✅ Problem 6 Analysis (FIT1058 Assignment 1, Semester 1 2025)
💡 What is the problem about?
Problem 6 asks you to write a one-line sed command that:
- Performs a local transposition of width 8 (
T₈)
- Uses a specific permutation derived from your student ID, which is
35175192
Essentially, you need to:
- Construct a permutation from your student ID using a set of steps.
- Use that permutation to write a
sed command that reorders blocks of 8 letters accordingly.
🔢 Step-by-step Solution
🧮 Step 1: Generate Your Student Permutation
Student ID = 35175192
Remove 8 and 9
→ Result: 3517512
Keep only the first occurrence of each digit
→ Result: 3512 (second 5 and 1 are removed)
This is the "reduced student number": 3512
Append missing digits from 0 to 7 in decreasing order
Missing digits in 3512: 0, 4, 6, 7
In decreasing order: 7, 6, 4, 0
→ Final permutation = 3512 + 7640 = 35127640
Interpret as permutation
🧪 Step 2: Understand How sed Will Apply the Permutation
- You are doing a local transposition in blocks of 8 letters.
- That means you’ll use regex with capture groups
\(...\) to grab 8 characters.
- Then reorder them using
\1 to \8 based on your permutation.
🧠 Your permutation (35127640) is:
0 1 2 3 4 5 6 7
↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
3 5 1 2 7 6 4 0 ← this maps index i in the original to index perm[i] in the output
This means:
- 1st output char = original char at position 4 (index 3)
- 2nd output char = original char at position 6 (index 5)
- 3rd output char = original char at position 2 (index 1)
- 4th output char = original char at position 3 (index 2)
- 5th output char = original char at position 8 (index 7)
- 6th output char = original char at position 7 (index 6)
- 7th output char = original char at position 5 (index 4)
- 8th output char = original char at position 1 (index 0)
🧾 Step 3: Write the sed Command
sed 's/\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)/\4\6\2\3\8\7\5\1/g'
🔍 Breakdown:
\(.\) — captures one character, repeated 8 times- Output:
\4 → position 3 (index 0-based)\6 → position 5\2 → position 1\3 → position 2\8 → position 7\7 → position 6\5 → position 4\1 → position 0
Which corresponds exactly to permutation 35127640.
✅ Final Answer for Problem 6
Create a file named prob6.txt and put this one line inside:
sed 's/\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)/\4\6\2\3\8\7\5\1/g'
Would you like me to help generate the answer for Problem 7 as well based on this?