Alternate cases in String - Python

The task of alternating the case of characters in a string in Python involves iterating through the string and conditionally modifying the case of each character based on its position. For example, given a string s = "geeksforgeeks", the goal is to change characters at even indices to uppercase and those at odd indices to lowercase, producing a string where the case alternates for each character.

Using list comprehension

When applied to the problem of alternating case in a string, list comprehension allows for a clean one liner that loops over the string, applying a conditional check on each character's index to determine whether to convert it to uppercase or lowercase. This method is efficient in terms of both time and space, as it processes each character exactly once and joins them into a string in one go.

s = "geeksforgeeks"

res = ''.join([s[i].upper() if i % 2 == 0 else s[i].lower() for i in range(len(s))])
print(res)

GeEkSfOrGeEkS

Explanation: list comprehension iterates over each character in s, converting it to uppercase for even indices and lowercase for odd indices and then ''.join() combines the resulting list of characters into a single string.

Using map()

map() can be used in conjunction with a lambda function to transform each character in the string based on its index. This method allows for a more functional programming approach, where we avoid explicit loops, but it may be less efficient than list comprehension for beginners.

s = "geeksforgeeks"

res = ''.join(map(lambda x: x[1].upper() if x[0] % 2 == 0 else x[1].lower(), enumerate(s)))
print(res)

GeEkSfOrGeEkS

Explanation: map() with a lambda function iterates over the string and alternates the case based on whether the index is even or odd and then result joined into a single string using ''.join().

Using str.format()

str.format() allows dynamic string formatting, making it a useful tool for alternating the case of letters in a string. By using str.format() inside a loop, we can conditionally check each character’s index and apply either uppercase or lowercase formatting based on whether the index is even or odd.

s = "geeksforgeeks"
res = ""

for idx, char in enumerate(s):
    res += '{}'.format(char.upper() if idx % 2 == 0 else char.lower())
print(res)

GeEkSfOrGeEkS

Explanation: for loop with enumerate() iterates over the string, using str.format() to alternate the case based on the index and the formatted characters are concatenated into the result string res.

Using for loop

A traditional for loop checks each character's index to alternate its case based on whether the index is even or odd. While simple , this method can be less efficient due to repeated string concatenation, leading to higher memory usage and slower performance for larger strings. However, it's a common approach for beginners to handle string manipulation tasks.

s = "geeksforgeeks"
res = ""

for idx in range(len(s)):
    if not idx % 2:
        res += s[idx].upper()
    else:
        res += s[idx].lower()
print(res)

GeEkSfOrGeEkS

Explanation: for loop iterates over the string by index, converting characters at even indices to uppercase and those at odd indices to lowercase, then concatenates them into the res string.