Cases

The elements of a collection matching a pattern.

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

Cases(collection, pattern)every element of $collection$ that matches $pattern$, in order.

Typed signatures

Type	Package	Applies
(collection<any>, any) -> collection	combinatorics	always

Domain: Collections

Details

• Matches compute-engine's own wildcards: a bare _ (or named, _a) matches any single element, __/___ match one-or-more/zero-or-more within a structural pattern, and a pattern built from a head applied to wildcards (e.g. $List(\_a)$) matches that structure.
• A plain value (no wildcard) matches by exact equality, same as Count.
• Only the top level of the collection is searched.

Examples

Basic #

live ↗

In$\mathrm{Cases}(\bigl\lbrack1, a, 2, b\bigr\rbrack, \operatorname{\_})$Out$\bigl\lbrack1, a, 2, b\bigr\rbrack$

Scope #

A plain value keeps only the elements equal to it

live ↗

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

A structural pattern keeps elements shaped like it

live ↗

In$\mathrm{Cases}(\bigl\lbrack\bigl\lbrack1\bigr\rbrack, 2, \bigl\lbrack3\bigr\rbrack\bigr\rbrack, \bigl\lbrack\operatorname{\_a}\bigr\rbrack)$Out$\bigl\lbrack\bigl\lbrack1\bigr\rbrack, \bigl\lbrack3\bigr\rbrack\bigr\rbrack$

Properties #

For a plain value, Cases and Count agree on how many

live ↗

In$\mathrm{Length}(\mathrm{Cases}(\bigl\lbrack1, 2, 1, 3, 1\bigr\rbrack, 1))=\mathrm{Count}(\bigl\lbrack1, 2, 1, 3, 1\bigr\rbrack, 1)$Out$\mathrm{True}$

Possible issues #

compute-engine's wildcards carry no type: _Integer is just a named wildcard, so it matches every element

live ↗

In$\mathrm{Length}(\mathrm{Cases}(\bigl\lbrack1, a, 2, b\bigr\rbrack, \operatorname{\_Integer}))$Out$4$

Wolfram's _Integer pattern only matches integers, here 2 (1 and 2).

About

Examples	5
Checked against	Wolfram Language 2 agree1 unevaluated
Crosswalk	FungrimWolfram Language

See also: Count, Select