While Loops with Sequences

Note

Source: Adapted from the C# edition (while/while-with-sequence.rst). One-char-per-line and vowel-counting examples are Python translations. The note about for loops is an original forward reference.

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):
    """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):
    """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):
    """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, target):
    """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.