LeetCode 194: Transpose File Solution in Shell Explained

Problem Statement

In LeetCode 194, you’re given a text file file.txt with rows of space-separated fields (e.g., names or numbers). Your task is to write a one-line bash command to:

• Read the file.
• Transpose it: turn rows into columns and columns into rows.
• Output the result with fields space-separated, maintaining order.

This differs from pattern matching like LeetCode 193: Valid Phone Numbers, focusing on matrix transformation rather than filtering.

Constraints

• File exists and is readable (file.txt).
• Each line has the same number of space-separated fields (rectangular matrix).
• Fields contain printable ASCII characters, no empty lines.

Example

Let’s see a case:

Copy codefile.txt:
name age
alice 21
bob 25

Output:
name alice bob
age 21 25
Explanation:
<ul>
<li>Row 1: "name age" → Column 1: "name", Column 2: "age".</li>
<li>Row 2: "alice 21" → Column 1: "alice", Column 2: "21".</li>
<li>Row 3: "bob 25" → Column 1: "bob", Column 2: "25".</li>
<li>Transposed: Column 1 becomes "name alice bob", Column 2 becomes "age 21 25".</li>
</ul>

Copy codefile.txt:
1 2 3 4
5 6 7 8

Output:
1 5
2 6
3 7
4 8
Explanation:
<ul>
<li>2x4 matrix → 4x2 matrix.</li>
</ul>

These examples show we’re transposing rows and columns.

Understanding the Problem

How do you solve LeetCode 194: Transpose File in a bash command? We need to:

• Read file.txt and interpret it as a matrix.
• Swap rows and columns: row i becomes column i, column j becomes row j.
• Output each new row with space-separated fields.

For "name age\nalice 21\nbob 25", a 3x2 matrix becomes a 2x3 matrix: "name alice bob" and "age 21 25". We need a command using tools like awk, cut, or paste to rearrange fields efficiently, not a bit-counting task like LeetCode 191: Number of 1 Bits. We’ll use: 1. awk Command: Versatile, concise—our best solution. 2. cut with paste: Alternative approach.

Let’s dive into the best solution.

Best Solution: awk Command Approach

Explanation

awk Command transposes the file by:

• Reading file.txt line by line.
• Storing each field in a 2D array (arr[row][col]).
• In the END block, printing columns as rows.
• Using space as the output field separator.

This ensures O(nm) time (n = rows, m = cols) for reading and storing, O(nm) for output, and O(n*m) space for the array, making it efficient and elegant in one command.

For "name age\nalice 21\nbob 25":

• Store: arr[1][1]="name", arr[1][2]="age", etc.
• Output: "name alice bob", "age 21 25".

Step-by-Step Example

Example 1: file.txt = "name age\nalice 21\nbob 25"

Goal: Return "name alice bob\nage 21 25".

• Step 1: Read file with awk.
• Line 1: "name age" → arr[1][1]="name", arr[1][2]="age".
• Line 2: "alice 21" → arr[2][1]="alice", arr[2][2]="21".
• Line 3: "bob 25" → arr[3][1]="bob", arr[3][2]="25".
• Step 2: Process in END block.
• NR=3 (rows), NF=2 (cols from first line).
• For j=1: arr[1][1] arr[2][1] arr[3][1] → "name alice bob".
• For j=2: arr[1][2] arr[2][2] arr[3][2] → "age 21 25".
• Step 3: Output with space separator.
• "name alice bob\nage 21 25".
• Finish: Output as required.

Example 2: file.txt = "1 2 3 4\n5 6 7 8"

Goal: Return "1 5\n2 6\n3 7\n4 8".

• Step 1: Read and store.
• Line 1: arr[1][1]="1", arr[1][2]="2", arr[1][3]="3", arr[1][4]="4".
• Line 2: arr[2][1]="5", arr[2][2]="6", arr[2][3]="7", arr[2][4]="8".
• Step 2: END block.
• NR=2, NF=4.
• j=1: "1 5".
• j=2: "2 6".
• j=3: "3 7".
• j=4: "4 8".
• Step 3: Output.
• "1 5\n2 6\n3 7\n4 8".
• Finish: Output as required.

