How is this possible to convert a long string to a number with less characters? I'm going to write a program (function) that can convert a long string to a number. For this, first I convert each character (letter) to a number; like a=0.01, b=0.02, c=0.03 ... . then for example I have:

abc  // I don't want to return 0.010.020.03 (because it is false in mathematic and the returned number length is more than the string length!), I want to have a certain number
  that just belong to this characters (abc) combination and be less than the length of string. For example for this return 54 (just for example)

For example I can combine (+) these numbers but there are many problems with this way, and no I will have problem with combinations. Like abc will be (0.01+0.02+0.03)=>0.06, but again bca, cba, bac,... (all combinations) will have same value (0.06)
Any suggestion or help about how is this possible? 

If the alphabet consists of $m$ characters, then there are $m^n$ possible strings of length $n$.  If you want to map each such string to a different nonnegative integer, then there are $m^n$ possible integers.  If $m > 10$, some of those integers will have length greater than $n$.
