select specific values from a matrix based on indeces

Hi all,

I have a matrxi of 4*37800. I want to keep specific values based on two indeces.

The first index shows the rows that i want to keep and the second the columns every 15 steps. I mean that i split the 37800 columns to groups of 15 the index shows which column of each group i want to keep.

How can i isolate the values based on the two indeces?

I attach the folders.

iMxR is the index showing the rows that i want. ix is the index showing which colum i need. freq is the matrix.

Thank you for your help.

[Tommy]

If the first index in ix is 14:

>> ix(1)

ans =

    14

and the first 15 indices in iMxR are the following:

>> iMxR(1:15)

ans =

     1     1     2     2     2     2     2     3     3     3     2     1     1     1     1

then which values should be pulled from the first 4x15 block of frequencies? Is the following correct?

frequencies(1, 14)
frequencies(1, 14)
frequencies(2, 14)
frequencies(2, 14)
frequencies(2, 14)
frequencies(2, 14)
frequencies(2, 14)
frequencies(3, 14)
...and so on

[Ilias Minas]

i want to start by keeping the appropriate rows using iMxR

freq(1,1)

freq(1,2)

freq(2,3) etc

It will give a single row matrix of 37800 columns, having the selected values

And using the ix, split the 37800 to groups of 15 columns and select the column that ix suggest.

Finally i will have 2520 columns and 1 row

Answer 1

[Tommy]

I believe this will give you the values you want:

temp = reshape(iMxR, 15, [])';
idx = sub2ind(size(temp), 1:size(temp,1), ix);
row = temp(idx); % row(i) is the row of the ith 4x15 block

col = ix; % col(i) is the column of the ith 4x15 block

temp = reshape(frequencies, 4, 15, []); % convert frequencies to a bunch of 4x15 blocks
idx = sub2ind(size(temp), row, col, 1:size(temp,3)); % convert row and col to linear indices of temp
val = temp(idx); % pull out the relevant values