How to fix number of elements in for loop?

Hi,

I have the following for loop I'm trying to create and plot the resulting equation over the range -6<a<6. However the way it is currently set up, the numbe rof elements on LHS and RHS do not equal. Any suggestions?

q= 0.2;

E = 200*10^8;

a= -6:6;

n=numel(a);

v=zeros(1,n);

for k=1:n

v(k)= ((-2*q)/pi*E)*(2.*a.*log(a)

end

Answer 1

[GT]

There are couple of things here however the first is to get your code working:

q= 0.2;
E = 200*10^8;
a= -6:6;
n=numel(a);
v=zeros(1,n);
for k=1:n
    v(k,:)= ((-2*q)/pi*E)*(2.*a.*log(a))
end

Notice that you were missing a bracket at the end of log(a)) and that the answer is actually an array, so when attributing the values of an array to a single place MATLAB errors, hence the v(k,:) meaning for row k, fill out all the values.

That being said I am not sure if this is what you want as you are repeating the values in each row, the for loop really does not make much sense as is. One could simply do:

q= 0.2;
E = 200*10^8;
a= -6:6;
n=numel(a);
v=zeros(1,n);
v= ((-2*q)/pi*E)*(2.*a.*log(a))

And if you want to plot v, then a simple

plot(v) 

I hope that this helps.

[Star Strider]

To use the loop as I believe you intend, your need to index ‘a’ as ‘a(k)’:

for k=1:n
   v(k)= ((-2*q)/pi*E)*(2.*a(k).*log(a(k)));
end

GK’s second version (without the loop) is the most efficient.

[Kelsea Miller]

Yes the for loop is redundant. Thanks for the assistance!