Define a function with the condition that it must be positive

Question

I have a function f(x) and I want to define a new function g(x) that is equal to f(x) only when f(x) is positive.

What is the right way to do that?

Sorry for this silly question, but I cannot find the proper way to accomplish this simple task in Mathematica.

Maybe g[x_]:=Max[f[x],0] ? (Provided I correctly remember the syntax of Max). But what should be the value of your new function when f is not positive? Is the zero value important? — Peltio, Commented Nov 9, 2013 at 18:17
@Peltio it's reasonable, but I'd like that the function g(x) was undefined when f(x) was negative. — psmith, Commented Nov 9, 2013 at 18:27
@s.s.o In this way g(x) is defined only for positive x, instead I want that it is defined only when f(x) is positive. — psmith, Commented Nov 9, 2013 at 18:54

s.s.o · Accepted Answer · 2013-11-09 18:53:46Z

4

You can check the Mathematica documentation about Condition /;

here is a simple example :

f[x_] := x + 5;
g[x_] := f[x] /; f[x] > 0

g[5]

10

g[-9]

g[-9]

answered Nov 9, 2013 at 18:53

s.s.o

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$\begingroup$ @Szabolcs, in fact it was what I suggested as comment but it was not what OP asked for. $\endgroup$
– s.s.o
Commented Nov 10, 2013 at 19:39

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