Infinite loop in blocks in ruby

The problem is that insert changes the original array:

a = [1,2,3,4,5,6,7,8,9,10]
a.each do |value|
  a.insert(value, 3)
  p a
end

# [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]                   # original, ^ marks current value
#  ^
# [1, 3, 2, 3, 4, 5, 6, 7, 8, 9, 10]                # inserted 3 at position 1
#     ^
# [1, 3, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10]             # inserted 3 at position 3
#        ^
# [1, 3, 3, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10]          # inserted 3 at position 2
#           ^
# [1, 3, 3, 3, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10]       # inserted 3 at position 2
#              ^
# [1, 3, 3, 3, 3, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10]    # inserted 3 at position 2
#                 ^
# [1, 3, 3, 3, 3, 3, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10] # inserted 3 at position 2
#                    ^
# ...                                               # continues forever ...

What you probably want instead is something like this:

a = [1,2,3,4,5,6,7,8,9,10]
a.each_index {|index| p a.dup.insert(index, 3) }
# [3, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
# [1, 3, 2, 3, 4, 5, 6, 7, 8, 9, 10]
# [1, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10]
# [1, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10]
# [1, 2, 3, 4, 3, 5, 6, 7, 8, 9, 10]
# [1, 2, 3, 4, 5, 3, 6, 7, 8, 9, 10]
# [1, 2, 3, 4, 5, 6, 3, 7, 8, 9, 10]
# [1, 2, 3, 4, 5, 6, 7, 3, 8, 9, 10]
# [1, 2, 3, 4, 5, 6, 7, 8, 3, 9, 10]
# [1, 2, 3, 4, 5, 6, 7, 8, 9, 3, 10]

• each_index iterates over the indices, not the values. This is likely the correct thing to do here, because insert takes an index as first argument.
• dup duplicates the array on every iteration so a remains unchanged.