for loop with multiple inputs

So I know I've done this a million times but it's been a few years since I've used MATLAB and I forget exactly how to go about this. I want to graph two equations as a variable changes, but it doesn't seem to be working. Here's the full problem:

"Graph amplification and phase angle for 0 <= r <= 2.5."

Here's my code:

for r = 0:0.1:2.5
X = (f_0/omega_n^2)/(sqrt((1-r^2)^2+(2*zeta*r)^2)); %Amplification Equation
phi = atan2((2*zeta*r),(1-r^2)); %Phase angle equation
end

However this is giving me a single value for both phase angle and amplification. (All the variables are correctly defined) How do I get it to spit out a matrix with every value for amplification and phase angle between 0 and 2.5? Do I need an if statement?

Thanks in advance.

Answer 1

[Guillaume]

If you use a for loop, you need to index X and phi otherwise you will of course overwrite the previous result. So you could do something like:

r = 0:0.1:2.5;
X = zeros(size(all_r));
phi = zeros(size(all_r));
for idx = 1:numel(r)
  X(idx) = (f_0/omega_n^2)/(sqrt((1-r(idx)^2)^2+(2*zeta*r(idx))^2)); %Amplification Equation
  phi(idx) = atan2((2*zeta*r(idx)),(1-r(idx)^2)); %Phase angle equation
end

But of course, matlab can operate on whole vectors at once so a loop is not needed at all:

r = 0:0.1:2.5;
X = (f_0/omega_n^2)./(sqrt((1-r.^2).^2+(2*zeta*r).^2)); %Amplification Equation
phi = atan2((2*zeta*r),(1-r.^2)); %Phase angle equation

Note the replacement of / by ./ and ^ by .^ for array operation