Output matrix for simple function

I'm brand new to Matlab, and have created a very simple function here to calculate an ion's equilibrium potential:

function y = equipotent(n,X1,X2)

y = (58/n) * log10(X1/X2);

I'd like to do two things: 1) vary X2 for a set of values (1-100) while keeping X1 and n constant. and 2) store all the outputs from the function in a vector for plotting X2 vs y.

Anything helps! Thanks!

Answer 1

[the cyclist]

If you change your code to

function y = equipotent(n,X1,X2)

y = (58/n) * log10(X1./X2);

then it will give a vector output y for vector input X2, and you should be all set.

Answer 2

[Sven]

Hi Derek,

MATLAB has some useful ways to do what you're trying to do. If you use the (.*) operator instead of (*), it will perform a vector multiplication.

Therefore you can adjust your function as follows:

function y = equipotent(n,X1,X2)

y = (58 ./ n) .* log10(X1 ./ X2);

Then you can just run commands:

y = equipotent(4, 3, X2)
figure
plot(X2,y)

Did that help you out?

[Derek]

I changed the code and each time I run the function, the new output overrides the previous. Any ideas on how to record each output and store each one in the same matrix?

[Sven]

Ah, this sounds like the following code would help you:

n = 4;

X2 = (1:100)'; % Let's make it a vertical (100-by-1) vector
X1_set = 1:5; % Let's use 5 different values for X1
% So the result(s) can be collected in a 100-by-5 matrix
y_set = zeros(length(X2), length(X1_set));
for i = 1:length(X1_set)
    y_set(:,i) = equipotent(n, X1_set(i), X2);
end
% And plot them
figure
plot(X2,y_set)
legend(num2str(X1_set'))

I'll update my answer accordingly. Is this what you were looking for?

Note that in your original question:

2) store all the outputs from the function in a vector for plotting X2 vs y

You only wanted to plot X2 (which is a 100-length vector) against y. In the original answer, putting the (.*) operator in the function lets you calculate y all-at-once so that y is also a 100-length vector. There should have been no need to run your function more than once.

In my comment above I've also showed how to store sets of y for different input via a loop. This is the only time where you'd need to run your function more than once (and therefore need to avoid overwriting your previous answer by putting y into columns of the y_set matrix.

[Evangelia Lo]

I have one similar question ... I want ti make a function which will take p, v, d and D as input, and will calculate the pressure drop register (Dp), for d / D from 1 to d / D (derived from the input data), and in steps of (1/100 ) * (d / D)... How can i do it ?

[Evangelia Lo]

The type of function is