How to take precalculated variables in a function, without defining the variables within said function?

my code looks like this

   function D = down(A, B, C, n)
A = (dependent on other variables);
B = (dependent on other variables);
C = (dependent on other variables);
n=20;
D = sqrt(A)/2;
for i = 1:n
DA = (D-A/4./sqrt(B^2+(C+D)*(C+D)))/(1+(C+D)*A/4./(B^2+(C+D)*(C+D))^1.5);
D = D - DA;
end
           end

A, B, and C are calculated in the script prior to the function being defined, and therefore they depend on other variables. However, if I were to include every variable that A, B and C depend on within the function, then I would basically have to include my entire script as the variables that A depends on, depend on other variables themselves and so on.

I want to be able to take the values that are precalculated for A, B and C (before the function is active), into the function to calculate the new value for D. Whenever I try to include the equations for A to C, I'm met with the error of undefined variables. Is there a way to do this?

I can not set A etc. to absolute values either, as they will change as I progress through a loop of mission legs. I hope I've been clear, thanks!

[dpb]

That's a bizzare definition of D in such a loop, but if it is, indeed, what is intended (why the magic number 20 and not 19 or 21 or 10?(*)), then I think the answer to your dilemma is that you're trying to factor the code in the wrong place.

It seems this code just should be directly in line in the calling routine and not be a separate function, anyway.

If you did think it must be a function for some reason, then the way to share scope of the other variables is by making it a local function by including its source in the script m-file instead of a standalone m-file function.

Then A thru C will be accessible and don't need to be in the argument list at all while D will be local and returned.

(*) As written, you're overwriting the passed value of variable n inside the function so either need to remove it from the argument list or delete the line that redefines it locally.

[Jan]

Whenever I try to include the equations for A to C, I'm met with the error of undefined variables.

Please post the code and a copy of the complete error message. The current description does not uniquely clarify what you do and what the error is.

[DGM]

Assign values to A,B,C,etc in the calling function/script and then pass them as arguments to the function down(). Once inside the scope of down(), you don't need to redefine them again unless they are going to change.

Nothing in the function down() requires parameters A, B, or C to change within its scope. If they need to change in a loop, do that external to down().

% this is a simplistic example
% this is not a working demo
n=20;
rootest=zeros(1,10);
for f=1:10
	% you can recalculate parameters
	% inside the loop, without needing to 
	% do it inside down()
	A = sin(f);
	B = 1*f;
	C = f/2;

	rootest(f) = down(A,B,C,n);
end

[Stephen23]

This code defines a function with four input arguments:

function D = down(A, B, C, n)
A = (dependent on other variables);
B = (dependent on other variables);
C = (dependent on other variables);
n=20;

Those four input arguments are then ignored and immediately replaced with four unrelated variables.

In general input arguments are supposed to by used within the function. Instead, you ignore the input arguments and then appear to be asking how to pass all of the relevant variables required to recalculate those values...

Wouldn't it be much simpler to just refer to the actual input arguments, rather than completely ignoring them?

[Bob Wake]

%P
W = ALL*9.81;
DRAG = D100*F^2/100/100*DR;
T = sqrt(W^2+DRAG^2)*BL;
G = atan(DRAG/W);
A = T/(0.5*R*V^2*AR);
if(F~= 0);
    M    = F/V;
    B   = M*cos(G);
    C   = M*sin(G);
    down;
end
    
function D = down(A, B, C)
A = %how should i write this part?;
B = %how should i write this part?;
C = %how should i write this part?;
D = sqrt(A)/2;
for i = 1:20
    DA = (D-A/4./sqrt(B^2+(C+D)*(C+D)))/(1+(C+D)*A/4./(B^2+(C+D)*(C+D))^1.5);
    D = D - DA
end
end

I've included the script beforehand in the hope its easier to understand what I'm trying to say. So as you can see, A, B and C are defined in the script BEFORE the function. They all depend on other variables i.e. A = T/(0.5*R*V^2*AL). Variables R, V etc have all been defined earlier in the script and there is no problem here. I've not included them above because they are not needed to understand the problem.

As you can also see A to C are also required for D, in the function 'down'. I want the function 'down' to take the values of A, B and C into the function 'down' when it is called. However, I'm not sure how to do this?

I think my problem is occuring because I don't know how to write A to C within the function. If i try to write them in the same form they are defined in the script e.i. A = T/(0.5*R*V^2*AL) then I am met with the error 'undefined variable T'. I want to know what I should write for A, B and C within the function down, so that they take the precalculated values before the function is called? Thanks.

P.S. 'n' was not a magic number, rather just a number i defined for the number of iterations. I've removed this now as I've realised it was pointless and not needed, thanks dpb.

[Stephen23]

"I want to know what I should write for A, B and C within the function down, so that they take the precalculated values before the function is called?"

Much less than you are doing now. You are overthinking the situation.

"I think my problem is occuring because I don't know how to write A to C within the function."

No. There are two problems, neither of them are "how to write A to C within the function":

1. you are not calling the function with any inputs (although the intent is that it has three inputs).
2. you are overwriting (and hence discarding) all of the input arguments within the function.

function D = down(A, B, C)
A = % get rid of this line. This is hindering you.
B = % get rid of this line. This is hindering you.
C = % get rid of this line. This is hindering you.

Call the function with all of its required input arguments:

down(A,B,C)

How to call functions with input arguments is shown here:

https://www.mathworks.com/help/matlab/learn_matlab/calling-functions.html

[Bob Wake]

thanks guys, works now. sorry for the poor description.

Answer 1

[Jan]

You have defined the function down() with input arguments. Then use them:

A = T/(0.5*R*V^2*AR);
if(F~= 0);
    M    = F/V;
    B   = M*cos(G);
    C   = M*sin(G);
    down(A, B, C);
    %    ^  ^  ^
    ...
        
function D = down(A, B, C)
D = sqrt(A)/2;
for i = 1:20
    DA = (D - A / 4 ./ sqrt(B^2 + (C + D) * (C + D))) /...
         (1 + (C + D) * A / 4 ./ (B^2 + (C + D) * (C + D))^1.5);
    D = D - DA;
end
end

You do the same, what you call sqrt with an input argument:

    A = 17
    sqrt()   % Nope
    sqrt(A)  % Yes!    

This is a duplicate of the explanations DGM has give already.

[Bob Wake]

Wow! This was so easy! Thank you, works perfectly.

[Bob Wake]

Apologies for my explanation.

[Jan]

You are welcome. There is no reason for apologies, because questions about Matlab are the topic of this forum. Of course any problem is trivial as soon at it is solved :-)