First¶

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description¶

First[expr] gives the first element of expr.

Examples¶

All examples below are verified against the current Mathilda build.

In[1]:= First[<|"a" -> 10, "b" -> 20|>]
Out[1]= 10

In[2]:= Rest[<|"a" -> 10, "b" -> 20, "c" -> 30|>]
Out[2]= <|"b" -> 20, "c" -> 30|>

In[3]:= Take[<|"a" -> 1, "b" -> 2, "c" -> 3|>, 2]
Out[3]= <|"a" -> 1, "b" -> 2|>

Implementation notes¶

builtin_first (in src/part.c) takes a single argument and returns a deep copy of its first element (args[0]). It returns NULL (unevaluated) when the argument is atomic or has no elements.

Attributes: none registered.

Implementation status¶

Stable — documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

References¶

Source: src/part.c
Specification: docs/spec/builtins/data-structures.md

Notes & additional examples¶

Worked examples¶

In[1]:= First[{a,b,c}]
Out[1]= a

Notes¶

First[expr] returns the first element (part 1) of any expression, not only lists.