index error: index out of range, It shouldnt be out of range though, i tried tracing it

Question

This is a code for encrypting the message like you do in playfair cipher.

def encrypt(input_message):
    r1=r2=c1=c2=0
    play = playfair()
    listed_ip = list(input_message)
    j=1
    listed_op=list('#####################################################################################################################################################')
    for i in range(len(listed_ip)-1):
        first_letter = listed_ip[i]
        second_letter = listed_ip[j]
        if first_letter =='j':
            listed_ip[i] = 'i'
        if second_letter == 'j':
            listed_ip[j] = 'i'
        for row in range(5):
            for col in range(5):
                if play[row][col] ==first_letter:
                    r1=row
                    c1=col
                if play[row][col] == second_letter:
                    r2=row
                    c2=col
            if r1 == r2:
                listed_op[i] = play[r1][(c1+1)%5]
                listed_op[j] = play[r2][(c2 + 1) % 5]
            elif c1 ==c2:
                listed_op[i] = play[(r1+1)%5][c1]
                listed_op[j] = play[(r2+1)%5][c2]
            else:
                listed_op[i]=play[r1][c2]
                listed_op[j]=play[r2][c1]
            i+=2
            j+=2
    output_text = ''.join(listed_op)
    print(output_text)
encrypt('Hithisisvishal')

I am supposed to get an output that changes this input message where evry letter is replaced with another letter according to its position in a matrix.

The other function where the matrix in which the position of each letter is checked with is:

import numpy as np


def playfair():
    key = input('Enter the key :')
    key += 'abcdefghiklmnopqrstuvwxyz '
    size = len(key)
    listed_key=list(key)

    for i in range(size):
        if listed_key[i] == 'j':
            listed_key[i] = 'i'
        for j in range(i+1, size):
            if listed_key[j] == listed_key[i]:
                for k in range(j, size-1):
                    listed_key[k] = listed_key[k+1]
                size -= 1
            else : j += 1
    final_key = ''.join(listed_key)
    play = np.zeros((5, 5), 'U1')

    play.flat[:32] = list(final_key)
    return play

The error i am getting is:

Enter the keysecc

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-6-dd6ef9446893> in <module>
     33     output_text = ''.join(listed_op)
     34     print(output_text)
---> 35 encrypt('Hithisisvishal')

<ipython-input-6-dd6ef9446893> in encrypt(input_message)
      7     for i in range(len(listed_ip)-1):
      8         first_letter = listed_ip[i]
----> 9         second_letter = listed_ip[j]
     10         if first_letter =='j':
     11             listed_ip[i] = 'i'

IndexError: list index out of range

Answer 1

Your i and j are not correctly assigned.

for i in range(len(listed_ip)-1):

The variable i will be 0, 1, 2, ... After you set i += 2, it will be reset when it loops back. The variable j is updated inside the second for loop. Therefore, you have index error.

def encrypt(input_message):
    r1 = r2 = c1 = c2 = 0
    play = playfair()
    listed_ip = list(input_message)
    j = 1
    listed_op = list('#'*150)

    for i in range(0, len(listed_ip)-1, 2):
        j = i + 1
        first_letter = listed_ip[i]
        second_letter = listed_ip[j]
        if first_letter == 'j':
            listed_ip[i] = 'i'
        if second_letter == 'j':
            listed_ip[j] = 'i'
        for row in range(5):
            for col in range(5):
                if play[row][col] == first_letter:
                    r1 = row
                    c1 = col
                if play[row][col] == second_letter:
                    r2 = row
                    c2 = col
            if r1 == r2:
                listed_op[i] = play[r1][(c1+1) % 5]
                listed_op[j] = play[r2][(c2 + 1) % 5]
            elif c1 == c2:
                listed_op[i] = play[(r1+1) % 5][c1]
                listed_op[j] = play[(r2+1) % 5][c2]
            else:
                listed_op[i] = play[r1][c2]
                listed_op[j] = play[r2][c1]
            # i += 2
            # j += 2
    output_text = ''.join(listed_op)
    print(output_text)

Answer 2

it looks like your problem is that your j value is getting too big for the array. Currently your j value is increasing by ten for every element in listed_ip. At that rate, it only needs two iterations to exceed the length of listed_ip and throw an IndexError. A possible fix for this would be setting your j value to 1 inside the for loop rather than before it. That way for every value in listed_ip, you start with j=1.

Heres my fix:

def encrypt(input_message):
    r1=r2=c1=c2=0
    play = playfair()
    listed_ip = list(input_message)
    print(listed_ip)
    
    listed_op=list('#####################################################################################################################################################')
    for i in range(len(listed_ip)-1):
        j=1
        first_letter = listed_ip[i]
        second_letter = listed_ip[j]
        if first_letter =='j':
            listed_ip[i] = 'i'
        if second_letter == 'j':
            listed_ip[j] = 'i'
        for row in range(5):
            for col in range(5):
                if play[row][col] ==first_letter:
                    r1=row
                    c1=col
                if play[row][col] == second_letter:
                    r2=row
                    c2=col
            if r1 == r2:
                listed_op[i] = play[r1][(c1+1)%5]
                listed_op[j] = play[r2][(c2 + 1) % 5]
            elif c1 ==c2:
                listed_op[i] = play[(r1+1)%5][c1]
                listed_op[j] = play[(r2+1)%5][c2]
            else:
                listed_op[i]=play[r1][c2]
                listed_op[j]=play[r2][c1]
            i+=2
            j+=2
    output_text = ''.join(listed_op)
    print(output_text)
encrypt('Hithisisvishal')

Its just a change to that one line, see if it works for you