# select specific values from a matrix based on indeces

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Ilias Minas on 4 Jun 2020 at 15:54
Answered: Tommy on 4 Jun 2020 at 18:13
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.

Tommy on 4 Jun 2020 at 16:20
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 on 4 Jun 2020 at 16:27
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

Tommy on 4 Jun 2020 at 18:13
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