Improving MATLAB code speed

Question

Would be grateful for some pointers. I am reading about 1M rows of data and it is taking almost 24 hours with the following code. How can I improve the execution time?

The array Day contains the value of the n^th day from the start and there are more then one record for a particular day. The program checks if a particular id (stored in unique_id) is repeated within 180 days.

%// calculating the number of repeats within 180 days
fid2 = 'data_050913/Unique_id_repeat_count1.xlsx';
fid1 = 'data_050913/data_050913_2000.csv';

fid_data = fopen(fid1);
data     = fgetl(fid_data); %// the first line, title line
ep       = 0; %// position point number

while 1
    data = fgetl(fid_data);
    if(length(data)<10)
      break;
    end
    ep = ep+1;
    id = find(data == ',');
    unique_id(ep) = str2num(data(1:id(1)-1)); 
    day(ep) = str2num(data(id(8)+1:id(9)-1)); 
end

repeat = zeros(ep,1);

tic
i = 1; 
count = 0;
while i <= ep
    j = i+1;
    while ( (j<=ep) && (day(j)<= day(i)+179) )
        if unique_id(i) == unique_id(j)
           count = 1;
           break;
        end
        j = j+1;
    end

    repeat(i,1) = count;

    count = 0;
    i = i+1;
end
toc

i = 1;
k = 1;
while i<=ep
    count = repeat(i,1);
    j=i;
    while (day(j) == day(i))
        count = repeat(j,1)+count;
        j = j+1;
        if j > ep
            break;
        end
    end

    day_final(k,1)= day(i);
    repeat_final(k,1) = count;

    k = k+1;
    i = j;
end

xlswrite(fid2,day_final,'Repeat_Count','A2');
xlswrite(fid2,repeat_final,'Repeat_Count','B2');

Thanks

Answer 1

if not already doing this, ensure you allocate all memory up-front where possible. I've seen Matlab scripts go from 24-hours to 8 minutes by doing this.

Use the zeros function to preallocate memory for all growing arrays (day, unique_id, repeat, day_final and repeat_final).

x = zeros(1000); %// Creates a 1000 element array of all zeros