decode-encode-rot.py

# ROT Encoder/Decoder now with input validation.
import sys

# Sets alphabet characters for comparison.

alphabet = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", 
    "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

# Performs ROTation based on user input.
    
def rotCalc(userStr):
    output = ""
    for i in range(len(userStr)):
    
        # Handles whitespace.
    
        if userStr[i] == " ":
            output = output + " "
            
        # Handles digits - in default ROT encoding these do not rotate.
        
        elif userStr[i].isdigit():
            output = output + userStr[i]
            
        # Handles alphabetical characters.
        
        elif userStr[i].isalpha():
            for j in range(len(alphabet)):
            
                # Handles uppercase characters.
                
                if userStr[i].isupper():
                    if userStr[i].lower() == alphabet[j]:
                        if encOrDec == 1:
                            if j + rotNum > 25:
                                    diff = (j + rotNum) - 25
                                    output = output + (alphabet[diff - 1].upper())
                            else:
                                output = output + (alphabet[j + rotNum].upper())
                        elif encOrDec == 2:
                            if j - rotNum < 0:
                                diff = (j - rotNum) + 25
                                output = output + (alphabet[diff + 1].upper())
                            else:
                                output = output + (alphabet[j - rotNum].upper())

                # Handles lowercase characters.
                                
                elif userStr[i] == alphabet[j]:
                    if encOrDec == 1:
                        if j + rotNum > 25:
                            diff = (j + rotNum) - 25
                            output = output + (alphabet[diff - 1])
                        else:
                            output = output + (alphabet[j + rotNum])
                    elif encOrDec == 2:
                        if j - rotNum < 0:
                            diff = (j - rotNum) + 25
                            output = output + (alphabet[diff + 1])
                        else:
                            output = output + (alphabet[j - rotNum])
                            
        # Handles anything else (special characters like !@#$, etc).
        
        else:
            output = output + userStr[i]
    return output   
    
# Main program loop requests encode/decode, ROTation number, and user string.  Then sends to rotCalc function.
# The main program implements input validation ensuring legal input from the user; quits on illegal input.
    
while True:
    encOrDec = raw_input("Enter 1 to ENCODE, 2 to DECODE, anything else to quit: ")
    userStr = raw_input("Enter your string: ")
    if encOrDec.isdigit() == False:
        print "Goodbye!"
        sys.exit()
    else:
        encOrDec = int(encOrDec)
        if encOrDec == 1 or encOrDec == 2:
            for rotNum in range(1,25):
                rotNum = int(rotNum)
                if rotNum >= 1 and rotNum < 26:                    
                    # Prints final encoded/decoded string
                   
                    if encOrDec == 1:
                        print userStr, ("ROT-" + str(rotNum)), "encoded:", rotCalc(userStr)
                    elif encOrDec == 2:
                        print userStr, ("ROT-" + str(rotNum)), "decoded:", rotCalc(userStr)
                else:
                    print "Goodbye!"
                    sys.exit()          
        else:
            print "Goodbye!"
            sys.exit()