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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.

Asked by .. Vamshi on 23 Nov 2012
```    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 by Harshit on 23 Nov 2012

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.

.. Vamshi on 26 Nov 2012

mmm... I want here to increment the count every time when the condition is met. Does the above piece of code satisfy my requirement?

Harshit on 26 Nov 2012

Yes it will just think about it. Whenver the condn is true 1 will be generated in the matrix and otherwise 0. sum will give the no of 1.

.. Vamshi on 26 Nov 2012

I followed your suggestion.. the counts have to be stored for every iteration.. but here am unable to do..

Answer by Akiva Gordon on 23 Nov 2012

1 Comment

.. Vamshi on 23 Nov 2012

Thank you A G. I think eval is fine here for concatenations, isn't it?