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

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!

[dpb]

As you've outlined that you're practicing, let's go at it as if were homework and provide hints to aid in learning, not just a canned solution.

Solution 1: Your above will work (remember to preallocate and address for performance, of course) in a loop; what if you make the step size in colon a variable???

[Jan]

"I somehow always end up with my array being just 1s" - then the best strategy in the forum is to post the failing code, such that the readers can point to the problem and fix it.

[CaraAkame]

Alright, so I think I tried what you're getting at dpb, at some earlier point. It kept giving me 1s, but someone answered what might have been the issue.

I wanted to post the failing code, but I could not for the life of me remember what I did hours ago, and at this point it's just a mess of commented out lines that are not connected.

It was something like this (which when I run it still gives me 1s, but I have not tried the other solution yet):

%   A=zeros(1000);
%   eggs=(1:1:1000)';
%   for n = eggs(:,1)
%       A(n:n:end)=1;
%   end

[CaraAkame]

I know what I did wrong now, thank you all for your help!

Answer 1

[Jan]

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:

https://www.mathworks.com/learn/tutorials/matlab-onramp.html

[CaraAkame]

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!

Answer 2

[DGM]

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.

[CaraAkame]

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!