How to extract columns of a matrix using a for loop?

Hi Everyone,

I have a 3x6 matrix and trying to extract 1st and 3rd, 2nd and 5th, 3rd and 6th column using a loop. The matrix is:

polygon_nf =
    0.0185    0.0192    0.0207    0.0185    0.0192    0.0207
    0.0241    0.0236    0.0239    0.0241    0.0236    0.0239
    0.0185    0.0192    0.0207    0.0241    0.0236    0.0239

The loop I have so far is:

for i=1:3
    x_nf=polygon_nf(:,i);
    y_nf=polygon_nf(:,i+3);
end

When I try to display x_nf and y_nf values from within the loop I get 6 columns that I want, however, when I display the results after the loop has finished, I only get 2 columns. And when I try to put x_nf(i) and y_nf(i) instead of x_nf and y_nf inside the loop, I get an error saying:

In an assignment  A(I) = B, the number of elements in B and I must be the same.

Any help on this issue would be highly appreciated.

Answer 1

[Star Strider]

You need to subscript the left-side variables in your assignments as well:

for i=1:3
    x_nf(:,i)=polygon_nf(:,i);
    y_nf(:,i)=polygon_nf(:,i+3);
end

[RG]

Excellent. Thanks very much!!

[Star Strider]

My pleasure!

There’s actually an easier way to do that, without the loop:

x_nf = polygon_nf(:,1:3);
y_nf = polygon_nf(:,4:6);

which is what I believe you want to do.

[RG]

Thanks for the alternative version.

The reason I thought of using loop was that in my code I want to run it more than three times (the input matrix is 3 x n), but again, thanks for the suggestion, it is highly appreciated.

[Star Strider]

My pleasure.