How to do an array conversion into a different size of array

Hello, I would like to try to make an array into a different size of array.

If I assume my array one is 1x16 array, and my desired one is 1x100 array at desired rows, how can I translate it?

For example, I can achieve it manually as decribed below (W - exsist array, WD desired array):

WD(1,34) = W(1,1);

WD(1,35) = W(1,2);

WD(1,36) = W(1,3);

WD(1,37) = W(1,4);

WD(1,44) = W(1,5);

WD(1,45) = W(1,6);

WD(1,46) = W(1,7);

WD(1,47) = W(1,8);

WD(1,54) = W(1,9);

WD(1,55) = W(1,10);

WD(1,56) = W(1,11);

WD(1,57) = W(1,12);

WD(1,64) = W(1,13);

WD(1,65) = W(1,14);

WD(1,66) = W(1,15);

WD(1,67) = W(1,16);

other rows of WD is 0.

However, when I try to do it by using for loops, it only reads the last value of W(1,16) onto the desired points on WD.

This is my for loops.

for i = 30:10:60
    for j = 4:7
        for k = 1:16
            WD(1,i+j) = W(1,k);
        end
            for l = 68:100
                WD(1,l) = 0;
            end
    end
end

I believe I have to modify the loop status somehow, but I found it difficult at the moment.

How can I fix it?

Any help would be much appreciated.

Thank you.

[Guillaume]

I forgot I was going to comment about your code to explain a few of the problems. In general, you should strive to avoid loops. Certainly copying elements from one array to another should never require a loop. Just use indexing.

Problems with your loops:

for k = 1:16
    WD(1,i+j) = W(1,k);
end

The destination here is the same for each step of the loop. So you're copying 16 different values into the same index, each time overwriting the previous one you've copied. Of course, that doesn't work. At the end, WD(1, i+j) is always W(1, 16).

Note that when using vectors, you should use linear indexing (ie W(16)) instead of 2d indexing. With 2d indexing you're forcing the vector to be in particular direction (in your case a row vector). With linear indexing, the code works regardless of the shape (row, column, whatever).

for l = 68:100
    WD(1,l) = 0;
end

That can be simplified to

WD(1, 68:100) = 0;  %no need for a loop when you can just pass all the indices at once.
%or
WD(68:100) = 0; 

[Dan Park]

Thank you for your help Guillaume!

You are giving me a very good advice beyond the problem. I must learn how to make the code compact.

'Certainly copying elements from one array to another should never require a loop. Just use indexing.'

Yes, I realised that I don't need any for loops for this problem. You've pointed out what I was exactly encountered, and now I see where my problem comes from. I will try fix it with much simpler codes.

Thank you again.

Dan

Answer 1

[Guillaume]

Even simpler, no loop needed:

starts = [34, 44, 54, 64];
span = 4;

WD = zeros(1, 100);
WD(starts + (0:span-1)') = W(1:numel(starts)*span);  %requires R2016b or later, or bsxfun for the +

[Stephan]

This is a pretty nice one! The + feature is new to me - Thanks for posting this answer Guillaume!

[Dan Park]

Thank you for both Guillaume, and Stephan.

Both of your answers are helpful to my problem.

Actually, this example is simplified to one case, so I need a general expression for this, and that's why I was using for loops.

Anyways, I will try work out with your suggestion!

Many thanks.

Dan

[Stephan]

To do it in a generalized way all you have to do is change the starts vector and the span value as needed.

[Dan Park]

Thank you for your help again, Stephan.

I will take that into account in my code.

Dan

[Guillaume]

If you tell us more about your actual use case, we can give you a more generalised code.

But certainly, if you can generate the list of indices with simple math operations (and automatic array expansion as I've done), it will be a lot simpler. Triple loops are certainly completely unnecessary and you should learn to avoid them.