How to select the row of an element in a vector and put it in the same row for a different matrix?

For example, vector A is a 3x1 and has the value 1 in its third row. I want to then take the sum of the entire vector A and put this sum in a 3x5 matrix B (currently filled with NaN's) in the first column third row. I want to keep doing this for each subsequent column-- so for a second round, if vector A (3x1) has the value 1 in the first row, I want to put the sum of all numbers into the second column first row of matrix B.

Answer 1

[Kirby Fears]

Here's an example of creating an index and using it appropriately:

A = [1;2;3];
B = NaN(3,5);

% find where A is equal to 1
idx1 = (A==1);

% Set B's column 1 to be the sum(A) in the same row where A is 1
B(idx1,1) = sum(A);

Hope this helps.

[lauuser1]

Thank you, that is definitely helpful.

What would happen if A is a randomized vector? (with different values every time it is initiated in the command window). Say you're looking for the greatest value in the vector. Would you just change the 3rd code to

idx1 = max(A)

?

[dpb]

No. Not if it's that location you're looking for for the position. See

doc max  % for why not and what it would be instead...

See my Answer for what happens if the values are/aren't in the vector (or, better yet, just do some 'spearmint-ing at the command line and see)

[Kirby Fears]

The max function can return the location of the first occurrence of the maximum value.

If the maximum may occur in more than one position and you want to find them all, you can simply use idx = (A==max(A)) to find all locations in A that equal the max.

% getting idx1 without actually storing the max value itself
[~,idx1] = max(A);

% getting idx1 for multiple max match
idx1 = (A==max(A));

[lauuser1]

Thanks!

Sum(A) would be one value, so what if you wanted to move over the whole vector A? For example, if I wanted to move sum(A) to the location where max(A) was in the vector, let's say max(A) was in the third row, thus sum(A) gets moved to the third row, first column. What if I wanted to move all 3 values of vector A to the third row of matrix B? (They would be placed in the third row, first, second and third column)

[dpb]

B(:,3)=A;

You really need to read the "Getting Started" documentation on Matlab syntax and addressing, etc., ...

<Getting started>

[lauuser1]

Thank you. I'll definitely take a look at that... However what I'm looking for is that all values of A would need to be moved to the third row of Matrix B because the max value of A was in the third row. If the max value of A was in the first row, I would need to move all values of A to the first row of Matrix B.

I understand if I was simply moving all values of A to a column or row I would simply use what you mentioned above. However, I need to place it in a certain location depending on the location of the max value.

[lauuser1]

I've figured it out.

Answer 2

[dpb]

Well, much is undefined but the indexing is pretty simple...

B(A==1,icol)=sum(A);

Now, you'll have to iterate or otherwise set the column index and regenerate A and it's presumed there will only be a single value of '1' in A (else't you'll get more than one value set in B or none if didn't happen to have any). Also A must be a vector of length<=size(B,1) or you'll also be changing the size of the B array.