Sum cumulative value from all files that is summoned in a loop

I want to add the row length of the variable 'time' in a file (length(dataset.time)) from multiple datasets that are summoned in a loop. My setup is a follows:

datafolder = ' Z:\Projects\data';

file = dir(datafolder);

for i = 3:105

patient = load(strcat(datafolder,'\',file(i).name));

dataset = patient.table

row_length = length(dataset.time)

sum_length = ?

For now, ' row_length' only outputs the number of rows from the last dataset.

How do I add the total number of rows from dataset 3:105?

Hope you can help

[Stephen23]

This line:

for i = 3:105 

is an anti-pattern, buggy attempt to ignore the dot directories:

https://www.mathworks.com/matlabcentral/answers/345841-unexplained-error-on-dir#answer_271627

https://www.mathworks.com/matlabcentral/answers/21150-function-call#comment_46042

https://www.mathworks.com/matlabcentral/answers/394807-create-text-file-containing-list-of-file-names#comment_556641

https://www.mathworks.com/matlabcentral/answers/288815-gui-for-loop-not-working-correctly#answer_224956

https://www.mathworks.com/matlabcentral/answers/484430-using-struct-dir-selpath-what-do-and-mean-as-struct-name#answer_395754

https://www.mathworks.com/matlabcentral/answers/40949-omitting-pointers-when-listing-folder-contents#answer_50568

https://www.mathworks.com/matlabcentral/answers/13978-list-of-files-sorting#answer_19278

As Walter Roberson wrote in that last link: "In short: if your code assumes that '.' and '..' are the first two entries in a directory, your code has a bug (even in MS Windows). If your code assumes that directory entries are returned in any sorted order, your code has a bug (in all OS.)"

Much better code uses ISMEMBER or SETDIFF.

[SRRellum]

Thanks, good point. This was indeed the easy way out. I will change it

Answer 1

[Davide Masiello]

Inside the loop write

row_length(i) = length(dataset.time);

After the loop, you can calculate the summ of all lengths with

sum_length = sum(row_length);

[SRRellum]

Thanks for yout help. It works, however it gives the message to consider preallocating. Will look into that.

Answer 2

[David Hill]

datafolder = ' Z:\Projects\data'; 
file = dir(datafolder); 
row_length = 0;
for i = 3:105 
  patient = load(strcat(datafolder,'\',file(i).name));
  dataset = patient.table 
  row_length = row_length+length(dataset.time)
end

[Stephen23]

Fragile/buggy code:

for i = 3:105

Lets try it with some simple, perfectly valid filenames:

csvwrite('+new.txt',1)
csvwrite('-old.txt',1)
csvwrite('test.txt',1)
dir()

+new.txt  -old.txt  .         ..        test.txt  

Fragile at best, buggy at worst.