A recursive function without returns keeps cycling?

Question

I was playing with a sudoku solver, and came across this interesting behavior. When I enter a number (0,5) the function will keep going up to the upper limit (5). Once the limit is reached it still keeps printing my number (a), but decreasing the value? (Shown below).

Why is this?

def solver(a):
    print("->",a)
    if a == 0 or a == 5:
        return
    else:
        a = a+1
        print("-->",a)
        solver(a)
    print("---")
    print(a)

gives

-> 2
--> 3
-> 3
--> 4
-> 4
--> 5
-> 5
---
5
---
4
---
3

It is just executing the part of the function that comes after returning from the recursed function call. — mkrieger1, Jun 26 '20 at 22:08
Does this answer your question? [Understanding how recursive functions work](https://stackoverflow.com/questions/25676961/understanding-how-recursive-functions-work) — mkrieger1, Jun 26 '20 at 22:10
Because `solver(5)` was called from `solver(4)`, and after returning from `solver(5)`, `solver(4)` continues. And since `solver(4)` was called from `solver(3)`, after `solver(4)` returns, the rest of `solver(3)` is executed, etc. — mkrieger1, Jun 26 '20 at 22:12

score 1 · Accepted Answer · answered Jun 26 '20 at 22:14

This may help you to see what is going on: I've added a variable recursionLevel so that you can see at what level in the recursion the numbers are printed.

def solver(a, recursionLevel):
    print("->",a, recursionLevel)
    if a == 0 or a == 5:
        return
    else:
        a = a+1
        print("-->",a, recursionLevel)
        solver(a, recursionLevel + 1)
    print("---")
    print(a, recursionLevel)
    

solver(2, 0)

This gives:

-> 2 0
--> 3 0
-> 3 1
--> 4 1
-> 4 2
--> 5 2
-> 5 3
---
5 2
---
4 1
---
3 0

As you can see, the lower numbers are from the shallower recursion levels that continue their execution after the recursive call returns. The higher values are local to those deeper calls and do not affect the value in the caller.

A recursive function without returns keeps cycling?

1 Answers1