# extract a number N of equally spaced rows within a matrix

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Alberto Acri on 22 Sep 2023
Commented: Alberto Acri on 24 Sep 2023
HI! I have a number N and a matrix with 100 rows and 2 columns.
How can I recover N rows of the matrix so that they are as equidistant from each other as possible?
For example:
• with N = 10 I will have 10 rows (row 1, 11, 21, 31,...)
• with N = 7 I will have 7 rows (row 1, 15, 29,...) In this case the steps are like 100/7 = 14.28 = 14.
a=1:1:100;
a=a';
b=0.1:0.1:10;
b=b';
M=[a,b]; % example matrix
N=10; % number of rows
step = height(M)/N;
step = 14; % round down
v = 1:step:height(M);
M_new = M(v,:);

Fangjun Jiang on 22 Sep 2023
Edited: Fangjun Jiang on 22 Sep 2023
M=repmat((1:100)',1,2);
N=7;
d=M(round(linspace(1,height(M),N)),:)
d = 7×2
1 1 18 18 34 34 51 51 67 67 84 84 100 100

Dyuman Joshi on 22 Sep 2023
Edited: Dyuman Joshi on 22 Sep 2023
You are on the right course -
a=(1:1:100)';
b=(0.1:0.1:10)';
M=[a,b]; % example matrix
N=7; % number of rows
step = height(M)/N
step = 14.2857
%Use floor to round down to nearest integer less than or equal to step
step = floor(step) % round down
step = 14
v = 1:step:height(M)
v = 1×8
1 15 29 43 57 71 85 99
M_new = M(v,:)
M_new = 8×2
1.0000 0.1000 15.0000 1.5000 29.0000 2.9000 43.0000 4.3000 57.0000 5.7000 71.0000 7.1000 85.0000 8.5000 99.0000 9.9000
Dyuman Joshi on 24 Sep 2023
Edited: Dyuman Joshi on 24 Sep 2023
"when height(M) is 100 and N is 10, it is hard to say which one is the right answer."
OP has specified what the expected outcome is and the logic behind it -
HI! I have a number N and a matrix with 100 rows and 2 columns.
How can I recover N rows of the matrix so that they are as equidistant from each other as possible?
For example:
• with N = 10 I will have 10 rows (row 1, 11, 21, 31,...)
• with N = 7 I will have 7 rows (row 1, 15, 29,...) In this case the steps are like 100/7 = 14.28 = 14.
Alberto Acri on 24 Sep 2023
Thanks for the replies!
@Fangjun Jiang Yes, it is actually impossible to make an "equally spaced" distribution for any value of N but your case is more useful for my purpose.
@Dyuman Joshi Your solution is certainly fine too. I gave an example to make it clear what I wanted to achieve as a final result. The distribution as you wrote it follows exactly what I wanted to obtain as a result in the question. But it was an example. You'll accept both as an answer but that's not possible.

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