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

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RG on 28 Apr 2016
Commented: Star Strider on 28 Apr 2016
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.

Star Strider on 28 Apr 2016
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

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Star Strider on 28 Apr 2016
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 on 28 Apr 2016
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 on 28 Apr 2016
My pleasure.