Incrementing characters in a string

Question

Suppose you have a string s and an integer array roll . I want to increment each of the letters of s by 1 depending on the number in roll . For example, if s = "abc" and roll = [1,2] , then the output would be s = "ccc" .

When I run the below code, I keep getting the original string s Why is this?

def  rollTheString(s, roll):
  for i in range(0, len(roll)):
     for j in range(0,i):
       s[j] = (chr(ord(s[j])+1));
  return s;

Assigning to a string will throw an error, as they are immutable. You should change it to a mutable type, like a list or bytearray. — Artyer
– Artyer, Commented Aug 25, 2017 at 19:51
As other comments stated, the question and the code snippet simply don't match. -1. — CristiFati
– CristiFati, Commented Aug 25, 2017 at 20:09
@CristiFati: You increment the first letter by 1 , so we get bbc. Then we increment the first two letters by 1 to get ccc. — NebulousReveal
– NebulousReveal, Commented Aug 25, 2017 at 20:30
@Trevor: I guessed that this would be the desired behavior. However when looking at the code (without running it), besides the TypeError thrown, it only increments the 1st char, resulting bbc. — CristiFati
– CristiFati, Commented Aug 25, 2017 at 20:45

Nabaz Maaruf · Accepted Answer · 2019-01-13 17:22:08Z

2

you can use this

def rollTheString(s, roll):
    increment = [0] * len(s)
    a_to_z = [chr(i) for i in range(97, 97 + 26)]
    for num in roll:
        for i in range(num):
            increment[i] += 1
    result = ""
    for i in range(len(s)):
        index = ord(s[i]) + increment[i] - 97
        index = index % 26
        result += a_to_z[index]

    return result

answered Jan 13, 2019 at 17:22

Nabaz Maaruf

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Comments

Alexander · Accepted Answer · 2017-08-25 20:13:40Z

1

Strings are immutable, so you cannot change them via slicing. You'll need to create a new string, preferably through a comprehension using join.

from itertools import izip_longest

s = "abc"
roll = [1, 2]

>>> "".join(chr(ord(c) + (n or 0)) for c, n in izip_longest(s, roll))
'bdc'

 # a + 1 = b, b + 2 = d, c + 0 = c

Looking at your nest loops, you may be creating a cumsum of sorts on the roll. You don't even appear to use the values in roll, just its length.

I would split your problem into two parts:

Determine the offset to add to each character from s.
Use the join method illustrated above to create a new string based on the offsets.

edited Aug 25, 2017 at 20:13

answered Aug 25, 2017 at 19:59

Alexander

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Comments

Davide Del Papa · Accepted Answer · 2017-08-25 20:34:57Z

1

The error you get, should be:

TypeError: 'str' object does not support item assignment

This is due to the fact that in Python strings are immutable, so you can't change their characters in-place.

As to your problem, consider the following code:

def rollTheString(s, roll):
    i = 0
    tmp = ''
    while i < len(s):
        ch_offset = roll[i] if ( i < len(roll)) else 0
        tmp += (chr(ord(s[i]) + ch_offset))
        i += 1
    return tmp

newString = rollTheString("abc", [2,1])
print (newString)

answered Aug 25, 2017 at 20:34

Davide Del Papa

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