S3Q2 · Reorder the Jumbled Lines

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⚡ Quick Reference

Type: File-in, stdout-out

Core idea: read all lines, parse the last line as new_pos-original_pos pairs, reorder and print.

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

content_lines = lines[:-1]                 # all lines except the last
order_line    = lines[-1].strip()          # "1-3,2-4,3-2,4-1"

# Parse pairs and sort by new position
pairs = [pair.split("-") for pair in order_line.split(",")]
ordered = sorted(pairs, key=lambda p: int(p[0]))

for _, orig in ordered:
    print(content_lines[int(orig) - 1], end="")

Key rules: - Last line format: new_pos-original_pos pairs, comma-separated - Sort pairs by new_pos to get the correct output order - Lines are 1-indexed - subtract 1 for list indexing

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Problem Statement

Problem (File I/O → stdout)

Read a file whose last line contains reorder instructions in new_pos-original_pos format. Print the preceding lines in the specified order.

Example:

Input file

first line
second line
third line
fourth line
1-3,2-4,3-2,4-1

Output

third line
fourth line
second line
first line

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Understanding the order line

1-3,2-4,3-2,4-1 breaks down as:

Pair	New position	Original line
1-3	1	line 3 → "third line"
2-4	2	line 4 → "fourth line"
3-2	3	line 2 → "second line"
4-1	4	line 1 → "first line"

Output order follows new positions 1 → 2 → 3 → 4: third line, fourth line, second line, first line ✓

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Tracing the example

lines = ["first line\n", "second line\n", "third line\n",
         "fourth line\n", "1-3,2-4,3-2,4-1"]

content_lines = lines[:-1]    # first 4 lines
order_line    = "1-3,2-4,3-2,4-1"

pairs = [["1","3"], ["2","4"], ["3","2"], ["4","1"]]
# sorted by new_pos (already in order here)

For each (new, orig):
  orig=3 → content_lines[2] = "third line\n"   → print
  orig=4 → content_lines[3] = "fourth line\n"  → print
  orig=2 → content_lines[1] = "second line\n"  → print
  orig=1 → content_lines[0] = "first line\n"   → print

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Solution approaches

PythonicExplanatory (step by step)Using a dict for direct lookupUsing lambda + sorted

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

content_lines = lines[:-1]
order_line    = lines[-1].strip()

pairs   = [p.split("-") for p in order_line.split(",")]
ordered = sorted(pairs, key=lambda p: int(p[0]))

for _, orig in ordered:
    print(content_lines[int(orig) - 1], end="")

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

# Separate content lines from the order instruction
content_lines = lines[:-1]
order_line    = lines[-1].strip()

# Parse "1-3,2-4,3-2,4-1" into (new_pos, orig_pos) pairs
pairs = []
for part in order_line.split(","):
    new_pos, orig_pos = part.split("-")
    pairs.append((int(new_pos), int(orig_pos)))

# Sort by new position to get output order
pairs.sort(key=lambda p: p[0])

# Print each line in the new order
for new_pos, orig_pos in pairs:
    line = content_lines[orig_pos - 1]   # 1-indexed → 0-indexed
    print(line, end="")

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

content_lines = lines[:-1]
order_line    = lines[-1].strip()

# Build a mapping: new_position → original_line_content
mapping = {}
for part in order_line.split(","):
    new, orig = map(int, part.split("-"))
    mapping[new] = content_lines[orig - 1]

# Print in new position order 1, 2, 3, ...
for key in sorted(mapping):
    print(mapping[key], end="")

Stores each line keyed by its new position, then iterates sorted(mapping) for the correct output order.

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

content_lines = lines[:-1]
parse = lambda p: tuple(map(int, p.split("-")))
pairs = sorted(map(parse, lines[-1].strip().split(",")))

for _, orig in pairs:
    print(content_lines[orig - 1], end="")

Lambda parses each "new-orig" pair into an integer tuple. sorted() on tuples sorts by first element (new position) naturally.

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Key takeaways

01

lines[:-1] and lines[-1] split cleanly

lines[:-1] gives all content lines; lines[-1] gives the order instruction. Works regardless of how many content lines there are - no hardcoded indices.

02

Sort by new position before printing

The pairs in the order line may not appear in new-position order. Always sort by new_pos before iterating - otherwise the output order would follow the order line's listing, not the intended sequence.

03

1-indexed to 0-indexed: subtract 1

The instruction uses 1-based line numbers; Python lists are 0-indexed. content_lines[orig - 1] converts correctly. Forgetting this offset is a common off-by-one error.