Consider preallocating for speed

Question

Hüseyin Uzun 2021-6-21

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/861435-consider-preallocating-for-speed

评论： Hüseyin Uzun 2021-6-21

Matlab tells me in line 11 to consider preallocating, but when I try to do it with line 7, matlab is still not happy with it. The value "Werte" is (1,16) in line 8 and in line 11 I add +1. I searched the forums but couldn't find a solution. What can i do?

while true
  tic
  data=importdata('C:\Users\Hüs\Documents\MATLAB\Datalogger\KlimaLoggProAuto.dat1');
  alle=[data(8,1);data(11,1);data(21,1);data(24,1);data(34,1);data(37,1);data(47,1);data(50,1);data(60,1);data(63,1);data(73,1);data(76,1);data(86,1);data(89,1);data(99,1);data(102,1)];
  string(alle);
  a=split(alle,'"');
  Werte=zeros(1,17); %preallocated value....might be unused.
  Werte=a(:,2)';
  Zeit=datetime('now');
 Zeit=string(Zeit);
 Werte=[Zeit Werte]; %Consider preallocating for speed.
 Werte=cellstr(Werte);
 [succes,message]=xlsappend('Datalogger.xlsx',Werte,1);
 pause(600-toc)
end

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Answer 1

Bjorn Gustavsson 2021-6-21

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https://ww2.mathworks.cn/matlabcentral/answers/861435-consider-preallocating-for-speed#answer_729770

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In your code you change the values and type of Werte 4 times. That seems a bit excessive. I'd try something like this:

while true
  tic
  data = importdata('C:\Users\Hüs\Documents\MATLAB\Datalogger\KlimaLoggProAuto.dat1');
  alle = [data(8,1);data(11,1);data(21,1);data(24,1);data(34,1);data(37,1);data(47,1);data(50,1);data(60,1);data(63,1);data(73,1);data(76,1);data(86,1);data(89,1);data(99,1);data(102,1)];
  string(alle);
  a = split(alle,'"');
  % Overwritten on the very next line: Werte=zeros(1,17); %preallocated value....might be unused.
  Werte = a(:,2)';
  Zeit = datetime('now');
 Zeit = string(Zeit);
 % Seems skippable: Werte=[Zeit Werte]; %Consider preallocating for speed.
 ZeitWerte_string = cellstr([Zeit,Werte]);
 [succes,message] = xlsappend('Datalogger.xlsx',ZeitWerte_string,1);
 pause(600-toc)
end