# Populate a 1000x1000 array with rows where a value is added every second, then third etc. column

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CaraAkame on 28 Apr 2022
Edited: dpb on 28 Apr 2022
I know how to make the individual rows:
% two = zeros (1,1000);
% two (2:2:end)=1;
and so on, but I can't do this for every row (this might be a roundabout way of doing it). How do I incorporate this in a loop? Preferably directly into an array. I've been trying for hours, but I haven't used matlab in years and was never the best at using it beyond graphs.
The array should look something like this:
1 1 1 1 1 1 1 1 1 1
0 1 0 1 0 1 0 1 0 1
0 0 1 0 0 1 0 0 1 0
0 0 0 1 0 0 0 1 0 0
(but for 1000x1000)
I somehow always end up with my array being just 1s when I try to incorporate it in a loop.
I've been trying to do some puzzles to get back into using matlab, and I managed fine for one that needed similar things, but with if/else; but for loops (which I think I need here) were always my kryptonite, so please help me! Thank you!
##### 4 CommentsShowHide 3 older comments
CaraAkame on 28 Apr 2022
I know what I did wrong now, thank you all for your help!

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### Accepted Answer

Jan on 28 Apr 2022
Edited: Jan on 28 Apr 2022
You want an output of size 1000x1000:
M = zeros(1000, 1000);
You want a loop to go from 1 to 1000:
for k = 1:1000
...
end
You want to set some elements to 1 in the k'th row of M:
M(k, a:b:c) = 1;
Here c is obviously 1000. Just find matching values for a and b.
You find an exhaustive tutorial here:
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CaraAkame on 28 Apr 2022
Thank you!
This was really helpful in understanding what my mistake was, since I did most of that (forgot the k), and overcomplicated other stuff (the k again), on instinct and half remembered coding sessions from uni, and could not figure out why it wasn't working. Thank you for taking the time to explain this in steps!

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### More Answers (1)

DGM on 28 Apr 2022
If you were getting a bunch of ones, you were probably pretty close.
N = 1000;
everyN = zeros(N);
for n = 1:N
everyN(n,n:n:end) = 1;
end
If you forgot the row index, the first pass would have set the entire array to 1.
##### 1 CommentShowHide None
CaraAkame on 28 Apr 2022
Oh wow, that looks very similar to what I did the first time around, I'm going to go try that right now. If I was really that close for the last few hours I'm going to be both proud and annoyed at myself!
.
.
It really was my biggest error. I mean, my starting statement for the for loop was also slightly wonky, but I'm sure I had the statement that way as well at some point as well. Thank you so much!

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