Wolfram Function Repository
Instant-use add-on functions for the Wolfram Language
Function Repository Resource:
Initialize a value unless it already satisfies a given condition
ResourceFunction["SetUnless"][expr,val,test] evaluates expr=val if test[expr] does not return True. | |
ResourceFunction["SetUnless"][val,test] represents an operator form of ResourceFunction["SetUnless"]. | |
ResourceFunction["SetUnless"][test] represents an operator form of ResourceFunction["SetUnless"]. |
Initialize a symbol:
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The value is not set if it already satisfies the given criteria:
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The expr does not need to be a symbol:
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Use the operator forms:
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Use Unevaluated to prevent evaluation of the second argument unless needed:
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Without using Unevaluated, the second argument will evaluate, even if not being used:
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Use an additional Unevaluated to get the same behavior as SetDelayed:
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Initialize a counter if it doesn’t already exist before incrementing:
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If the counter ends up with an invalid value, Increment will no longer return an integer:
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Use SetUnless to ensure a valid value before incrementing:
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Create a function that stores data in an association and initializes it if necessary:
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Check the data:
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Clear the data:
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Now save will reinitialize the association and start again:
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Saving will reset the association if it is no longer valid:
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If test[expr] does not explicitly yield True, it will be considered False:
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SetUnless has the SequenceHold attribute:
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Wolfram Language 11.3 (March 2018) or above
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