CODEWITHSUNDEEP
matlabsymbolic-math
ZK Zhao
ZK Zhao

Reputation: 21573

Matlab: Solving equation with different input variables

The equation I want to solve is 

syms w v;
rho_air = 1.25;
equ = w == 0.5 * rho_air * v^2

The problem is that sometimes, I want to solve w from v, an sometimes from v to w.

How can I do it?

I only know to do it like this(This is now not working, and I don't know why):

syms v;
rho_air = 1.25;
w = 1;
equ = w == 0.5 * rho_air * v^2;

But then I have to change it to solve v, like

syms w;
rho_air = 1.25;
v = 1;
equ = w == 0.5 * rho_air * v^2;

which is quite repetative. Is there anyway to solve it more elegantly?

Upvotes: 1

Views: 439

Answers (2)

Luis Mendo
Luis Mendo

Reputation: 112759

In old Matlab versions you need to define the equation as a string; then you can apply solve:

>> syms w v;
rho_air = 1.25;
>> equ = 'w = 0.5 * rho_air * v^2';

>> solve(equ, w)
ans =
0.5*rho_air*v^2

>> solve(equ, v)
ans =
  (2^(1/2)*w^(1/2))/rho_air^(1/2)
 -(2^(1/2)*w^(1/2))/rho_air^(1/2)

Upvotes: 1

Benoit_11
Benoit_11

Reputation: 13945

Here is a way using a function, in which the inputs are the variable to solve for (as a string) and rho_air. It's quite self-explanatory. And I don't know why your above code does not work; you are just missing the call to solve and you're good to go :)

function [ out ] = SolveFor(Var2Solve,rho_air)

if strcmp(Var2Solve,'v')

    syms Var2Solve
    v = Var2Solve

    w = 1;
    equ = w == 0.5 * rho_air * v^2;
    out = solve(equ,v)

elseif strcmp(Var2Solve,'w')

    syms Var2Solve
    w = Var2Solve

    v = 1;
    equ = w == 0.5 * rho_air * v^2;
    out = solve(equ,w)

end


end

TESTS (in command window):

SolveSym('v',1.25)

out =

  (2*10^(1/2))/5
 -(2*10^(1/2))/5

and

SolveSym('w',1.25)

out =

5/8

Upvotes: 1

Related Questions