为什么这个双循环不起作用？

Question

First of all, here's the code:

def check_sudoku(n):
    d=len(n)
    i=0
    s=1
    while i<d:
        print "i=",i
        while s<d:
            print "s=",s
            if n[0][i]==n[s][i]:
                return False
            s=s+1
            i=i+1
         return True 

what I want to do is that after the value of s changes from 1 to d , then it loops again and the value of i changes. But in my code the value of i is not changing at all.

Just to be clear of what I want to do, say

n =[[1,2,3,4],
    [2,3,1,3],
    [3,1,2,3],
    [4,4,4,4]]

i want the following to happen:

• first it should check for

   n[0][0]==n[1][0] n[0][0]==n[2][0] n[0][0]==n[3][0] 

  after that value of i should increase by 1

• then it should go like this:

   n[0][1]==n[1][1] n[0][1]==n[2][1] n[0][1]==n[3][1] 

after this value of i will increase again and this same loop will run.

This is not happening and I am not sure why. Please tell me what changes I should make to make it run the way I want to.

Answer 1

You don't reset s in the outer loop, so the inner loop will only execute once. After that, s < d will never be true again.

Answer 2

Your inner loop increments s until it is greater than d , but you never set it back to 1 . Also, with your current indentation, you also increment i in the inner loop, so it'll reach d inside the inner loop causing the outer loop to only be run once too.

So, if d is 4, your values will go:

i = 0, s = 1
i = 1, s = 2
i = 2, s = 3
# exit inner loop
return True

You'd be much better off using for loops using range() , avoiding your mistakes altogether:

def check_sudoku(n):
    for i in range(len(n)):
        for s in range(1, len(n)):
            if n[0][i]==n[s][i]:
                return False
    return True

With range the inner loop will always start at 1 , range through to len(n) every time without explicit resets needed.

If you use the any() function you can collapse the whole test into one line:

def check_sudoku(n):
    return not any(n[0][i] == n[s][i] for i in range(len(n)) for s in range(1, len(n)))

Answer 3

Further to unwind's answer, this is the modified code in a way that should work. I've changed the whitespace in the code and the operators in a way which most Python programmers will consider more conventional.

def check_sudoku(n):
    d = len(n)
    i = 0
    s = 1

    while i < d:
        print "i=",i
        while s < d:
            print "s=",s
            if n[0][i] == n[s][i]:
                return False
            s += 1
        i += 1
        s = 1
    return True 

n =[[1,2,3,4],
    [2,3,1,3],
    [3,1,2,3],
    [4,4,4,4]]

check_sudoku(n)

Answer 4

I think you should move the statement i=i+1 over the second loop follow as:

def check_sudoku(n):
d=len(n)
i=0
s=1
while i<d:
    print "i=",i
    while s<d:
        print "s=",s
        if n[0][i]==n[s][i]:
            return False
        s=s+1
    i=i+1
    return True