Soundex algorithm in Python (homework help request)

Question

The US census bureau uses a special encoding called “soundex” to locate information about a person. The soundex is an encoding of surnames (last names) based on the way a su

Answer 1

surname = input("Enter surname of the author: ") #asks user to input the author's surname

while surname != "": #initiates a while loop thats loops on as long as the input is not equal to an empty line

    str_ini = surname[0] #denotes the initial letter of the surname string
    mod_str1 = surname[1:] #denotes modified string excluding the first letter of the surname

    import re #importing re module to access the sub function
    mod_str2 = re.sub(r'[aeiouyhwAEIOUYHW]', '', mod_str1) #eliminating any instances of the given letters


    mod_str21 = re.sub(r'[bfpvBFPV]', '1', mod_str2)
    mod_str22 = re.sub(r'[cgjkqsxzCGJKQSXZ]', '2', mod_str21)
    mod_str23 = re.sub(r'[dtDT]', '3', mod_str22)
    mod_str24 = re.sub(r'[lL]', '4', mod_str23)
    mod_str25 = re.sub(r'[mnMN]', '5', mod_str24)
    mod_str26 = re.sub(r'[rR]', '6', mod_str25)
                #substituting given letters with specific numbers as required by the soundex algorithm

    mod_str3 = str_ini.upper()+mod_str26 #appending the surname initial with the remaining modified trunk

    import itertools #importing itertools module to access the groupby function
    mod_str4 = ''.join(char for char, rep in itertools.groupby(mod_str3))
                #grouping each character of the string into individual characters
                #removing sequences of identical numbers with a single number
                #joining the individually grouped characters into a string

    mod_str5 = (mod_str4[:4]) #setting character limit of the modified string upto the fourth place

    if len (mod_str5) == 1:
        print (mod_str5 + "000\n")
    elif len (mod_str5) == 2:
        print (mod_str5 + "00\n")
    elif len (mod_str5) == 3:
        print (mod_str5 + "0\n")
    else:
        print (mod_str5 + "\n")
                #using if, elif and else arguments for padding with trailing zeros

    print ("Press enter to exit") #specification for the interactor, to press enter (i.e., equivalent to a new line for breaking the while loop) when he wants to exit the program
    surname = input("Enter surname of the author: ") #asking next input from the user if he wants to carry on

exit(0) #exiting the program at the break of the while loop

Answer 2

This is hardly perfect (for instance, it produces the wrong result if the input doesn't start with a letter), and it doesn't implement the rules as independently-testable functions, so it's not really going to serve as an answer to the homework question. But this is how I'd implement it:

>>> def soundex_prepare(s):
        """Prepare string for Soundex encoding.

        Remove non-alpha characters (and the not-of-interest W/H/Y), 
        convert to upper case, and remove all runs of repeated letters."""
        p = re.compile("[^a-gi-vxz]", re.IGNORECASE)
        s = re.sub(p, "", s).upper()
        for c in set(s):
            s = re.sub(c + "{2,}", c, s)
        return s

>>> def soundex_encode(s):
        """Encode a name string using the Soundex algorithm."""
        result = s[0].upper()
        s = soundex_prepare(s[1:])
        letters = 'ABCDEFGIJKLMNOPQRSTUVXZ'
        codes   = '.123.12.22455.12623.122'
        d = dict(zip(letters, codes))
        prev_code=""
        for c in s:
            code = d[c]
            if code != "." and code != prev_code:
                result += code
         if len(result) >= 4: break
            prev_code = code
        return (result + "0000")[:4]

Answer 3

I would suggest you try the following.

• Store a CurrentCoded and LastCoded variable to work with before appended to your output
• Break down the system into useful functions, such as
  1. Boolean IsVowel(Char)
  2. Int Coded(Char)
  3. Boolean IsRule1(Char, Char)

Once you break it down nicely it should become easier to manage.