Kenneth and Recursion

Question

Kenneth mentions this function is a good example of recursion:

def get_locations(cells):

    monster = random.choice(cells)
    door = random.choice(cells)
    player = random.choice(cells)

    if monster == door or monster == player or door == player:
        monster, door, player = get_locations(cells)

    return monster, door, player

We call get_locations() again if any of the values are the same, which makes sense. But why do we set it equal to monster, door, and player? Why wouldn't calling just get_locations(cells) have done the trick since that is what the function is doing?

Answer 1 · 2017-04-13T18:29:13Z

April 13, 2017 6:29pm

Because the original call needs to return the correct result.

Recursion is very tricky. Pretend the outermost function call recurses.

Main function call -> Recursion Call |               Return value received from call
                                     v              ^
                                       Return value |

Note that the recursion call's return isn't the same as the main function call's return.

The recursion call's return value is passed on to where the function was called, in this case the main function call. Then the main function call returns to who called that function, which is somewhere outside the function.

If the function goes deeper, it might look like this:

Outermost call | ---------------------------------------------------- Return to call outside of function       
               v ---------------------------------------------------- ^
               Inner call | ---------------------------- Return value |
                          v ---------------------------- ^
                          Innermost call -> Return value |

Does this make a little more sense?

I hope this helps.

~Alex

Answer 2 · 2017-04-13T18:36:12Z

April 13, 2017 6:36pm

In this case, the function is designed get three random locations (monstor, door and player), so when any two are the same, we call the same function again and it tries to set all three again. It is not designed to set one at a time.

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Kurt Maurer

Kurt Maurer

Kenneth and Recursion

2 Answers

Alexander Davison

Alexander Davison

Because the original call needs to return the correct result.

Kurt Maurer

Kurt Maurer

Alexander Davison

Alexander Davison

Christopher Shaw

Christopher Shaw