How to import numbers or protected signs from .txt?

Question

I'm new here, so please apologize any formal errors.

I stumbled upon a somewhat simple problem. I want to import plain numbers or protected signs like Pi, Cos, Sin from .txt files. Somehow Mathematica does not understand that the read-in Pi is the protected Pi (= 3.14...). Even more simple examples seem not to work, e.g. reading in the number 5 from a .txt-file does not allow for using that 5 for further calculations.

My approach was:

1. Saving the sign/number in a txt file. E.g. Pi.txt contains just "Pi" (without quotation marks).
2. My Mathematica code is

SetDirectory["C:\\Code"];
x := Import["Pi.txt"]
f[t_] = Sin[t];
Evaluate[f[x]]

Thank you for your help.

Answer 1

The problem description sounds as if you're in fact trying to import something into Mathematica that is more appropriately classified as a .m format, i.e., a "Package". Let's say your text file contains the single entry

Pi

and its name is Pi.txt - then you could simply import it as follows:

variable = Import["Pi.txt", "Package"];

Now variable contains the value Pi. Moreover, your text file can be an entire Mathematica program such as

var1 = Sin[3];
var2 = 4;
var1 + var2

and its file name could be prog.txt - then in Mathematica you could do this:

Import["prog.txt", "Package"]

(* ==> 4 + Sin[3] *)

var1

(* ==> Sin[3] *)

The "Package" format is just a text file containing valid Mathematica expressions.

Things can be made even simpler if you adopt the standard extension for such Mathematica packages by calling the program file prog.m instead of prog.txt. Then the Import statement will be

Import["prog.m"]

and you're done.