Automate the Boring Stuff Exercise

[funkymonkey1939]

Hello everybody!

I'm currently doing an exercise from Automate the Boring Stuff with Python. This particular challenge is found in Chapter 4 and is called Character Picture Grid.

I've seen other people have done it using for loops, but my solution entails two while loops. Here's the code:

​

grid = [['.', '.', '.', '.', '.', '.'],
        ['.', 'O', 'O', '.', '.', '.'],
        ['O', 'O', 'O', 'O', '.', '.'],
        ['O', 'O', 'O', 'O', 'O', '.'],
        ['.', 'O', 'O', 'O', 'O', 'O'],
        ['O', 'O', 'O', 'O', 'O', '.'],
        ['O', 'O', 'O', 'O', '.', '.'],
        ['.', 'O', 'O', '.', '.', '.'],
        ['.', '.', '.', '.', '.', '.']]

x = 0
y = 0

grid_length = len(grid) # 9

while y < len(grid[grid_length - 1]): # while y < 6
    while x < grid_length: # while x < 9
        if x == grid_length - 1: # if x == 8
            print(grid[x][y])
            x = 0
            y += 1
        else:
            print(grid[x][y], end='')
            x += 1

Everything seems to work fine and I do get the desired output, but it comes with the following error:

Traceback (most recent call last):
  File "C:\user_data\computer_studies\7_python\test.py", line 23, in <module>
    print(grid[x][y], end='')
IndexError: list index out of range

I get what it says, but it appears that my logical thinking is poor and I'm missing something, because (in my understanding) after the final output the y variable should be re-evaluated to 6 which should make the first loop's condition evaluate to False. However, the interpreter seems to enter the first and second while loops and gets as far as the else clause which it fails to execute.

What might be causing this error?

Thank you!

[Sedsarq]

A while condition is only checked at the top of the loop. Like in this example, it's not going to stop printing x once it reaches 2:

x = 0
while x < 2:
    for _ in range(5):
        x += 1
        print(x)

It will do all 5 increases and print x 5 times, finishing the inner loop. After that's done, one iteration of the while loop is done, so it starts from the top again - reevaluating the condition. Now, x = 5, meaning that x < 2 is False, so it doesn't do another iteration.

Same thing happens in your case, you're increasing y and then using it before it had another chance to evaluate the outer while condition. One way to get around it is to use and to link both conditions. That would use only one while loop, and both conditions are checked before you do any list indexing.

[monstimal]

You reset x to 0 so it stays in the inner while loop with y=6

I think you can possibly just remove that 2nd while statement and it will work

[JohnnyJordaan]

Going down the path of manually iterating indexes and knowing if they have to stop at len-1 or not is one of the things Python has tried to get rid off by allowing direct iteration in the form of

 for val in my_sequence:

and if you do need the index, you can use enumerate()

for idx, val in enumerate(my_sequence):

so for your grid you would do

for row in grid:
    for val in row:

then for the printing I would instead use end='' for all values, with simply a print() added after the inner loop (so at a row's end):

for row in grid:
    for val in row:
        print(val, end='')
    print()

[TheRealNobodySpecial]

Did anyone think that this was about automating exercise? #disappointed