How improve for cycle with sequential variables name and time evaluate ?

Question

Stefano Alberti 2016-2-11

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https://ww2.mathworks.cn/matlabcentral/answers/267683-how-improve-for-cycle-with-sequential-variables-name-and-time-evaluate

编辑： Stephen23 2019-6-19

I've this for cycle:

for k =1:3 %numel(D)
      fid = fopen(fullfile(D(k).name), 'rt');
      filename = D(k).name ;
      a=filename(11:end);
      b=regexp(a,'__','split');
      out1=cellfun(@(x) datestr(datenum(x(1:10),'yyyy_mm_dd'),'yyyy/mm/dd'),b,'un',0);
      out2=cellfun(@(x) datestr(datenum(x(12:end),'HH_MM'),'HH:MM'),b,'un',0);
      out=[out1'  out2'];
      clearvars out1 out2;
      fclose(fid);
  end

I want to assign a cumulative name at the variables 'out' like 'out-1', 'out-2', ... for each for cycle. I try do that with this code:

t1 = cell(numel(D), 1);
 t2 = cell(numel(D), 1);
 for j=1:numel(D);
 t1{j} = sprintf('TimeStart %u', j);
 t2{j} = sprintf('TimeFinish %u',j);
 end

But I don't now how put this code in the for cycle.

I need to evaluate che difference between two dates & times, and for do that I use this code:

prova = [out{1,1}, ' ', out{1,2}];
prova2 = [out{2,1}, ' ', out{2,2}];
format shortg;
t1 = datevec(prova,'yyyy/mm/dd HH:MM');
t2 = datevec(prova2,'yyyy/mm/dd HH:MM');
e=etime(t2, t1)

I need to do that with a iterative cycle, for each cell and create a file with the column of results.

Thanks in advance.

Stefano

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Stephen23 2016-2-11

Read the answers, and then use a cell array or structure.

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Answer 1

Stephen23 2016-2-11

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/267683-how-improve-for-cycle-with-sequential-variables-name-and-time-evaluate#answer_209461

编辑：Stephen23 2019-6-19

http://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

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Stephen23 2016-2-12

编辑：Stephen23 2016-2-12

在 MATLAB Online 中打开

@Stefano Alberti: Your code needs some improvement. For example you fopen and fclose the file, but you never use this file while it is open. Do not make code more complicated than it needs to be!

I notice that in the last two days you have asked many question around this topic, but you never really told us what you are trying to achieve. It is usually more effective to describe what you want to achieve (i.e. you task) and then we can show you an efficient way of doing it. Your task seems to be to read the file names and calculate the elapsed time based on the timestamps in each filename. Here is a efficient way of doing this:

C = {'Daily_cum_2013_05_30_09_59__2013_05_31_10_05.asc';...
     'Daily_cum_2015_02_11_08_16__2015_02_11_08_42.asc';...
     'Daily_cum_1999_12_31_23_59__2000_01_01_00_01.asc'};
[mtc,spl] = regexp(C,'\d{4}(_\d{2}){4}','match','split');
mtc = vertcat(mtc{:});
%spl = vertcat(spl{:});
dtv = datevec(mtc(:),'yyyy_mm_dd_HH_MM')
out = etime(dtv(1+end/2:end,:),dtv(1:end/2,:))

and its output (the time differences in seconds):

Stephen23 2016-2-12

编辑：Stephen23 2016-2-12

在 MATLAB Online 中打开

Don't use my code inside the loop. Just give it a cell array of the filenames:

C = {D.name};
% my code here
for k = 1:numel(D)
    out(k) % the time difference
    % your code here
end

and it will work. Putting my code before the loop will be faster and neater, and within the loop you can access the elements of out using indexing:

out(k)

The reason your code does not work is because you misunderstand what

D.name

does. It is explained in the documentation for non-scalar structures. In short this:

D.name

creates a comma-separated list.:

D(1).name, D(2).name, D(3).name, ...,D(end).name

However it does not create a cell array of strings! So when you use it as an input to regexp it is equivalent to lots of separate inputs:

regexp(D(1).name, D(2).name, ...,, D(end).name, ...)

as multiple inputs, exactly as the documentation describes. Those extra inputs are interpreted as optional arguments, but do not match any options and cause an error. If you want to have those strings in a cell array, then put your list into a cell array:

{D.name}

to make a cell array of strings. It is easy to check this behavior yourself:

>> S = struct('name',{'A','B','C'});
>> S.name % comma-separated list = separate variables!!
ans =
A
ans =
B
ans =
C
>> {S.name} % put them in a cell array
ans = 
    'A'    'B'    'C'