Basic Examples (3) 

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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Scope (2) 

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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Applications (2) 

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Saving will reset the association if it is no longer valid:

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Properties and Relations (2) 

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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