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Mastering Swift by Jon Hoffman

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Optional variables

All of the variables that we have looked at so far are considered to be nonoptional variables. This means that the variables are required to have a non-nil value; however, there are times when we want or need our variables to contain nil values. This can occur if we return a nil from a function whose operation failed or if a value is not found.

In Swift, an optional variable is a variable that we are able to assign nil (no value) to. Optional variables and constants are defined using ? (question mark). Let's look at the following Playground; it shows us how to define Optional and shows what happens if we assign a nil value to a Non-Optional variable:

Notice the error we receive when we try to assign a nil value to the nonoptional ...

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