String index not working? - Python

Question

I'm Making a sort of cipher that requires a string to be read backwards and i get an indexing error even though the index I reference is well within range:

M = str(input("Input Message: "))
M = M.upper()
L = len(M)
A = ["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"]

def DECRYPT():
    global L
    global M
    global A

    if L%2 != 0:
        POS = False
    else:
        POS = True

    i = L-1
    NM = ""
    while 1:
        if M[i] != " ":
            INDEX = A.index(M[i])
            if POS == True:
                INDEX += (i + 1)
            else:
                INDEX -= (i)
            INDEX %= 26        
            NM = NM+A[INDEX]
            i += 1
            if POS == True:
                POS = False
            else:
                POS = True
    print("\n"+NM)

def ENCRYPT():
    global L
    global A
    global M

    POS = True
    M = M[::-1]
    i = 0
    NM = ""

    while 1:
        if i == L:
            break
        if M[i] != " ":
            INDEX = A.index(M[i])
            if POS == True:
                INDEX += (i + 1)
            else:
                INDEX -= (i + 1)
            INDEX %= 26        
            NM = NM+A[INDEX]
        i += 1
        if POS == True:
            POS = False
        else:
            POS = True
    print("\n"+NM)

while 1:
    C = int(input("\nWhat do you want to do:\n1) Encrypt Something\n2)Decrypt Something\n\n"))
    if C == 1:
            ENCRYPT()
    if C == 2:
            DECRYPT()

where i is a placeholder value. I run it and get this:

Input Message: ABC

What do you want to do:
1) Encrypt Something
2)Decrypt Something

2
Traceback (most recent call last):
  File "C:\Users\Danny\Google Drive\SHIFT.py", line 67, in <module>
    DECRYPT()
  File "C:\Users\Danny\Google Drive\SHIFT.py", line 19, in DECRYPT
    if M[i] != " ":
IndexError: string index out of range

I have tried changing value of i to no avail.

The error is happening in the parts of the code you are not showing. — idjaw
– idjaw, Commented Nov 12, 2016 at 17:17
Could you give some more code? This looks correct but maybe you stripped out the part that poses a problem. Also, stylewise: in Python, write your variables like this var_name and not VarName. The latter is used for classes — user1361491
– user1361491, Commented Nov 12, 2016 at 17:20
If you have an empty string e.g: String = '' then the value of the assignment i = len(String) - 1 is -1. There's no offset, positive or negative in an empty string so String[i] which is the same as String[-1] will yield IndexError: string index out of range — Darragh Enright
– Darragh Enright, Commented Nov 12, 2016 at 17:21

cdlane · Accepted Answer · 2016-11-13 03:03:32Z

There are multiple problems with your code: as fernand0 noted, your index runs the wrong way; your inversion of POS happens at different levels in the code, the encryptor does it on every character, the decryptor does it on every letter -- they should work the same; five of your six global declarations aren't needed; you don't deal with word breaks correctly so the decryption won't match the original; once you encrypt, there's no way to decrypt in the same session as the local NM doesn't feedback into the global M.

Below is my rework of your code addressing the above problems and some style issues. The key phrase here is 'simplify'. I've kept your odd uppercase variable names but expanded them from single characters to what they represent:

MESSAGE = input("Input Message: ").upper()
ALPHABET = list("ABCDEFGHIJKLMNOPQRSTUVWXYZ")
ALPHABET_LENGTH = len(ALPHABET)

def DECRYPT(MESSAGE):

    LENGTH = len(MESSAGE)
    IS_NEGATIVE = True

    NEW_MESSAGE = ""

    for I in range(LENGTH):

        if MESSAGE[I] in ALPHABET:
            INDEX = ALPHABET.index(MESSAGE[I])
            if IS_NEGATIVE:
                INDEX -= (I + 1)
            else:
                INDEX += (I + 1)
            INDEX %= ALPHABET_LENGTH
            NEW_MESSAGE += ALPHABET[INDEX]

            IS_NEGATIVE = not IS_NEGATIVE
        else:
            NEW_MESSAGE += MESSAGE[I]

    return NEW_MESSAGE[::-1]

def ENCRYPT(MESSAGE):

    MESSAGE = MESSAGE[::-1]

    LENGTH = len(MESSAGE)
    IS_POSITIVE = True

    NEW_MESSAGE = ""

    for I in range(LENGTH):

        if MESSAGE[I] in ALPHABET:
            INDEX = ALPHABET.index(MESSAGE[I])
            if IS_POSITIVE:
                INDEX += (I + 1)
            else:
                INDEX -= (I + 1)
            INDEX %= ALPHABET_LENGTH
            NEW_MESSAGE += ALPHABET[INDEX]

            IS_POSITIVE = not IS_POSITIVE
        else:
            NEW_MESSAGE += MESSAGE[I]

    return NEW_MESSAGE

while True:
    print("\nWhat do you want to do:")
    print("1) Encrypt Message")
    print("2) Decrypt Message")

    CHOICE = int(input("\n"))

    if CHOICE == 1:
        MESSAGE = ENCRYPT(MESSAGE)
    if CHOICE == 2:
        MESSAGE = DECRYPT(MESSAGE)

    print("\n" + MESSAGE)

TEST

> python3 file.py
Input Message: An opportunity to teach is an opportunity to learn

What do you want to do:
1) Encrypt Message
2) Decrypt Message

1

OPDAQ HB OEWAGIBFXIU JD RI JZEZZ GC NFVBFJAGWJT KC

What do you want to do:
1) Encrypt Message
2) Decrypt Message

2

AN OPPORTUNITY TO TEACH IS AN OPPORTUNITY TO LEARN

What do you want to do:
1) Encrypt Message
2) Decrypt Message

fernand0 · Accepted Answer · 2016-11-12 17:48:12Z

0

You are starting i at the end of the string and then, you are increasing it. I think the line i += 1 should be i -= 1

answered Nov 12, 2016 at 17:48

fernand0

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2 Comments

WhoGivesAFlux Over a year ago

I have corrected the syntax but it had no effect on the error

fernand0 Over a year ago

Well, there are more issues. You decrement i only when you enter in the if, so this is a problem. Moreover, the condition in the while is 1, so the i will become, eventually -1 and this is out of range. Try putting print("i",i,L-1) under the while 1 and you will see what's happening.

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