# Recommended steps to deal with a slow loop

Asked by Danielle Leblanc on 27 Jul 2012
Latest activity Commented on by Danielle Leblanc on 27 Jul 2012

Hi,

If a "for loop" is slow and each iteration is taking 2 hours what are the recommended steps that I should follow to deal with this issue?

## Products

No products are associated with this question.

I would recommend that you first focus on what happens inside the for loop... why is it taking so long? Get a complete breakdown of how much time is spend at every step, then you can decide where you can possibly optimise.

Further details would be great.

Answer by Sean de Wolski on 27 Jul 2012
• Listen to mlint. If it is warning you about a variable changing size within a loop, that will slow it down immensely
• Remove every calculation from every loop that does not depend on the loop.

Danielle Leblanc on 27 Jul 2012

below is my code. It is taking ages for each loop. I am a beginner and took me a lot of effort to write it . but still it is not feasible to implement it as it needs a lot of time to be executed and most probably my laptop will burn before running these codes. Any help would be a life saver. cd C:\Users\Danielle\Documents\MATLAB\CS\CSV

```for i=1:114
%% Import the data, extracting spreadsheet dates in MATLAB datenum format
fullFileName=sprintf('%s%d%s', 'C:\Users\Danielle\Documents\MATLAB\CS\CSV\',i, '.csv') ;
fid = fopen(fullFileName, 'rt');
fclose(fid);
X=M{1,1};
fieldnames = regexp( X(1,:), ',', 'split');
V= regexp( X(2:end,1), ',', 'split');
ncol=[4 5 10 18];%numeric columns
scol=[1 2 3 4 6 7 11 12 13];%string columns
variablestxt=fieldnames{1,1};
v2=nan(size(V,1)-1,size(ncol,2));
s2=num2cell(nan(size(V,1)-1,size(scol,2)));
for j=2:length(V)
try
v0=[V{j,1}];
v2(j-1,1)=datenum(v0(1,4),'mm/dd/yyyy');
str = [v0{1,4}, ' ', v0{1,5}];
v2(j-1,2) = datenum(str, 'mm/dd/yyyy HH:MM:SS');
v2(j-1,3:length(ncol))=str2double(v0(1,ncol(3:end)));
s2(j-1,1:length(scol))=v0(1,scol);
catch
'error' %#ok<NOPTS>
end
end
try
ID=unique(s2(:,1));
catch
xe3=find(cellfun(@(x)~ischar(x),s2(:,1)));
s2(xe3,:)=[];
v2(xe3,:)=[];
ID=unique(s2(:,1));
end
Count=0;
for k=1:length(ID)
x0=find(ismember(s2(:,1),ID(k,1)));
v3=v2(x0,:);
s3=s2(x0,:);
x1=find(max(v3(:,2)));
v(Count+1:Count+size(x1,1),1:size(v2,2))=v3(x1,:);
s(Count+1:Count+size(x1,1),1:size(s2,2))=s3(x1,:);
Count=Count+size(x1,1);
xe=find(ismember(s(:,5),'C'));
s(xe-1:xe,:)=[];
v(xe-1:xe,:)=[];
end
filename=[num2str(i) '.mat'];
cd C:\Users\Danielle\Documents\MATLAB\CS\Step1
save (filename,'s','v','stxt','vtxt')
```
Sean de Wolski on 27 Jul 2012

Use the profiler to identify the bottlenecks. Just run two or three files with profiler and it will show you where to focus your efforts. Some obvious things:

• try/catch is slow. I would try and write the code to avoid those.
• Skip the call to find here:
` x0=find(ismember(s2(:,1),ID(k,1))); `