Using an array as an element in a function.

Hello,

I am having trouble getting an array and a function to co-operate. I am trying to setup array "V=[1:4]" and have the equations run for all values of %error. Without an array my at_volume is returning as 1 always. Below is my goal, code, and error.

code to call (that ive been using to test) IdealGasLaw(1, 300, [1:4])

% Define your function file here

function [p_Ideal, p_Waals] = IdealGasLaw(n, T, V);

R = 0.08206;

a = 1.39;

b = 0.0391;

V = [1:4]

p_Ideal = (n*R*T) / V;

p_Waals = ((n*R*T)/(V-n*b)) - ((n^2)*a)/V^2;

error = (p_Ideal - p_Waals) / p_Waals;

max_error = max(error)

[max_error, idx_of_max] = max(error)

Answer 1

[Stephen23]

Use element-wise division ./ not matrix division /

You will probably also need to use element-wise power .^, not matrix power ^

https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

Learning the difference between matrix operations (which you were incorrectly using) and array operations (which you should be using) is critical for writing MATLAB code.

Note that square brackets around one vector/matrix/scalar/array do nothing, get rid of them.

IdealGasLaw(1, 300, 1:4) % got rid of those pointless square backets

ans = 1×4

24.6180 12.3090 8.2060 6.1545

% and here we use array operations, not matrix operations.
function [p_Ideal, p_Waals] = IdealGasLaw(n, T, V)
R = 0.08206;
a = 1.39;
b = 0.0391;
p_Ideal = (n*R*T) ./ V;
p_Waals = ((n*R*T)./(V-n*b)) - ((n^2)*a)./V.^2;
end

Tip: if you are not doing linear algebra, you probably should be using array operations.

[Michael]

Thank you! That was perfectly explained. Follow up. I got the code to run and ID the location of the volume that max error occurs. Now im trying to assemble the matrix and I am not receiving error in the code but the grading system is saying my resulting matrix is the wrong size (and all of my variable values are incorrect to boot). I tried doing a 1X4 thinking that it had to have the same number of rows as V, which is one (from screenshot in original post). I populated that with the row corresponding to max error. Am I missing something glaringly obvious as to what the dimensions and values should be? I even tried a 4x4 after seeing that it stated "corresponding rows" using this for output.

output = [V(1:4)', p_Ideal(1:4)', p_Waals(1:4)', error(1:4)']

IdealGasLaw(1, 300, 1:4)

% and here we use array operations, not matrix operations.

function [p_Ideal, p_Waals] = IdealGas(n, T, V)

R = 0.08206;

a = 1.39;

b = 0.0391;

p_Ideal = (n*R*T) ./ V; %ideal gas over array V

p_Waals = ((n*R*T)./(V-n*b)) - ((n^2)*a)./V.^2; %waals over array V

error = (p_Ideal - p_Waals)./(p_Waals); % percent error over array V

max_error = max(error); %maximum error value

[max_error, idx_of_max] = max(error); %location of maximum error value

at_volume = V(idx_of_max); %volume corresponding to max error

out = [V(1), p_Ideal(1), p_Waals(1), error(1)] %output array with same number of rows as V

end

[Stephen23]

The assignment specifies that the function should have three outputs, but your function only returns two outputs (and those are not even in the right output positions).You need to provide exactly the outputs requested by the assignment, perhaps something like this:

[out, mx_err, at_vol] = IdealGasLaw(1, 300, 1:4)

out = 4×4

1.0000 24.6180 24.2297 0.0160 2.0000 12.3090 12.2069 0.0084 3.0000 8.2060 8.1599 0.0056 4.0000 6.1545 6.1284 0.0043

mx_err = 0.0160

at_vol = 1

function [output, max_error, at_volume] = IdealGasLaw(n, T, V)
R = 0.08206;
a = 1.39;
b = 0.0391;
p_Ideal = (n*R*T) ./ V; %ideal gas over array V
p_Waals = ((n*R*T)./(V-n*b)) - ((n^2)*a)./V.^2; %waals over array V
%
error = (p_Ideal - p_Waals)./(p_Waals); %  percent error over array V
output = [V(:), p_Ideal(:), p_Waals(:), error(:)]; %output array with same number of rows as V
[max_error, idx_of_max] = max(error); %location of maximum error value
at_volume = V(idx_of_max); %volume corresponding to max error
end