extract a number N of equally spaced rows within a matrix

6 views (last 30 days)
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,:);

Accepted Answer

Fangjun Jiang
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

More Answers (1)

Dyuman Joshi
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
  8 Comments
Dyuman Joshi
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
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.

Sign in to comment.

Categories

Find more on Shifting and Sorting Matrices in Help Center and File Exchange

Products


Release

R2021b

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!