🔒 help with recursive function

[tall vault]

@safe rapids

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[placid quest]

!code

[tall vault]

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[safe rapids]

i have the following snippet from my sudoku solving algorithm using backtracking in python. i understand the premises of how it works, and according to my code, it should return False the second the backtracking fails and reaches aa dead end. however, the code instead persists and continues until it returns the right answer. i am happy it does so, but i am not sure why. here is the code:

def solve():
    for y1 in range(9):
        for x1 in range(9):
            if board[y1][x1] != 0: continue
            for N1 in range(1, 10):
                if move_allowed(N1, x1, y1):
                    board[y1][x1] = N1
                    if solve(): return True
                    board[y1][x1] = 0
            # Tried all possible in empty pos. None worked; messed up before
            return False
    return True

if solve():
    print(board)
else:
    print("NO SOL")

[placid quest]

Are you expecting return False to end all recursive calls?

[safe rapids]

if you'd like the remaining code for context, please let me know

[safe rapids]

yes, i think so because it continues till the dead end, at which it returns False, which should end the solve() entirely. however, it does not do this.

[placid quest]

Yes. Note that recursion is not special. If function a calls function b, and b executes return False, does that cause a to also return?

[safe rapids]

ohh so the return False is in context of the if solve(): return True. since that outputs false, it goes back to the outside if move_allowed... statement which continues to replace board[y1][x1] = 0 because N1 does not satisfy it

[placid quest]

When you return from a recursive call, that returns from that one call, and execution returns to the previous recursive call that made the recursive call that was returned from. If that makes sense.

Just like with all functions, when you return, execution picks up where it left off in the function that made the call. It doesn't matter if that was a recursive call or a "normal" function call.

[safe rapids]

got it, you cleared up a really foggy part of this code for me, i really appreciate your help

!close

[tall vault]

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