How to increment/decrement characters in python in cyclic manner?

Question

I had created a function where I can increment and decrement character according to certain conditions and it's working absolutely fine for incrementing before "z" and decrementing characters that exist after "a" Here is my working code for that :

str="a b c d e"
Posnew=""
Negnew=""
for i in range(len(str)):
    Posnew+=chr(ord(str[i])-i)
    Negnew+=chr(ord(str[i])+i)
print(Posnew)
print(Negnew)

but I don't know what logic should I create to cyclically rotate it which means if I decrement "a" then it should give "z" and if I increment "z" then it should give "a".

Desired output would be something like if i decrement "a" by 2 then it should return "y" or if I increment "z" by 5 then it should return "e"

Thanks in Advance

Answer 1

The following script increments and decrements each character:

• by 1 for training and illustrative purposes initially, and then
• by i (ie by its position in the input string):

---

import sys
str="a b c e z" if len(sys.argv) == 1 else ''.join(sys.argv[1:])

Posnew=""
Negnew=""
for i in range(len(str)):
    ordstrP = ordstrN = ord(str[i])
    ordstri = ordstrP - 97                   #  97 == ord('a')
    if ordstri in range(26):                 #  26 == ord('z')-ord('a')+1
        ordstrP = ((ordstri +  1) % 26) + 97
        ordstrN = ((ordstri + 25) % 26) + 97
    Posnew+=chr(ordstrP)
    Negnew+=chr(ordstrN)

print('orig:', str)
print('or+1:', Posnew)
print('or-1:', Negnew)

print('-')

Posnew=""
Negnew=""
for i in range(len(str)):
    ordstrP = ordstrN = ord(str[i])
    ordstri = ordstrP - 97                   #  97 == ord('a')
    if ordstri in range(26):                 #  26 == ord('z')-ord('a')+1
        ordstrP = ((ordstri + (i % 26)) % 26) + 97
        ordstrN = ((ordstri + 26 - (i % 26)) % 26) + 97
    Posnew+=chr(ordstrP)
    Negnew+=chr(ordstrN)

print('orig:', str)
print('or+i:', Posnew)
print('or-i:', Negnew)

Output : \\SO\\64788535.py

orig: a b c e z
or+1: b c d f a
or-1: z a b d y
-
orig: a b c e z
or+i: a d g k h
or-i: a z y y r