LeetCode-194: Transpose File

194. Transpose File

Given file.txt:

1name age
2alice 21
3ryan 30

Desired Output:

1name alice ryan
2age 21 30

Solution

 1cat file.txt | awk '{
 2    for (i = 1; i <= NF; i++) {
 3        if (NR == 1) {
 4            cols[i] = $i
 5        } else {
 6            cols[i] = cols[i] " " $i
 7        }
 8    }
 9}
10END {
11    for (i = 1; i <= length(cols); i++) {
12        print cols[i]
13    }
14}'

Transposing a File with awk

Transposing a file means converting rows into columns and columns into rows. This is useful in various data manipulation tasks.

awk Basics

• NF (Number of Fields): The number of fields in the current line (record). (column)
• NR (Number of Records): The line number or the number of records read so far. Incremented automatically with each new line. (row)

awk Script for Transposing Explanation

1{
2    for (i = 1; i <= NF; i++) {
3        if (NR == 1) {
4            cols[i] = $i
5        } else {
6            cols[i] = cols[i] " " $i
7        }
8    }
9}

• The {} block is executed for each record (line) read from the input.

Loop Through Fields

1for (i = 1; i <= NF; i++) {

• NF (Number of Fields): Number of fields in the current record.
• Loops through each field from 1 to NF.

First Record Handling

1if (NR == 1) {
2    cols[i] = $i
3}

• NR (Number of Records): The current record number.
• For the first record, initialise the cols array with fields from the first line.

Subsequent Records

1else {
2    cols[i] = cols[i] " " $i
3}

• For subsequent lines, append the current field $i to the existing string in cols[i], separated by a space.

Handling the End of Input

1END {
2    for (i = 1; i <= length(cols); i++) {
3        print cols[i]
4    }
5}

• The END block executes after all lines have been processed.
• Prints each column (now a row in the transposed format).

Example Workflow

Given file.txt:

1name age
2alice 21
3ryan 30

Processing Each Record

First Record (NR == 1)

• NF is 2 (two fields: name and age).
• The loop runs from i = 1 to i = 2.

1cols[1] = "name"
2cols[2] = "age"

Second Record (NR == 2)

• NF is 2 (two fields: alice and 21).
• The loop runs from i = 1 to i = 2.

1cols[1] = "name alice"
2cols[2] = "age 21"

Third Record (NR == 3)

• NF is 2 (two fields: ryan and 30).
• The loop runs from i = 1 to i = 2.

1cols[1] = "name alice ryan"
2cols[2] = "age 21 30"

Final Output

1name alice ryan
2age 21 30

Debugging the awk Script

You can add print statements inside your awk script to debug and see what’s happening at each step.

Debugging Script

 1cat file.txt | awk '{
 2    print "Processing line:", NR
 3    for (i = 1; i <= NF; i++) {
 4        if (NR == 1) {
 5            cols[i] = $i
 6        } else {
 7            cols[i] = cols[i] " " $i
 8        }
 9        print "cols[" i "] =", cols[i]
10    }
11}
12END {
13    print "\n[RESULT]"
14    for (i = 1; i <= length(cols); i++) {
15        print cols[i]
16    }
17}'

Output with Debugging Statements

 1Processing line: 1
 2cols[1] = name
 3cols[2] = age
 4Processing line: 2
 5cols[1] = name alice
 6cols[2] = age 21
 7Processing line: 3
 8cols[1] = name alice ryan
 9cols[2] = age 21 30
10
11[RESULT]
12name alice ryan
13age 21 30