How the Code Works (awk Command) – Detailed Line-by-Line Explanation

Here’s the bash command with a thorough breakdown (as a one-liner):

Copy code# Command: awk '{for(i=1;i<=NF;i++)arr[NR,i]=$i{ END{for(j=1;j<=NF;j++){for(i=1;i<=NR;i++)printf("%s%s",arr[i,j],i==NR?"\n":" "){ {' file.txt

# Line 1: awk processes file
awk '{
    # Line 2: Store fields in 2D array
    for(i=1;i<=NF;i++)
        arr[NR,i]=$i
        # NR=row, i=col, $i=field (e.g., arr[1,1]="name")
    { 
    # Line 3: Transpose and print in END block
    END{
        for(j=1;j<=NF;j++){
            # Iterate columns (e.g., j=1 for "name alice bob")
            for(i=1;i<=NR;i++)
                # Iterate rows (e.g., i=1,2,3 for "name", "alice", "bob")
                printf("%s%s",arr[i,j],i==NR?"\n":" ")
                # Print field, space or newline (e.g., "name ", "alice ", "bob\n")
        {
    {' 
# Line 4: Input file
file.txt
    # Source file (e.g., "name age\nalice 21\nbob 25")

This detailed breakdown clarifies how the awk command efficiently transposes the file.

Alternative: cut with paste Approach

Explanation

cut with paste transposes by:

• Using cut to extract each column into separate files.
• Using paste to combine columns into rows.
• Cleaning up temporary files (multi-line version).

It’s a practical alternative, O(nm) time (n=rows, m=cols), O(nm) space (temp files), but less elegant and requires multiple steps or temp files, making it less ideal as a one-liner.

For "name age\nalice 21\nbob 25":

• Columns: "name alice bob", "age 21 25".
• Paste: "name alice bob\nage 21 25".

Step-by-Step Example (Alternative)

For the same example:

• Step 1: cut columns.
• cut -d' ' -f1: "name\nalice\nbob".
• cut -d' ' -f2: "age\n21\n25".
• Step 2: paste into rows.
• paste -d' ': "name alice bob\nage 21 25".
• Finish: Output as required (multi-line needs temp files).

How the Code Works (cut with paste)

Copy code# Multi-line (not one-liner):
for i in $(seq 1 $(head -n1 file.txt | wc -w)); do cut -d' ' -f$i file.txt > tmp$i; done && paste -d' ' tmp* && rm tmp*

Complexity

• awk Command:
• Time: O(n*m) – read and print.
• Space: O(n*m) – array.
• cut with paste:
• Time: O(n*m) – cut and paste.
• Space: O(n*m) – temp files.

Efficiency Notes

awk Command is the best solution with O(nm) time and O(nm) space, offering a single-line, in-memory approach—cut with paste matches time complexity but uses O(n*m) disk space and multiple steps, making it less elegant and harder to fit as a one-liner.

Key Insights

• awk: In-memory transpose.
• cut/paste: Column extraction.
• Space: Separator handling.

Additional Example

Copy codefile.txt:
a b c
d e f
Output:
a d
b e
c f
Explanation: 2x3 → 3x2.

Edge Cases

• Single Row: Transpose to column.
• Single Column: Transpose to row.
• Large File: Both handle, awk more memory-efficient.

Both solutions handle these, but awk is cleaner.

Final Thoughts

LeetCode 194: Transpose File in shell is a clever text-manipulation challenge. The awk Command solution excels with its efficiency and brevity, while cut with paste offers a multi-step alternative. Want more shell? Try LeetCode 193: Valid Phone Numbers for filtering or explore Python Basics for coding skills. Ready to practice? Solve LeetCode 194 on LeetCode with "name age\nalice 21\nbob 25", aiming for the transposed output—test your skills now!