Pre Allocating for Speed

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Nathan McClean
Nathan McClean on 4 Apr 2019
Hi,
My Matlab code comes up with an error regaring pre-allocating for speed. Would anybody be able to help me preallocate the following:
An example would be nice.
Thanks
%% Newmarks Method
a1=(1/(beta*dt^2))+((gamma/(beta*dt))*2*zeta*w_n);
a2=(1/(beta*dt))+(((gamma/beta)-1)*2*zeta*w_n);
a3=((1/(2*beta))-1)+((dt*2*zeta*w_n)*((gamma/(2*beta))-1));
wn_2=(w_n)^2+a1;
u_t(1)=0;
dudt(1)=0;
d2udt2(1)=0;
for n=1:length(Acc_g)-1
p_hat(n+1)=(Acc_g(n+1))+(a1*u_t(n))+(a2*dudt(n))+(a3*d2udt2(n));
u_t(n+1)=p_hat(n+1)/wn_2;
dudt(n+1)=((gamma/(beta*dt))*(u_t(n+1)-u_t(n)))+((1-(gamma/beta))*dudt(n))+(dt*(1-(gamma/(2*beta)))*d2udt2(n));
d2udt2(n+1)=((1/(beta*dt^2)))*(u_t(n+1)-u_t(n))-((1/(beta*dt))*dudt(n))-(((1/(2*beta))-1)*d2udt2(n));
end
  2 Comments
John D'Errico
John D'Errico on 4 Apr 2019
+1 for a pretty accurate answer by Adam.

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Answers (1)

Andreas Bernatzky
Andreas Bernatzky on 4 Apr 2019
Hey Nathan,
if a vector is not preallocated it means that it grows in every Iteration.
This means you have to expand your vector, shift all the entries and shrink it back. Long Story short unnecessary memory usage. Here a small example:
%% Vector without preallocating
loops=20;
Vector1=[];
for(a=1:1:loops)
Vector1(end+1)=a;
end
%% Vector with preallocating
loops=20;
Vector2=ones(1,loops);
for(a=1:1:loops)
Vector2(a)=a;
end
Especially for your example preallocate your vector with ones or zero (depends on the application).
In your example i did it for p_hat.
%% Newmarks Method
a1=(1/(beta*dt^2))+((gamma/(beta*dt))*2*zeta*w_n);
a2=(1/(beta*dt))+(((gamma/beta)-1)*2*zeta*w_n);
a3=((1/(2*beta))-1)+((dt*2*zeta*w_n)*((gamma/(2*beta))-1));
wn_2=(w_n)^2+a1;
u_t(1)=0;
dudt(1)=0;
d2udt2(1)=0;
p_hat=ones(1,length(Acc_g)-1);
for n=1:length(Acc_g)-1
p_hat(n)=(Acc_g(n+1))+(a1*u_t(n))+(a2*dudt(n))+(a3*d2udt2(n));
u_t(n+1)=p_hat(n+1)/wn_2;
dudt(n+1)=((gamma/(beta*dt))*(u_t(n+1)-u_t(n)))+((1-(gamma/beta))*dudt(n))+(dt*(1-(gamma/(2*beta)))*d2udt2(n));
d2udt2(n+1)=((1/(beta*dt^2)))*(u_t(n+1)-u_t(n))-((1/(beta*dt))*dudt(n))-(((1/(2*beta))-1)*d2udt2(n));
end
Hope I could help.

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