a function including a constraint condition

I have a function. for example f(x)=x^3+3*x^2+g(x) and g(x)=7*x+5

I have my own special algorithm in order to minimize the function f(x),so it takes lots of different x values in the algorithm loop.

I have a constraint condition: "g(x) must be always less than zero, if it is not less than zero,then do not evaluate the function f(x) until g(x) takes a negative value."

my question is: "how can i write this constraint condition with logical oparators or with an 'if-else' or 'for-else' or etc. in order not to evaluate f(x) in the loop if the constraint condition is not satisfied? so my special algorithm will search for new x values in the loop until satisfy the constraint condition."

thank you.

Answer 1

[KSSV]

g = @(x) 7*x+5 ;
f = @(x) x.^3+3*x.^2+g(x) ;
% g will be negative for these values of x 
x = linspace(-5/7,-10) ; % shoul dstart from -5/7 and end at inf

g = g(x) ;
f = f(x) ;

Answer 2

[Torsten]

if g(x) <= 0 
  fval = f(x);
end  % evaluate f only if g(x) <=0.

Best wishes

Torsten.