for loop - Why do I need a temporary value for the columns in the 2D Fourier Transform (Matlab) -

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here matlab code 2d discrete fourier transform. code transcribed from:

scilab textbook companion digital image processing s. jayaraman, s. esakkirajan , t. veerakumar.

i not going on during second loop, , why have use temporary value f_2.

what happening during stage of script?

thank you.

function [ftransformed2d]= dft2d(f)      [rows,cols]=size(f);   k=1:cols       f_1(:,k)=exp(-2*1j*pi*(0:nrows-1)'*(0:rows-1)/rows)*f(:,k); end   j=1:rows      f_2_a=exp(-2*1j*pi*(0:ncols-1)'*(0:ncols-1)/cols)*(f_1(j,:)).';     f_2(j,:)=f_2_a.';    end  ftransformed2d = circshift(f_2,floor(size(f_2)/2)); end

the first loop performs fft on each column, , second loop performs fft on each row. concerns second loop,

 f_2_a=exp(-2*1j*pi*(0:cols-1)'*(0:cols-1)/cols)*(f_1(j,:)).';  f_2(j,:)=f_2_a.';

the use of f_2_a not necessary may have been included ease of reading. have been written instead 

 f_2(j,:) = ( exp(-2*1j*pi*(0:cols-1)'*(0:cols-1)/cols)*(f_1(j,:)).' ).';

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