How can I reduce the time complexity of this matlab code.. Its taking lot of time to execute when an image is fed as input.

Question

.. Vamshi 2012-11-23

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/54561-how-can-i-reduce-the-time-complexity-of-this-matlab-code-its-taking-lot-of-time-to-execute-when-an

    cnt1=0; cnt2=0; cnt3=0; cnt4=0; cnt5=0; cnt6=0; cnt7=0; cnt8=0;
    p = 1;
    row = 1;
    while p<(size(Theta_deg_img,1))
        q = 1;   
        while q<(size(Theta_deg_img,2))
            for i=p:p+8             
                for j=q:q+8                                                 
                    if Theta_deg_img(i,j)>=-180 && Theta_deg_img(i,j)<=-135                    
                        cnt1 = cnt1+1;                
                    elseif Theta_deg_img(i,j)>=-134 && Theta_deg_img(i,j)<=-90                   
                        cnt2 = cnt2 + 1;                                    
                    elseif Theta_deg_img(i,j)>=-89 && Theta_deg_img(i,j)<=-45                    
                        cnt3 = cnt3 + 1;                
                    elseif Theta_deg_img(i,j)>=-44 && Theta_deg_img(i,j)<=0                    
                        cnt4 = cnt4 + 1;                
                    elseif Theta_deg_img(i,j)>=1 && Theta_deg_img(i,j)<=45                    
                        cnt5 = cnt5 + 1;                
                    elseif Theta_deg_img(i,j)>=46 && Theta_deg_img(i,j)<=90                   
                        cnt6 = cnt6 + 1;                
                    elseif Theta_deg_img(i,j)>=91 && Theta_deg_img(i,j)<=135                 
                        cnt7 = cnt7 + 1;                                
                    else                        
                        cnt8 = cnt8 + 1;
                    end                    
                    eval([fsrc '(row,1) = cnt1;']);
                    eval([fsrc '(row,2) = cnt2;']);
                    eval([fsrc '(row,3) = cnt3;']);
                    eval([fsrc '(row,4) = cnt4;']);
                    eval([fsrc '(row,5) = cnt5;']);
                    eval([fsrc '(row,6) = cnt6;']);
                    eval([fsrc '(row,7) = cnt7;']);
                    eval([fsrc '(row,8) = cnt8;']);                                     
                end                               
            end           
            q = q + 9;  row = row+1;        
            cnt1=0; cnt2=0; cnt3=0; cnt4=0; cnt5=0; cnt6=0; cnt7=0; cnt8=0;                    
        end         
        p = p + 9;        
    end

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Answer 1

Harshit 2012-11-23

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Vectorize your code. Example you don't need inner loops you can use sum to get the value of counts. In the outer two loops you are calculating size again and again. Store it in a variable.