Replace a single element in an array

p array.map { |x| x == 4 ? 'Z' : x }

# => [1, 2, 3, 'Z']

You can do it as:

array[array.index(4)] = "Z"

If the element is not necessarily in the array, then

if i = array.index(4)
  array[i] = "Z"
end