Slow for loop string comparison

Hi,

I have a for loop which detects the exact position of a specific string in the 'Time' row of a timetable, however it is really slow... anybody with ideas to speed up this process? My code is shown below, and the Time elements format is, e.g. 30-Sep-2019 08:58:08.000

for j=1:500001

   app.v = string(app.ttable.Time(j));
                   
     if app.v==string('30-Sep-2019 08:58:08.000')
                      
     app.SelectfileDropDown.Items = string(app.ttable.Time(j));
                       
     end              
                   
end  

Answer 1

[Stephen23]

I assume that

app.ttable.Time

is actually a datetime array. In that case, your loop very inefficiently converts each datetime individually to a string, and then compares that string against some characters. A much more efficient approach would be to use MATLAB indexing:

d = datetime(2019,9,30,8,58,8) % 30-Sep-2019 08:58:08.000
X = d==app.ttable.Time;
app.SelectfileDropDown.Items = string(app.ttable.Time(X));

Answer 2

[KSSV]

Obviously what you have shown will be dead slow and also may not work.

You can striaght aways use strcmp, contains. Read about them. Aslo you need not to convert the date into string, rather you can have them into Datetime class and striaght away work on them.

Answer 3

[Mathieu NOE]

hello

for / if loops will make this very slow

here my suggestion / demo code for faster method :

t1 = datetime(2000,11,1,8,0,0);
t2 = datetime(2020,11,5,8,0,0);
t = (t1:t2)';

%% 1st method 
tic
for j=1:numel(t)
                   
     if string(t(j)) == string('08-Feb-2001 08:00:00')
                      
     disp(' found')
                       
     end              
                   
end
toc
% Elapsed time is 6.430877 seconds.

%% 2nd method 
tic

n = datenum(t); % list of dates 
test_date = datenum('08-Feb-2001 08:00:00'); % test date 
ind = find(n == test_date);
date_found = t(ind)

toc
% Elapsed time is 0.036663 seconds.