# extract all rows of a matrix except 'r' (vector) rows

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Alberto Acri on 8 Sep 2023
Answered: MarKf on 8 Sep 2023
Hi! I need to extract all rows of 'matrix' except 'r' rows.
matrix = [54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65];
r = [1; 2; 3; 4: 9];
The result is:
matrix_out = [58; 59; 60; 61; 63; 64; 65];

Stephen23 on 8 Sep 2023
The most efficient approach:
matrix = [54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65];
r = [1; 2; 3; 4; 9];
matrix_out = matrix;
matrix_out(r) = []
matrix_out = 7×1
58 59 60 61 63 64 65

Dyuman Joshi on 8 Sep 2023
Edited: Dyuman Joshi on 8 Sep 2023
matrix = [54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65];
r = [1; 2; 3; 4; 9];
%Method 1
%remove directly
matrix(r,:)=[];
%or
%Method 2
%remove indirectly
idx = 1:size(matrix,1);
ix = setdiff(idx,r);
matrix = matrix(ix,:);
Note that Method 1 won't work when there is an index not in the range of the dimensions of the array matrix.
For such cases, use Method 2.

MarKf on 8 Sep 2023
mat = magic(8)
mat = 8×8
64 2 3 61 60 6 7 57 9 55 54 12 13 51 50 16 17 47 46 20 21 43 42 24 40 26 27 37 36 30 31 33 32 34 35 29 28 38 39 25 41 23 22 44 45 19 18 48 49 15 14 52 53 11 10 56 8 58 59 5 4 62 63 1
r_not = [1 3:5 8];
mat(r_not,:) = [] %use mat_o = mat; to keep the original mat;
mat = 3×8
9 55 54 12 13 51 50 16 41 23 22 44 45 19 18 48 49 15 14 52 53 11 10 56
There are some issues with your example:
matrix = [54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65];
r = [1:3, 6, 9]; %btw this did not work with those dimensions;
% also not the way you wanted with those indices (matrix_out just removed 1:3)
matrix_out = matrix;
matrix_out(r,:) = []
matrix_out = 7×1
57 58 60 61 63 64 65
Since yours is a simple vector, matrix_out(r) = []; would have worked easily too

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