While Loops with Sequences#

Strings are sequences of characters that we can access by index. Before the for loop chapter, we use while loops with an index variable to process strings one character at a time.

One Character Per Line#

To print each character of a string on its own line:

def one_char_per_line(s: str) -> None:
    """Print each character of s on a separate line."""
    i = 0
    while i < len(s):
        print(s[i])
        i += 1

one_char_per_line("bug")

Output:

b
u
g

Following the loop-planning rubric:

Changing variable: i (the index)
Initial value: 0
Continuation condition: i < len(s) (valid indices are 0 through len(s)-1)
Update: i += 1

Counting Vowels#

def count_vowels(s: str) -> int:
    """Return the number of vowel characters in s."""
    vowels = "aeiouAEIOU"
    count = 0
    i = 0
    while i < len(s):
        if s[i] in vowels:
            count += 1
        i += 1
    return count

print(count_vowels("Hello World"))   # 3

Checking for All Digits#

def all_digits(s: str) -> bool:
    """Return True if every character of s is a digit."""
    i = 0
    while i < len(s):
        if not s[i].isdigit():
            return False
        i += 1
    return True

print(all_digits("12345"))   # True
print(all_digits("123a5"))   # False

Note the early return False — as soon as a non-digit is found, there is no need to check further.

Finding a Character#

Search for the first occurrence of a target character:

def find_char(s: str, target: str) -> int:
    """Return the index of the first occurrence of target in s, or -1."""
    i = 0
    while i < len(s):
        if s[i] == target:
            return i
        i += 1
    return -1

This is essentially what s.find(target) does internally.

Note

The for loop (covered in the next chapter) makes iterating over a string much cleaner — for ch in s: instead of managing an index manually. The while forms above are shown to build understanding of how indexing works; in practice you will almost always use for.