# A problem related to a self-defined function

ABCDSeries[x0_] = Module[{x=x0, s1, s2, s3, str, tmp},
s1 = "R";
s2 = "C";
s3 = "L";
str = StringTake[SymbolName[t], 1];
tmp = Which[str=="R", x, str=="C", 1/(I*W*x), str=="L", (I*W*x)];
tmp
];

Code Above is used for generate the impedence for circuit analysis

There are three types of input: Resistor R, Capacitor C and Inductor L

And the output of function is R, 1/(I W C) and I W L, respectively.

The line str = StringTake[SymbolName[t], 1]; is used so as if we input Ls,

then the function will output I W Ls.

However, I type ABCDSeries[Ls] and receive x, not I W Ls.

I have no idea how to fix it. I think maybe the problem is related to local variable

How can I solve the problem, thanks for your help

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• Define your function with a := instead of a =. Also, when you define str it refers to some variable t - if you change that to x or x0 it works better. Commented Feb 22, 2016 at 10:36
• But one more thing, if Ls is defined to be a number instead of just a symbol, you will get an error. Commented Feb 22, 2016 at 10:38
• I've done some changes (j->I, w->W, ab ->a b) in your question.
– Kuba
Commented Feb 22, 2016 at 10:48

ClearAll[ABCDSeries];
ABCDSeries[x0_Symbol] := Module[{ str},
str = StringTake[SymbolName[x0], 1];
Switch[str,
"R", x0,
"C", 1/(I*W*x0),
"L", (I*W*x0)
]
];

ABCDSeries[Ls]
i Ls W

I put there x0_Symbol since SymbolName is not HoldFirst, if you need to have this function working for Ls where Ls may have a value, then SetAttributes HoldFirst to your function and change to SymbolName @ Unevaluated @ x0.

Search documentation for references to those things to understand what is going on. Moreover, what you need to read about:

• Note also that Kuba has removed unnecessary temporary variables and used Switch rather than Which. These are nice enhancements to your original code. Commented Feb 22, 2016 at 14:52