First

The first element of a collection.

✓ checked against that system · ! checked, with disagreements (hover a chip)Wolfram LanguageFirst!↗

First(collection)the first element of the collection.

First(collection, default)default when the collection is empty, instead of the first element.

First(expr)the first operand of any expression, not just a collection.

Typed signatures

Type	Package	Applies
(xs: indexed_collection<any>) -> any	compute-engine	always
(any, any?) -> any	combinatoricsreplaces compute-engine	always

Domain: Collections

Details

• $First(c) = At(c, 1)$. See At.
• Positional indexing from the front (index 1) and back (negative indices).
• On an empty collection with no default given, returns the symbol $Missing$ rather than raising an error.
• A second argument supplies a default for an empty collection, instead of $Missing$.
• Works on the operands of any expression, not just a collection's elements.

Examples

4 more kept as data, checked against the oracles

Basic #

live ↗

In$\mathrm{First}(\bigl\lbrack1, 2, 3\bigr\rbrack)$Out$1$

live ↗

In$\mathrm{First}(\bigl\lbrack10, 20, 30\bigr\rbrack)$Out$10$

live ↗

In$\mathrm{First}(\bigl\lbrack a, b, c\bigr\rbrack)$Out$a$

An empty list yields the supplied default instead of failing

live ↗

In$\mathrm{First}(\bigl\lbrack\bigr\rbrack, x)$Out$x$

Scope #

A second argument supplies a default for an empty collection

live ↗

In$\mathrm{First}(\bigl\lbrack\bigr\rbrack, 99)$Out$99$

Of a matrix, the first row

live ↗

In$\mathrm{First}(\bigl\lbrack\bigl\lbrack a, b\bigr\rbrack, \bigl\lbrack c, d\bigr\rbrack\bigr\rbrack)$Out$\bigl\lbrack a, b\bigr\rbrack$

A lazy Range works too

live ↗

In$\mathrm{First}(5..10)$Out$5$

Any expression, not just a list: the first term of a sum

live ↗

In$\mathrm{First}(a^2+b^2)$Out$a^2$

A default is ignored when the collection has a first element

live ↗

In$\mathrm{First}(\bigl\lbrack a, b\bigr\rbrack, x)$Out$a$

live ↗

In$\mathrm{First}(\mathrm{SparseArray}(1..100))$Out$1$

Properties #

$First(c) = At(c, 1)$. See At

live ↗

In$\mathrm{First}(\bigl\lbrack1, 2, 3\bigr\rbrack)=\bigl\lbrack1, 2, 3\bigr\rbrack[1]$Out$\mathrm{True}$

Possible issues #

An empty collection has no first element; compute-engine returns the symbol $Missing$

live ↗

In$\mathrm{First}(\bigl\lbrack\bigr\rbrack)$Out$\mathrm{Missing}$

A power is an expression with parts too: the first part of $a^{-2}$ is its base

live ↗

In$\mathrm{First}(\frac{1}{a^2})$Out$a$

About

Examples	17
Checked against	Wolfram Language 16 agree1 differmathlib4 4 agree1 error
Crosswalk	Wolfram Language

Implementation

mappedmathlib4packages/oracle/src/mappings.ts

The head of a list; an empty list fails to elaborate (ours is Missing) rather than defaulting, and so does an expression that is not a list.

See also: Last, At