LeetCode-194: Transpose File

194. Transpose File

Given file.txt:

1
2
3

name age
alice 21
ryan 30

Desired Output:

1 2	name alice ryan age 21 30

Solution

cat file.txt | awk '{
    for (i = 1; i <= NF; i++) {
        if (NR == 1) {
            cols[i] = $i
        } else {
            cols[i] = cols[i] " " $i
        }
    }
}
END {
    for (i = 1; i <= length(cols); i++) {
        print cols[i]
    }
}'

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

{
    for (i = 1; i <= NF; i++) {
        if (NR == 1) {
            cols[i] = $i
        } else {
            cols[i] = cols[i] " " $i
        }
    }
}

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

Loop Through Fields

1	for (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

1
2
3

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

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

Subsequent Records

1
2
3

else {
    cols[i] = cols[i] " " $i
}

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

Handling the End of Input

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

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:

1
2
3

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

Second Record (`NR == 2`)

NF is 2 (two fields: alice and 21).

The loop runs from i = 1 to i = 2.

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

Third Record (`NR == 3`)

NF is 2 (two fields: ryan and 30).

The loop runs from i = 1 to i = 2.

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

Final Output

1 2	name alice ryan age 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

cat file.txt | awk '{
    print "Processing line:", NR
    for (i = 1; i <= NF; i++) {
        if (NR == 1) {
            cols[i] = $i
        } else {
            cols[i] = cols[i] " " $i
        }
        print "cols[" i "] =", cols[i]
    }
}
END {
    print "\n[RESULT]"
    for (i = 1; i <= length(cols); i++) {
        print cols[i]
    }
}'

Output with Debugging Statements

Processing line: 1
cols[1] = name
cols[2] = age
Processing line: 2
cols[1] = name alice
cols[2] = age 21
Processing line: 3
cols[1] = name alice ryan
cols[2] = age 21 30

[RESULT]
name alice ryan
age 21 30