``` --- Catch multiple messages: ```wl In[1]:= Check[Sin[0 ^ 0], err, {General::infy, General::indet, General::argx}] ``` Power::indet: Indeterminate expression 0^0 encountered. ```wl Out[1]= err In[2]:= Check[1 / 0, err, {General::infy, General::indet, General::argx}] ``` Power::infy: Infinite expression 1/0 encountered. ```wl Out[2]= err ``` --- Check for a message from a named group: ```wl In[1]:= f = Compile[{{x, _Real}}, x] Out[1]= CompiledFunction[{11, 14.1, 5470}, {_Real}, {{3, 0, 0}, {3, 0, 0}}, {}, {0, 0, 1, 0, 0}, {{1}}, Function[{x}, x], Evaluate] In[2]:= Check[f[I], "message issued", "Compiler"] ``` CompiledFunction::cfsa: Argument I at position 1 should be a machine-size real number. ```wl Out[2]= "message issued" In[3]:= Check[f[I], "message issued", "Graphics"] ``` CompiledFunction::cfsa: Argument I at position 1 should be a machine-size real number. ```wl Out[3]= I ``` ### Applications (1) Use ``Check`` and ``Quiet`` to provide values for arguments where a function is not applicable: ```wl In[1]:= f[x_] := Quiet[Check[StringTake[x, 4], x, StringTake::take], StringTake::take] ``` This function simply returns its argument if its argument is a string of length less than four: ```wl In[2]:= f /@ {"abcdef", "ab", "eg3j"} Out[2]= {"abcd", "ab", "eg3j"} ``` Without the ``Check`` and ``Quiet``, the middle input would have returned unevaluated with a message: ```wl In[3]:= StringTake["ab", 4] ``` StringTake::take: Cannot take positions 1 through 4 in "ab". ```wl Out[3]= StringTake["ab", 4] ``` ### Properties & Relations (3) ``Check`` does not prevent messages from being issued: ```wl In[1]:= Check[Sin[x, y], "too many", General::argx] ``` Sin::argx: Sin called with 2 arguments; 1 argument is expected. ```wl Out[1]= "too many" ``` --- ``Check`` does not detect messages suppressed with a inner ``Quiet`` : ```wl In[1]:= Check[Quiet[1 / 0], error] Out[1]= ComplexInfinity ``` However, the operation of ``Check`` is not affected by being inside ``Quiet`` : ```wl In[2]:= Quiet[Check[1 / 0, error]] Out[2]= error ``` --- ``Check`` does not report messages that have been turned off: ```wl In[1]:= Off[Power::infy] In[2]:= Check[1 / 0, "message issued"] Out[2]= ComplexInfinity ``` Restore the message: ```wl In[3]:= On[Power::infy] ``` ### Possible Issues (1) A nested ``Check`` does not silence a message for an enclosing ``Check`` : ```wl In[1]:= Check[Check[1 / 0, inner, Power::infy], outer, Power::infy] ``` Power::infy: Infinite expression 1/0 encountered. ```wl Out[1]= outer ``` Use an intermediate ``Quiet`` to prevent the enclosing ``Check`` from reacting to the message: ```wl In[2]:= Check[Quiet[Check[1 / 0, inner, Power::infy], Power::infy], outer, Power::infy] Out[2]= inner ``` ## See Also * [`Catch`](https://reference.wolfram.com/language/ref/Catch.en.md) * [`ConfirmQuiet`](https://reference.wolfram.com/language/ref/ConfirmQuiet.en.md) * [`Failsafe`](https://reference.wolfram.com/language/ref/Failsafe.en.md) * [`MessageList`](https://reference.wolfram.com/language/ref/MessageList.en.md) * [`\$MessageList`](https://reference.wolfram.com/language/ref/$MessageList.en.md) * [`Message`](https://reference.wolfram.com/language/ref/Message.en.md) * [`Quiet`](https://reference.wolfram.com/language/ref/Quiet.en.md) * [`\$MessageGroups`](https://reference.wolfram.com/language/ref/$MessageGroups.en.md) * [`Indeterminate`](https://reference.wolfram.com/language/ref/Indeterminate.en.md) * [`TimeConstrained`](https://reference.wolfram.com/language/ref/TimeConstrained.en.md) * [`CheckAbort`](https://reference.wolfram.com/language/ref/CheckAbort.en.md) * [`Assert`](https://reference.wolfram.com/language/ref/Assert.en.md) ## Tech Notes * [Messages](https://reference.wolfram.com/language/tutorial/TextualInputAndOutput.en.md#12413) * [Loops and Control Structures](https://reference.wolfram.com/language/tutorial/EvaluationOfExpressions.en.md#11091) ## Related Guides * [`Messages`](https://reference.wolfram.com/language/guide/Messages.en.md) * [`Conditionals`](https://reference.wolfram.com/language/guide/Conditionals.en.md) * [Package Development](https://reference.wolfram.com/language/guide/PackageDevelopment.en.md) * [Scoping Constructs](https://reference.wolfram.com/language/guide/ScopingConstructs.en.md) * [Robustness & Error Handling](https://reference.wolfram.com/language/guide/RobustnessAndErrorHandling.en.md) * [Tuning & Debugging](https://reference.wolfram.com/language/guide/TuningAndDebugging.en.md) ## Related Workflows * [Suppress Error Messages](https://reference.wolfram.com/language/workflow/SuppressErrorMessages.en.md) * [Set Up Error Checking and Messages in a Function](https://reference.wolfram.com/language/workflow/SetUpErrorCheckingAndMessagesInAFunction.en.md) * [Understand Error Messages](https://reference.wolfram.com/language/workflow/UnderstandErrorMessages.en.md) ## History * Introduced in 1988 (1.0) \| [Updated in 2007 (6.0)](https://reference.wolfram.com/language/guide/SummaryOfNewFeaturesIn60.en.md) ▪ [2008 (7.0)](https://reference.wolfram.com/language/guide/SummaryOfNewFeaturesIn70.en.md)