Error in fzero for solving numerical inverse function

Hello

I am trying to find the inverse of an function, g, numerically, as the explicit form of it is complex. Here I wrote the inverse function by solving through the fzero command, however, I don't know why it is giving me this error:

Error using fzero (line 328)
Function value at starting guess must be finite and real.

I really appreciate your help,

Thank you

syms x
g = 1.02*x + 0.6*exp(-8.7*x)-0.292

gx = @(x) g

gval = 0.1 % or 0.2 or...0.6
guess = 0.5
inv_fun = @(x) (gx(x) - gval)
S = fzero (@(x) inv_fun(x) , guess)

Answer 1

[Benjamin]

I think I was able to figure it out. it seems fzero can only solve functions in the form of function handle, which matlabFunction can be used for the functions defined as symbolic

syms x
g = 1.02*x + 0.6*exp(-8.7*x)-0.292
gx = matlabFunction(g)
gval = 0.1 % or 0.2 or...0.6
guess = 0.5
inv_fun = @(x) (gx(x) - gval)
S = fzero (@(x) inv_fun(x) , guess)

Answer 2

[James Tursa]

These lines don't do what you think they do

syms x
g = 1.02*x + 0.6*exp(-8.7*x)-0.292
gx = @(x) g

The x in g is symbolic. The x in gx is a dummy input argument that is ignored because it isn't used in g. That is, the x dummy input argument in gx is different from the symbolic x in g.

It looks like you really want just a simple function handle:

gx = @(x) 1.02*x + 0.6*exp(-8.7*x)-0.292

E.g.,

>> gx = @(x) 1.02*x + 0.6*exp(-8.7*x)-0.292
gx =
  function_handle with value:
    @(x)1.02*x+0.6*exp(-8.7*x)-0.292
>> gval = 0.1;
>> guess = 0.5;
>> inv_fun = @(x) (gx(x) - gval)
inv_fun =
  function_handle with value:
    @(x)(gx(x)-gval)
>> S = fzero (@(x) inv_fun(x) , guess)
S =
    0.3583
>> inv_fun(S)
ans =
  -2.7756e-17
>> gx(S)
ans =
    0.1000

If you insist on starting with a symbolic expression then you need to convert it to a function handle properly. E.g.,

>> gx = matlabFunction(g)
gx =
  function_handle with value:
    @(x)x.*(5.1e+1./5.0e+1)+exp(x.*(-8.7e+1./1.0e+1)).*(3.0./5.0)-7.3e+1./2.5e+2

Answer 3

[Walter Roberson]

gx = matlabFunction(g);

or change your solving method

syms gval x
g = 1.02*x + 0.6*exp(-8.7*x)-0.292
inv_formula = solve(g == gval, x);
inv_fun = matlabFunction(inv_formula);
inv_fun(0.1)