Error in loop: left and right sides have a different number of elements

I wanna make a loop out of the following lines:

dt=1;
a0= 0.01*sqrt(d/D)*Fr/D*u*dt; 
a1=0;
a2=7.07;
a3=4.45/tan(phi);
a4=0.786/(tan(phi)*tan(phi));
p=[a4 a3 a2 a1 a0];
Zvs1=real(roots(p))*D

dt=2;
a0= 0.01*sqrt(d/D)*Fr/D*u*dt;
a1=0;
a2=7.07;
a3=4.45/tan(phi);
a4=0.786/(tan(phi)*tan(phi));
p=[a4 a3 a2 a1 a0];
Zvs2=real(roots(p))*D

dt=3;
a0= 0.01*sqrt(d/D)*Fr/D*u*dt;
a1=0;
a2=7.07;
a3=4.45/tan(phi);
a4=0.786/(tan(phi)*tan(phi));
p=[a4 a3 a2 a1 a0];
Zvs3=real(roots(p))*D

dt=4;
a0= 0.01*sqrt(d/D)*Fr/D*u*dt; 
a1=0;
a2=7.07;
a3=4.45/tan(phi);
a4=0.786/(tan(phi)*tan(phi));
p=[a4 a3 a2 a1 a0];
Zvs4=real(roots(p))*D

For every Zvs i get for solutions (Zvs (4,1)). I tried like this:

dt = [1:1:4];
Zvs = NaN(length(dt),4);
for i=1:length(dt)
a0= 0.01*sqrt(d/D)*Fr/D*u*dt;
a1=0;
a2=7.07;
a3=4.45/tan(phi);
a4=0.786/(tan(phi)*tan(phi));
p=[a4 a3 a2 a1 a0];
Zvs (i)=real(roots(p))*D
end

I get this Error:

Unable to perform assignment because the left and right sides have a
different number of elements.

Error in Skript (line 26)
Zvs (i)=real(roots(p))*D

I know that the problem ist the different size from Zvs (i), its 1x1 and not 1x4. But i don't know how to fix it.

Answer 1

[RAJA SEKHAR BATTU]

Hi @Miriam

You can pre assign the variables to empty or zeros before loop to save the values efficiently. I will make some modifications please check if it works.

a0, p and Zvs variables are changing every iteration

dt = [1:1:4];

Zvs = zeros(4,length(dt));

a0 = zeros(1,length(dt));

p = zeros(length(dt),5);

for i=1:length(dt)

a0(i)= 0.01*sqrt(d/D)*Fr/D*u*dt(i);

a1=0;

a2=7.07;

a3=4.45/tan(phi);

a4=0.786/(tan(phi)*tan(phi));

p(i,:)=[a4 a3 a2 a1 a0(i)];

Zvs(:,i)=real(roots(p(i,:)))*D;

end

[Miriam]

i had to change it to this than it worked:

dt = [1:1:4];
Zvs = zeros(1,length(dt));
a0 = zeros(1,length(dt));
p = zeros(5,length(dt));
for i=1:length(dt)
a0(i)= 0.01*sqrt(d/D)*Fr/D*u*dt(i);
a1=0;
a2=7.07;
a3=4.45/tan(phi);
a4=0.786/(tan(phi)*tan(phi));

p=[a4 a3 a2 a1 a0(i)];
Zvs(i, :)=real(roots(p))*D

end

but just for dt=[1:1:4] if i change dt to [1:1:6] (or something else) it doesn't work anymore. I get the same Error again. What can i do?

[VBBV]

Zvs = zeros(4,length(dt)); %preallocate this size
Zvs(:,i)=real(roots(p))*D; % make these change , i have updated my answer

[RAJA SEKHAR BATTU]

Hi @Miriam,

The matrices of pre allocation should match the matrices inside the loop.

Please check the size of the matrices. This is the point of pre allocation.

Please check and modify your code accordingly.

[Miriam]

I got it, now it works. Thanks!