How to make my number board start at 1 end at 9 and arrange itself in 3 rows

Question

I’m trying to understand this code from "12 Beginner Python Projects – Coding Course" on the FreeCodeCamp Youtube channel and I really don’t understand this code to print a board for the Tic Tac Toe project. I’ve looked around on some other vids and I didn’t like how they set up their board. My first idea on how to solve this before I watched the vid fell through so I went with her code on how to generate the board but I don’t actually understand what its doing.

Using the code:

number_board = [[str(i) for i in range(j*3, (j+1)*3)] for j in range(3)]
for row in number_board:
    print('[' + ']['.join(row) + ']')

has the output:

[0][1][2]
[3][4][5]
[6][7][8]

I want it the output to be basically the same except starting at 1 and ending at 9 and the part I don’t understand is: (j*3, (j+1)*3)] for j in range(3)

I understand that the final range(3) dictates how many rows there are and if I change that to 5, it would be 5 rows and go up to 14. I thought I understood the "start: stop: step" concept but when I alter the start to be (j*3+1, (j+1)*3)] the output is:

[1][2]
[4][5]
[7][8]

Which I think is due to the stop not adding up to 10 but If I change it to something like [[str(i) for i in range(j*3+1, 10)] for j in range(3)] it starts at 1 but doesn’t end where I want it to. I tried a bunch of different combinations to get what I want but even if I guessed the correct one I still wouldn’t know why it works.

[1][2][3][4][5][6][7][8][9]
[4][5][6][7][8][9]
[7][8][9]

Maybe I’m missing something really obvious since this is my first month of learning so my apologies if this is an obvious question but I just don’t understand how the start: stop: step modifiers work(not even sure if they’re called modifiers or what the proper name for them is)

Asked By: new2codingitshard

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Source

Answer 1

This style of loops are called list comprehensions in python. Do a short search and you will get lots of information about them.
What happens in your code is practically same with the following:

number_board = []
for j in range(3):
    board_row = []
    for i in range(j*3, (j+1)*3):
        board_row.append(str(i))
    number_board.append(board_row)

for row in number_board:
    print('[' + ']['.join(row) + ']')

So first code iterates over 3 rows
For each row it needs to generate 3 numbers. This numbers are decided by range(j*3, (j+1)*3). So

when j=0, numbers are between 0 and 3
when j=1, numbers are between 3 and 6

Please check out python range documentation
As stated in documentation, format for range is
range(start, stop, step)

Once the board is created, it is printed out with another for loop

Answered By: drx

How to make my number board start at 1 end at 9 and arrange itself in 3 rows

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