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I have a function defined as

a[x_]:=If[x==0,const,f[x]]

where $f(0)\neq const$.

How do I evaluate $a(x)$ such that I just get $f(x)$ without having to put in a specific x value greater than 0? I've tried things like $/;$, Assumptions and Assuming, but haven't been able to do it. This seems like it should be a simple thing, any ideas?

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    $\begingroup$ This answer sheds some light on why this might not be as simply as it first seems. $\endgroup$ – Quantum_Oli Jun 8 '17 at 16:00
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You can use TrueQ:

a[x_] := If[TrueQ[x == 0], const, f[x]]

Or the fourth parameter of If:

a[x_] := If[x == 0, const, f[x], f[x]]

Or formulate your function in two definitions:

a[x_] /; x == 0 = const;
a[x_] := f[x]

Recommended reading:

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  • $\begingroup$ Or If[x === 0, ...] which is what I usually do. $\endgroup$ – Carl Woll Jun 8 '17 at 16:13
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    $\begingroup$ @CarlWoll ...and what I expressly warn about in the final link in my answer ;^) $\endgroup$ – Mr.Wizard♦ Jun 8 '17 at 16:14

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