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S3Q2 · Fill the First n Blanks with 1 to n

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Type: File-in, stdout-out

Core idea: read n from the first line, then replace each _ in order with 1, 2, ..., n; leave remaining _ unchanged.

import tempfile, sys

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

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

n = int(lines[0].strip())
print(lines[0].strip())     # print n as first line

counter = 1
for line in lines[1:]:
    result = []
    for ch in line:
        if ch == '_' and counter <= n:
            result.append(str(counter))
            counter += 1
        else:
            result.append(ch)
    print("".join(result), end="")

Key rules: - Print the first line (n) unchanged - Replace _ sequentially with 1, 2, ..., n across all remaining lines - After n replacements, leave remaining _ as-is - Preserve all other characters and newlines exactly

Problem Statement

Problem (File I/O → stdout)

Read n from the first line. Replace the first n underscores in the subsequent text with 1 through n sequentially. Print all lines including the first.

Example:

Input file 
11
a_bc__d___e
f_g__h__i__j
k___l_m__n
Output 
11
a1bc23d456e
f7g89h1011i__j
k___l_m__n

Tracing the example

n = 11, counter starts at 1.

Line 2: a_bc__d___e

Char _? Counter ≤ 11? Output
a ❌ - a
_ ✅ ✅ (1) 1, counter→2
b,c ❌ - bc
_ ✅ ✅ (2) 2, counter→3
_ ✅ ✅ (3) 3, counter→4
d ❌ - d
_,_,_ ✅×3 ✅ (4,5,6) 456, counter→7
e ❌ - e

→ a1bc23d456e ✓

Line 3: f_g__h__i__j (counter=7)

_ → 7, _ → 8, _ → 9, _ → 10, _ → 11, remaining __ → unchanged

→ f7g89h1011i__j ✓

Line 4: k___l_m__n (counter=12 > 11 → all _ unchanged)

→ k___l_m__n ✓

Solution approaches

import tempfile, sys

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

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

n = int(lines[0].strip())
print(lines[0].strip())

counter = 1
for line in lines[1:]:
    result = []
    for ch in line:
        if ch == '_' and counter <= n:
            result.append(str(counter))
            counter += 1
        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())

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

n = int(lines[0].strip())
print(lines[0].strip())

counter = 1
for line in lines[1:]:
    while '_' in line and counter <= n:
        line = line.replace('_', str(counter), 1)
        counter += 1
    print(line, end="")

str.replace('_', replacement, 1) replaces only the first _ per call. The while loop repeats until no _ remain or counter exceeds n.

import tempfile, sys

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

with open(filename) as f:
    content = f.read()

lines    = content.split('\n', 1)    # split into first line and rest
n        = int(lines[0].strip())
rest     = lines[1] if len(lines) > 1 else ""

counter = 1
result  = []
for ch in rest:
    if ch == '_' and counter <= n:
        result.append(str(counter))
        counter += 1
    else:
        result.append(ch)

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

Key takeaways

01

Counter persists across lines

The replacement counter must not reset between lines - blanks in line 3 continue from where line 2 left off. Initialise it once before the line loop and never reset it.

02

Check counter ≤ n before replacing

Once the counter exceeds n, remaining _ must be preserved. The condition ch == '_' and counter <= n handles both - extra underscores pass through unchanged.

03

Multi-digit numbers replace a single underscore

10 and 11 are two-character strings but they replace a single _. Building the result as a list of strings and joining at the end handles variable-width replacements cleanly.