S3Q2 · Uppercase Every k-th Vowel in a File

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Type: File-in, stdout-out

Core idea: scan character by character across all lines; maintain a global vowel counter; uppercase every k-th vowel, lowercase all others.

import tempfile, sys

_, filename = tempfile.mkstemp(prefix="case")
with open(filename, 'w') as f:
    f.write(sys.stdin.read())

VOWELS = set("aeiou")
with open(filename) as f:
    k       = int(f.readline().strip())
    count   = 0
    for line in f:
        result = []
        for ch in line:
            if ch.lower() in VOWELS:
                count += 1
                result.append(ch.upper() if count % k == 0 else ch.lower())
            else:
                result.append(ch)
        print("".join(result), end="")

Key rules: - Global vowel counter across all lines - count % k == 0 → uppercase | otherwise → lowercase - Consonants keep their original case unchanged - print(..., end="") preserves newlines already in each line

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Problem Statement

Problem (File I/O → stdout)

Read k from the first line. Process the rest of the file: uppercase every k-th vowel (globally), lowercase all other vowels, leave consonants unchanged.

Example (k=3):

Input file

3
This is a sample text.
it has many vowels.
every third one becomes uppercase.

Output

This is A sample tExt.
it has mAny vowels.
Every third One becOmes uppErcase.

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Tracing the example (k=3)

Line 1: "This is a sample text."

Char	Vowel?	Count	Count%3	Output
T	❌	-	-	T
h	❌	-	-	h
i	✅	1	1	i (lower)
s	❌	-	-	s
i	✅	2	2	i (lower)
s	❌	-	-	s
a	✅	3	0	A (UPPER)
...
a	✅	4	1	a
e	✅	5	2	e
e	✅	6	0	E (UPPER)

→ This is A sample tExt. ✓ (count continues to line 2)

Line 2 starts at count=6, line 3 at count=9 after "vowels"...

---

Solution approaches

Pythonic (character loop)Read whole file at onceUsing lambda + map

import tempfile, sys

_, filename = tempfile.mkstemp(prefix="case")
with open(filename, 'w') as f:
    f.write(sys.stdin.read())

VOWELS = set("aeiou")

with open(filename) as f:
    k     = int(f.readline().strip())
    count = 0
    for line in f:
        result = []
        for ch in line:
            if ch.lower() in VOWELS:
                count += 1
                result.append(ch.upper() if count % k == 0 else ch.lower())
            else:
                result.append(ch)
        print("".join(result), end="")

import tempfile, sys

_, filename = tempfile.mkstemp(prefix="case")
with open(filename, 'w') as f:
    f.write(sys.stdin.read())

VOWELS = set("aeiou")

with open(filename) as f:
    lines  = f.readlines()

k     = int(lines[0].strip())
count = 0
result = []

for line in lines[1:]:
    for ch in line:
        if ch.lower() in VOWELS:
            count += 1
            result.append(ch.upper() if count % k == 0 else ch.lower())
        else:
            result.append(ch)

print("".join(result), end="")

import tempfile, sys

_, filename = tempfile.mkstemp(prefix="case")
with open(filename, 'w') as f:
    f.write(sys.stdin.read())

VOWELS = set("aeiou")

with open(filename) as f:
    k   = int(f.readline().strip())
    text = f.read()

count  = [0]
transform = lambda ch: (
    [count.__setitem__(0, count[0]+1), ch.upper() if count[0]%k==0 else ch.lower()][1]
    if ch.lower() in VOWELS else ch
)

print("".join(map(transform, text)), end="")

---

Key takeaways

01

Global counter across lines

The vowel counter must persist across all lines - don't reset between lines. Initialise count = 0 once before the line loop; it increments with each vowel found anywhere in the file.

02

ch.lower() in VOWELS - case-insensitive vowel check

Lowercasing the character before checking membership handles both 'A' and 'a' with one set. The VOWELS set only needs lowercase letters.

03

Non-vowels keep original case - don't touch them

Only vowels are transformed. Consonants, spaces, punctuation, and newlines are appended to result unchanged. The else: result.append(ch) handles this correctly for all non-vowel characters including \n.