Python Variable starting point for nested for loop

Question

I am attempting to create a nested for loop where the inner loop will have a different range the first time the loop runs and then use a different range for each subsequent loop.

The program is a sudoku solver. And it works by taking a position on the 9x9 board (board[k][l]), checking a condition, and then moving to the board position directly to the left (board[k][l-1]).

If l is 0 than we need to move to the previous row (k-1) and the farthest position to the right where l equals 8.

The problem I am having is on the first iteration of the inner loop the loop will not always begin with l equal to 8.

For example a user my select the square board[3][3]. The function should then check board[3][2] then board[3][1] then board[3][0] then board[2][8]

etc.

The code below only works if l=8

for i in range(k, -1, -1):
    for j in range(l, -1, -1):

For clarity, I can achieve the desired result using multiple for loops, but I am trying to make this code more concise:

k = user selection
l = user selection

for j in range(l, 0, -1):
    test(k,j)
for i in range(k-1, -1, -1):
    for j in range(9, 0 , -1):
        test(i,j)

I don't like this for two reasons, first we encounter a problem if either k or l starts at 0, second it seems unnecessary to use two for loops here.

@DeveshKumarSingh sorry if it wasn’t clear k does not alway =9 and l does not always = 9 — tdammon
– tdammon, Commented May 6, 2019 at 16:41

Rolf of Saxony · Accepted Answer · 2019-05-06 17:04:03Z

1

Isn't it just a matter of putting an if statement in there?

>>> k = 8
>>> l = 3
>>> run_one = True
>>> for i in range(k, -1, -1):
...  if run_one:
...     run_one = False
...     for j in range(l, -1, -1):
...       print(i, j)
...  else:
...     for j in range(8, -1, -1):
...       print(i, j)
... 
8 3
8 2
8 1
8 0
7 8
7 7
7 6
7 5
7 4
7 3
7 2
7 1
7 0
6 8
6 7
6 6
6 5
6 4
6 3
6 2
6 1
6 0
5 8
5 7
5 6
5 5
5 4
5 3
5 2
5 1
5 0
4 8
4 7
4 6
4 5
4 4
4 3
4 2
4 1
4 0
3 8
3 7
3 6
3 5
3 4
3 3
3 2
3 1
3 0
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
2 0
1 8
1 7
1 6
1 5
1 4
1 3
1 2
1 1
1 0
0 8
0 7
0 6
0 5
0 4
0 3
0 2
0 1
0 0
>>>

answered May 6, 2019 at 17:04

Rolf of Saxony

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Comments

Chris Maes · Accepted Answer · 2019-05-06 17:10:52Z

0

You can manage this in one single loop if you use an extra if (or a ternary expression like here):

l = 2
k = 5
for i in range(l, -1, -1):
  max = k if i == l else 9
  for j in range(max, 0, -1):
    print("{},{}".format(i,j))

gives:

2,5
2,4
2,3
2,2
2,1
1,9
1,8
1,7
1,6
1,5
1,4
1,3
1,2
1,1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1

answered May 6, 2019 at 17:10

Chris Maes

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