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If

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If[cond, pos, neg]

returns pos if cond evaluates to True, and neg if it evaluates to
False.

If[cond, pos, neg, other]

returns other if cond evaluates to neither True nor False.

If[cond, pos]

returns Null if cond evaluates to False.

• • If[1<2, a, b]

If the second branch is not specified, Null is taken:

• • If[1<2, a]

• • If[False, a] // FullForm

You might use comments inside (* and *) to make the branches of If
more readable:

• • If[a, (*then*) b, (*else*) c];

Since one or more arguments to a boolean operation could be symbolic, it is possible that an If cannot be evaluated. For example:

• • Clear[a, b]; If [a < b, a, b]

To handle this, If takes an optional fourth parameter:

• • If [a < b, a, b, "I give up"]