Is there a method of significant preallocation beyond zeros?

Question

Kelly Catlin 2018-3-24

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/390455-is-there-a-method-of-significant-preallocation-beyond-zeros

评论： John D'Errico 2018-3-24

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

I have been calling a function with 83000 iterations (as below):

>>[t1data,t2data,Tdata,Ddata,lamdata] = example_run(2,0.1,25,1.01)

Yet I have let it run for the better part of a day without the program terminating. Is there a way (beyond preallocation with zeros, as I have already done) to decrease the run time significantly?

function [t1data,t2data,Tdata,Ddata,lamdata] = example_run(E0,sigma,F,tau)
%E0=2;sigma=0.1;F=25;tau=1.01; standard reference
%preallocation
Tdata = zeros(1, 100000);
t1data = zeros(1, 100000);
t2data = zeros(1, 100000);
kdata = zeros(1, 100000);
Ddata = zeros(1, 100000);
lamdata = zeros(1, 100000);
[Tc,Ts]=findTcTs(E0,sigma,F,tau);
%T=Ts+0.5;
%[t1,t2,lambda,D]=findt1t2lambdaD(E0,sigma,F,tau,T,Tc);
hold on;
T = Ts;
T0 = Tc;
for k = 1:83001
    [t1,t2,lambda,D] = findt1t2lambdaD(E0,sigma,F,tau,T,T0);
    Tdata = [Tdata,T];
    t1data = [t1data,t1];
    t2data = [t2data,t2];
    kdata = [kdata,k];
    Ddata = [Ddata,D];
    lamdata = [lamdata, lambda];
    T0 = t2;
    T = T + 0.1;
    plot(T,real(D), 'o')
    %iterate by increments to use valid initial guess for t2 for each increment in findt1t2...
end
hold off;

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Answer 1

Walter Roberson 2018-3-24

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/390455-is-there-a-method-of-significant-preallocation-beyond-zeros#answer_311747

在 MATLAB Online 中打开

Tdata = [Tdata,T];

Don't do that. You already used

Tdata = zeros(1, 100000);

so each iteration you are expanding Tdata by putting a single new element after all of the zeros. That is not preallocation! Preallocation would be if you had used

Tdata = zeros(1, 100000);

with

Tdata(k) = T;

(It is not obvious why your 100000 does not match your 83001)