Best Way to Implement a Custom lower() method in Python

August 15, 2022

I have an assignment where I’m not allowed to use the .lower() function so I’m creating my own. At the moment I have two ideas on how to do this but would love some input on what the most efficient way would be.

Method 1: Using if elif function

This is the most basic way I could think of, simply passing a character to the method and checking through each letter and returning its lowercase value. I thought this method might be quicker if I put the vowels higher up on the list as they occur the most:

def lowerCase(character):
    if character == "A":
        return 'a'
    elif character == "E":
        return 'e'

Method 2: Using two Lists:

The second idea I had was to have a list of capital letters and a corresponding list of lowercase letters and would return the index value of the lowecase array at the same position:

def lowerCase(character)

    lowercase_characters = ['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'
    ]

    uppercase_characters = ['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'
    ]

    for i in range(len(allowed_characters)-1):
        if character == uppercase_charactes[i]:
            return lowercase_characters[i]

I would love to know peoples thoughts on the most efficient way of doing this. Would it be quicker to go with method 2 and implement a quicksort algorithm or what would be the best way?

Thanks!

>Solution :

Use ord to convert a character into its ASCII value, and chr to convert an ASCII value to its corresponding character. This allows you to translate a character from the uppercase range into the lowercase range just by adding and subtracting:

>>> def lower(char):
...     if 'A' <= char <= 'Z':
...         return chr(ord(char) - ord('A') + ord('a'))
...     else:
...         return char
...
>>> lower('R')
'r'
>>> lower('t')
't'

If you were going to implement it using a lookup table instead of doing ASCII algebra, the more efficient way to build the lookup table would be as a dict, since that gives you constant-time lookups:

>>> import string
>>> upper_to_lower = dict(zip(string.ascii_uppercase, string.ascii_lowercase))
>>> upper_to_lower
{'A': 'a', 'B': 'b', 'C': 'c', 'D': 'd', 'E': 'e', 'F': 'f', 'G': 'g', 'H': 'h', 'I': 'i', 'J': 'j', 'K': 'k', 'L': 'l', 'M': 'm', 'N': 'n', 'O': 'o', 'P': 'p', 'Q': 'q', 'R': 'r', 'S': 's', 'T': 't', 'U': 'u', 'V': 'v', 'W': 'w', 'X': 'x', 'Y': 'y', 'Z': 'z'}
>>> def lower(char):
...     return upper_to_lower.get(char, char)
...
>>> lower('R')
'r'