Select range of rows and store them

Hello everyone,

that's the issue:

I have a matrix, let's call it Ch = 1000 x 33. In column 33, I have spotted some values using a threshold (MinPeakHeight function), which returns the location (lc) of these rows (let's call them rows_X). Now, using these indices, I would like to retain all the values within 3 rows before rows_X and 4 rows after every rows_X (the value inside rows_X must be retained as well). Any help will be very appreciated!!

Ex: let's take the following matrix. I have spotted all the values in column 6 greater than 7.Now I know these values lies in rows 3 and 8. I want to retain/store in different variables all the values in rows 3 and 8, plus all the values in the 2 rows before rows 3 and 8, and in 2 rows after rows 3 and 8.

Ch:

1 4 5 4 8 4

2 5 5 4 9 4

3 5 5 5 5 8

4 6 5 6 5 4

5 6 5 6 5 4

6 6 5 6 5 5

7 7 5 6 5 5

8 6 7 6 5 8

9 3 3 3 3 3

10 3 3 4 4 3

the final output should look like this:

rows_a =

1 4 5 4 8 4

2 5 5 4 9 4

3 5 5 5 5 8

4 6 5 6 5 4

5 6 5 6 5 4

rows_b =

6 6 5 6 5 5

7 7 5 6 5 5

8 6 7 6 5 8

9 3 3 3 3 3

10 3 3 4 4 3

[Matt J]

in the 2 rows before rows 3 and 8, and in 2 rows after rows 3 and 8.

Earlier, you said 3 rows before and 4 rows after. Why is it now 2 and 2?

[enzo]

@Matt J the first one was the actual scenario. The sample, where I asked minus 2 ans plus 2 rows from row_x is just for demonstrational purpose and in order to be easier to be shown to the matlab community

[Enzo]

@Matt J do you have any tips? please, fell free to share any doubt about this task

Answer 1

[David Hill]

Use of cell array for storage incase the sizes are not equal. If the sizes will always be the same, you could store in 3D-matrix.

Ch=[1  4  5  4  8  4
2  5  5  4  9  4
3  5  5  5  5  8
4  6  5  6  5  4
5  6  5  6  5  4
6  6  5  6  5  5
7  7  5  6  5  5
8  6  7  6  5  8
9  3  3  3  3  3
10 3 3  4  4  3];
idx=find(Ch(:,6)>7);
for k=1:length(idx)
    S{k}=Ch(max(1,idx(k)-2):min(size(Ch,1),idx(k)+2),:);%max and min to protect against to low or high of indexing
end
S{1}

ans = 5×6

1 4 5 4 8 4 2 5 5 4 9 4 3 5 5 5 5 8 4 6 5 6 5 4 5 6 5 6 5 4

S{2}

ans = 5×6

6 6 5 6 5 5 7 7 5 6 5 5 8 6 7 6 5 8 9 3 3 3 3 3 10 3 3 4 4 3

[Enzo]

hi @David Hill, unfortunately your code does not provide the desired output. Do you have any other tips? it would be really appreciated

[David Hill]

It matches the output above other than assigning different variables (should index into a single variable and not have dynamic variable names). What are you trying to do once you have the various matrices?

[Enzo]

hi @David Hill thanks for your time. I have tried again and, as I am using huge data files, matlab send me this error code

Out of memory.

reasonably, I have to keep it simple without killing the ram