Question

Say I have a function

a = b / c

and I ask the user to input two of these variables, either b and a or c and a and I want it to calculate the unknown variable without needing to write a function for every variable

In this case I would use:

pseudo-code
if input is a & b
then c = b / a
if input is a & c
then b = a * c
if input is b & c
then a = b / c

I know this is a function with only three variables so it is easy to put it in an if-statement and voilà BUT I want to apply it to a system containing lots of equations (a jet engine for example). I used TkSolver before and it was really great, you throw as many equations as you want at it (a jet engine was an example!), you only need to give a number of known quantities and in a few seconds, all the unknowns are calculated (even if I had a known quantity in one side of the equation and unknown on another side mixed with known ones, it will do the maths!)

So, is there a way to do this in MatLab or perhaps python which I’m learning?

---

Edit to the question, thanks for directing me to use the Symbolic toolbox, it’s great, I have another problem:

I couldn’t think of a way to let the program know which of the variables is entered. I can do this:

syms f a b c
f = a * c - b %(I want to write a = b / c)
c = 10; a = 5;
X = eval(solve(f,b))

What I want now is a way of letting the user enter two knowns (e.g c & a) and the code will recognise them and solve to the unknown variable (e.g b).

---

Edit 2: I managed to get what I want, it’s a bit long and there might be another way of achieving the same thing.

clear, clc
syms a b c
var = [{'a'}, {'b'}, {'c'}];
var1n = 0;
var2n = 0;

while isvarname(var1n) == 0
    var1n = input('Which variable is known: ','s');
    while ( any( strcmpi(var1n,var)  ) )== 0
        fprintf('\nThe variable entered is wrong, please enter a, b, or c')
        var1n = input('\nWhich variable is known: ', 's');
    end
end
fprintf('\nPlease enter the value of %s', var1n)
var1v = input(': ');
eval([var1n '=' num2str(var1v)]);

while isvarname(var2n) == 0
    var2n = input('What is the second known variable: ', 's');
    while ( any( strcmpi(var2n,var) ) ) == 0
        fprintf('\nThe variable entered is wrong, please enter a, b, or c')
        var2n = input('\nWhat is the second known variable: ', 's');
    end
end
fprintf('\nPlease enter the value of %s', var2n)
var2v = input(': ');
eval([var2n '=' num2str(var2v)]);


var3n = char (  var ( find( strcmpi(var1n, var) == strcmpi(var2n, var) ) )  );

var3v = solve(a - b / c);
eval([var3n '=' char(var3v)]);

Answer 1

You could use this: http://www.mathworks.de/help/toolbox/symbolic/solve.html but you have to have the symbolic math toolbox (:

EDIT: On the documentation page of solve there’s a sentence:

If the right side of an equation is 0, specify the left side as a
symbolic expression or a string:

That means, if you want to solve a = b/c for the value which is NOT set, simply rewrite the equation so that there is a zero on the right hand side:, i.e. a - b/c = 0, than you can use:

syms a b c
% E.g. assign values to a and c and automatically let matlab chose the 'free' variable (in this case, it's b:
a = input('Input var a: ')
a = input('Input var c: ')
solve(a - b/c)

which than gives you b (e.g. entering a = 10 and c = 40 gives you b = a * c = 400). Input function is explained here: http://www.mathworks.de/help/techdoc/ref/input.html! Hope that helps!