puts vs return

Let’s use a simple code snippet to explain the difference between puts and return.

list = [1, 2, 3]

def mutate(array)
  array.pop
end

p "Before mutate method: #{list}" # "Before mutate method: [1, 2, 3]"
p mutate(list)                    # 3
p "After mutate method: #{list}"  # "After mutate method: [1, 2]"

The result should not be surprising if we know the following:

  • Ruby methods always return the evaluated result of the last line that is executed (i.e., we could have written return array.pop in our method)
  • Difference between Pass by Reference vs Pass by Value#

Now let’s modify our code a little bit and include the Kernel method puts:

list = [1, 2, 3]

def mutate(array)
  puts array.pop                  # 3
end

p "Before mutate method: #{list}" # "Before mutate method: [1, 2, 3]"
p mutate(list)                    # nil
p "After mutate method: #{list}"  # "After mutate method: [1, 2]"

The order of the comments above is a bit confusing ; the results come up this way:

"Before mutate method: [1, 2, 3]"
3
nil
"After mutate method: [1, 2]"

What is surprising the first time we encounter it is the nil result on the penultimate line, although the result inside the method mutate is 3. This is because the Kernel method puts output what is passed to it but returns nil.

Good to know: the method p output what is passed to it but also returns the result. p array.pop would have output 3 and returned 3 here.