EC
IndexError
RubyERRORNotableCollection

Index out of range or invalid

Quick Answer

Use Array#[] for nil-safe access or Array#fetch with a default block to handle missing indices explicitly.

What this means

Raised when an index is invalid for a collection. It is the parent of KeyError and StopIteration. Array#fetch raises IndexError for out-of-bounds indices, unlike [] which returns nil.

Why it happens
  1. 1Calling Array#fetch with an index that does not exist
  2. 2Using a negative index beyond the start of the array

Fix

Use fetch with a default

Use fetch with a default
arr = [1, 2, 3]
value = arr.fetch(10, 'default')   # => "default"
# or
value = arr.fetch(10) { |i| "index #{i} not found" }

Why this works

fetch with a default value or block returns a fallback instead of raising IndexError.

Code examples
Array#fetch out of boundsruby
[1, 2, 3].fetch(5)
# IndexError: index 5 outside of array bounds: -3...3
Array#[] returns nil safelyruby
[1, 2, 3][5]   # => nil (no error)
Rescue IndexErrorruby
begin
  val = arr.fetch(idx)
rescue IndexError => e
  val = nil
  puts e.message
end
Sources
Official documentation ↗

Ruby Core Documentation

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