• ← Extract
• → FirstCase

First

WMA link

First[expr]

returns the first element in expr.

First[expr, def]

returns the first element in expr if it exists or def otherwise.

First[expr] is equivalent to expr[[1]].

• • First[{a, b, c}]

The first argument need not be a list:

• • First[a + b + c]

However, the first argument must be Nonatomic when there is a single argument:

• • First[x]

Or if it is not, but a second default argument is provided, that is evaluated and returned:

• • First[10, 1+2]

• • First[{}]

  • {} has zero length and no first element.

  • $$\mathrm{First}\left[\left\{\right\}\right]$$

As before, the first argument is empty, but a default argument is given, evaluate and return the second argument:

• • First[{}, 1+2]