Find Duplicate Elements In An Array In Ruby
Detect which values appear more than once in an array using group_by or tally.
Description
Duplicate detection comes up in data validation, deduplication pipelines, and form processing. Ruby’s tally method (3.0+) counts occurrences in one pass — filter for count > 1 to get duplicates. For broader compatibility, group_by with a size check works identically.
Sample input:
values = [1, 2, 3, 2, 4, 1, 5]
Sample Output:
[1, 2]
Answer
# Ruby 3.0+ — tally counts occurrences
values = [1, 2, 3, 2, 4, 1, 5]
duplicates = values.tally.filter_map { |val, count| val if count > 1 }
# => [1, 2]
# Broader compatibility — group_by
duplicates = values.group_by { |v| v }
.select { |_, group| group.size > 1 }
.keys
# => [1, 2]
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