How to preallocate memory for this program?

Question

I am trying to preallocate memory for the below program to increase the speed of computation.

global k1 k2 k3 k4 k5
k1=50;
k2=2.0e+03;
k3=5e+01;
k4=.0001;
k5=200;
dt=0.0001;
x(1)=0.9999;
y(1)=0.38462;
z(1)=0.1995;
t(1)=0.0;
for i=1:100000
    t(i+1)=t(i)+dt;
    x(i+1)=x(i)+dt*(-k5*(k1+k3)*x(i)+ k3*x(i)*z(i) + k4*z(i) + k2*y(i) );
    y(i+1)=y(i)+dt*(k1*x(i) - k2*y(i));
    z(i+1)=z(i)+dt*(-k3*x(i)*z(i) - k4*z(i) + k3*k5);
end
%plot(t,x,'b');
%plot(t,y,'g');
plot(t,z,'r');
%plot3(x,y,z);
%legend('x','y','z');
%hold on;
grid on;
%axis([0 10 0 10 0 10]);
%end

Kindly help. Thanks in advance.

Shai has already answered the question, so I'll just ask you something: Why do you use global variables? — Stewie Griffin, Nov 19 '14 at 07:26

score 2 · Answer 1 · edited May 23 '17 at 12:12

2

Since you know you'll have 100000 + 1 entries in each array, it is easy to pre-alloc

n = 100001;
x = zeros(n,1); %//pre-allocation
y = zeros(n,1);
z = zeros(n,1);
t = zeros(n,1);
x(1)=0.9999; %//setting init-values
y(1)=0.38462;
z(1)=0.1995;
t(1)=0.0;
for ii=1:( n - 1 )
    t(ii+1)=t(ii)+dt;
    x(ii+1)=x(ii)+dt*(-k5*(k1+k3)*x(ii)+ k3*x(ii)*z(i) + k4*z(ii) + k2*y(ii) );
    y(ii+1)=y(ii)+dt*(k1*x(ii) - k2*y(ii));
    z(ii+1)=z(ii)+dt*(-k3*x(ii)*z(ii) - k4*z(ii) + k3*k5);
end

PS,
It is best not to use i as variable name in Matlab.

edited May 23 '17 at 12:12

Community

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answered Nov 19 '14 at 07:23

Shai

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I suggest you change the loop to `for ii=1:n-1`, so the pre-allocation will always fit the number of iterations. – hbaderts Nov 19 '14 at 09:53

How to preallocate memory for this program?

1 Answers1