Defining a new command using a conditional

Question

I want to define a new command (to be used only in mathmode), the execution of which depends on the input given to it. Particularly, I want to define F as
newcommand{F}[1]{F#1}

if the first character in its input is ( , and as
newcommand{F}[1]{F(#1)}

if the first character in its input is not ( .
As an example, I would like F{x} to output F(x) and F{(x)} to output F(x).
I have no idea how to go about this.

Steven B. Segletes · Answer

No packages required.
documentclass{article}
newcommandF[1]{Faux#1endFaux}
makeatletter
newcommandFaux{@ifnextchar(FnoaddFadd}
makeatother
defFadd#1endFaux{F(#1)}
defFnoadd#1endFaux{F#1}
begin{document}
$F{(x)}$ $F{(G(x))}$

$F{x}$ $F{G(x)}$ 
end{document}

G. de Man · Answer

You can achieve this using the following code:
usepackage{xstring}
usepackage{ifthen}
newcommand{F}[1]{
    StrLeft{#1}{1}[firstletter]%
    StrRight{#1}{1}[lastletter]%
    ifthenelse{equal{(}{firstletter} AND equal{)}{lastletter}}{F#1}{F(#1)}%
}

It basically checks what the first and last characters of the argument are, and if those are '(' and ')', it will output the argument without extra parentheses.

Defining a new command using a conditional

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