Is the Zeroes Function here necessary?

Question

Andew 2018-2-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/383877-is-the-zeroes-function-here-necessary

评论： Stephen23 2018-2-21

采纳的回答： Greg

Here is the code:

nuclei=zeros(1,100);

t=nuclei;

nuclei(1)=input('Enter initial number of nuclei: ');

t(1)=0;

t_c=input('Enter time constant: ');

dt=input('Time step: ');

for i=1:99

nuclei(i+1)=nuclei(i)-(nuclei(i)/t_c)*dt;

t(i+1)=t(i)+dt;

end

plot(t,nuclei,'-ok','MarkerFaceColor','k')

xlabel('Number of Nuclei','FontWeight','bold')

ylabel('time(s)','FontWeight','bold')

title('Radioactive Decay Number of nuclei versus time')

str1=['Time Constant= ' num2str(t_c) ' s '];

str2=['Time Step= ' num2str(dt) ' s'];

text(t(50),nuclei(10),str1,'FontWeight','bold','FontSize',14)

text(t(50),nuclei(20),str2,'FontWeight','bold','FontSize',14)

I have plots that result from this code, and this code with that first line removed, and they appear to be the same. Is that first line necessary? If so, why?

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Stephen23 2018-2-21

No code is "necessary".

In this situation zeros makes that code much more efficient: array preallocation means that MATLAB does not waste time moving data around in memory:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html

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Answer 1

Greg 2018-2-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/383877-is-the-zeroes-function-here-necessary#answer_306221

It's called pre-allocation. It is always a good idea to pre-allocate. If you need a quick proof, throw a tic / toc around the code with and without the zeros line. It'll get noticeably faster as the number of loop iterations grows.