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Hash#rassoc

Updated May 19, 2026· Hash Methods
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Hash#rassoc searches a hash for a given value and returns the matching key-value pair as a two-element array [key, value]. It is the reverse of Hash#assoc — where assoc searches by key, rassoc searches by value.

Basic Usage

scores = { alice: 95, bob: 87, carol: 92 }

scores.rassoc(87)
# => [:bob, 87]

scores.rassoc(100)
# => nil

When a match is found, the key and value are returned together. When no match exists, rassoc returns nil.

Signature

rassoc(value)

Returns the first key-value pair where the value matches the given argument. The search proceeds in insertion order and stops on the first match.

Searching with Different Value Types

products = {
  "SKU-001" => "Widget",
  "SKU-002" => "Gadget",
  "SKU-003" => "Widget Pro"
}

products.rassoc("Widget")
# => ["SKU-001", "Widget"]

products.rassoc("Gadget")
# => ["SKU-002", "Gadget"]

Works with strings, numbers, symbols, arrays, or any object type that supports ==.

Comparing with Other Hash Methods

MethodSearches byReturns
assockey[key, value] or nil
rassocvalue[key, value] or nil
keyvaluekey or nil
fetchkey (with default)value or default
dignested key pathvalue or nil

Use rassoc when you have a value and need the corresponding key. This comes up when building inverse mappings or looking up records by a non-key field.

Limitations

Only returns the first match. If multiple entries share the same value, only the first one in insertion order is returned:

names = { alice: "Engineer", bob: "Designer", carol: "Engineer" }

names.rassoc("Engineer")
# => [:alice, "Engineer"]
# carol is not reached

Linear search. The hash is scanned from the beginning, making this O(n) regardless of hash size. For large hashes with frequent lookups, consider maintaining a reverse index.

No block form. Unlike select or find, there is no variant that accepts a block.

See Also