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Tenth Line

Easy

Asked by:

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Topics:

Strings

Given a text file file.txt, print just the 10th line of the file.

Example:

Assume that file.txt has the following content:

 Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 7 Line 8 Line 9 Line 10

Your script should output the tenth line, which is:

 Line 10

Note:
1. If the file contains less than 10 lines, what should you output?
2. There's at least three different solutions. Try to explore all possibilities.

Try coding it in LeetCode

Tenth Line

Easy

Asked by:

31 views

Topics:

Strings

Given a text file file.txt, print just the 10th line of the file.

Example:

Assume that file.txt has the following content:

 Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 7 Line 8 Line 9 Line 10

Your script should output the tenth line, which is:

 Line 10

Note:
1. If the file contains less than 10 lines, what should you output?
2. There's at least three different solutions. Try to explore all possibilities.

Try coding it in LeetCode

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What is the expected size of the file? For instance, could it be too large to fit entirely into memory?
What should be the output if the file contains fewer than ten lines? Should I return an empty string, null, or raise an error?
Should the returned content of the tenth line include the trailing newline character, or should it be trimmed?
If the tenth line exists but is an empty line (i.e., just a newline character), what should be returned?
How are lines delimited in the file? Can I assume a standard newline character (\n), or should I handle other line endings like carriage returns (\r\n)?

Brute Force Solution

Approach

The simplest way to find the tenth line is to read the file from the very beginning, one line at a time. We'll just keep a count of which line we are on, and when we get to the tenth one, that's our answer.

Here's how the algorithm would work step-by-step:

Start reading the file from the very first line.
Keep a mental counter, beginning with the number one for the first line.
After reading the first line, move to the second line and change your counter to two.
Continue this process, reading one line and then increasing your counter by one each time.
After reading each line, check if your counter has reached the number ten.
The moment your counter hits ten, the line you just read is the one you are looking for.
Present that specific line as the solution and you can stop.

Code Implementation

def get_tenth_line(file_path):
    # Initialize a counter to track which line number we are currently processing.

    current_line_number = 0
    with open(file_path, 'r') as file_to_read:
        # Iterating line-by-line is memory-efficient for large files, matching the step-by-step read.

        for line_content in file_to_read:
            current_line_number += 1

            # This check is the core of the algorithm, stopping exactly when we've read the target line.

            if current_line_number == 10:
                return line_content

    # This handles the edge case where the file has fewer than ten lines, returning an empty string.

    return ""

Big(O) Analysis

Time Complexity

O(1) – Let n be the total number of lines in the file. The cost of this solution is driven by the number of lines read from the file. The algorithm reads the file one line at a time and stops immediately after processing the tenth line. This means the process performs a maximum of 10 read operations, regardless of whether the file has 10 lines or 10 million lines. Since the total number of operations is bounded by a fixed constant and does not grow with the input size n, the time complexity simplifies to O(1).

Space Complexity

O(1) – The auxiliary space complexity is determined by the memory needed beyond the input storage. This algorithm uses a single integer variable as a counter and another variable to temporarily hold the string content of the current line being read. Let N be the total number of lines in the file. The amount of memory for the counter and the single line buffer does not depend on N, as these variables are simply reused for each line processed. Since the memory usage remains fixed regardless of the file's size, the space complexity is constant.

Optimal Solution

Approach

Instead of loading the entire file at once, which is inefficient for large files, the optimal approach is to read it one line at a time. We simply keep a running count of the lines and stop as soon as we find the tenth one.

Here's how the algorithm would work step-by-step:

Start at the very beginning of the file.
Read the first line and count it as line one.
Read the next line and update the count to two.
Continue reading the file one line at a time, increasing your count for each new line you see.
When the count reaches ten, you have found the line you're looking for.
Display that tenth line.
Immediately stop processing the file, as there is no need to read any further.

Code Implementation

line_counter = 0

with open('file.txt', 'r') as file_to_read:
    # Reading line-by-line is memory-efficient, avoiding loading the entire file at once.

    for current_line in file_to_read:
        line_counter += 1

        # This check is performed for every line to identify exactly when we reach the tenth one.

        if line_counter == 10:
            print(current_line.rstrip())

            # Processing stops immediately after the target line is found for optimal performance.

            break

Big(O) Analysis

Time Complexity

O(1) – The time complexity is constant because the algorithm's execution time does not depend on the size of the input file. Let n be the total number of lines in the file; the process involves reading the file one line at a time. The operation will always stop after reading at most 10 lines, regardless of whether n is 10 or one million. Since the number of operations is capped at a small, fixed constant, the total work done is constant, which simplifies to O(1).

Space Complexity

O(1) – The algorithm uses a constant amount of auxiliary space. This space consists of a single integer variable for the running count and a temporary buffer to hold the content of the current line being read. Since we only process one line at a time and do not store previous lines, the memory required does not grow with the total number of lines (N) in the file.

Edge Cases

Case	How to Handle
File has fewer than ten lines	The program should produce no output or an empty string as the requested line does not exist.
File is extremely large and does not fit in memory	The solution must read the file line-by-line instead of loading the entire content at once to avoid memory exhaustion.
The tenth line itself is an empty line	The program must correctly output the empty line as valid content rather than treating it as the end of file.
File does not exist or user lacks read permissions	The program should handle the resulting I/O error gracefully, for instance by printing a message to standard error.
File uses different line endings (e.g., Windows CRLF)	The line-reading logic should correctly interpret any standard line ending format as a line separator.
The tenth line has no trailing newline character	The solution must return the line's content even if it is the last line and lacks a terminating newline character.
Input path is a directory, not a regular file	The file open operation will fail, and this specific error should be caught and handled cleanly without crashing.
File contains non-textual (binary) data	Reading a binary file as text may result in a decoding error or produce meaningless output.

File has fewer than ten lines

How to Handle:

The program should produce no output or an empty string as the requested line does not exist.

File is extremely large and does not fit in memory

How to Handle:

The solution must read the file line-by-line instead of loading the entire content at once to avoid memory exhaustion.

The tenth line itself is an empty line

How to Handle:

The program must correctly output the empty line as valid content rather than treating it as the end of file.

File does not exist or user lacks read permissions

How to Handle:

The program should handle the resulting I/O error gracefully, for instance by printing a message to standard error.

File uses different line endings (e.g., Windows CRLF)

How to Handle:

The line-reading logic should correctly interpret any standard line ending format as a line separator.

The tenth line has no trailing newline character

How to Handle:

The solution must return the line's content even if it is the last line and lacks a terminating newline character.

Input path is a directory, not a regular file

How to Handle:

The file open operation will fail, and this specific error should be caught and handled cleanly without crashing.

File contains non-textual (binary) data

How to Handle:

Reading a binary file as text may result in a decoding error or produce meaningless output.

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What is the expected size of the file? For instance, could it be too large to fit entirely into memory?
What should be the output if the file contains fewer than ten lines? Should I return an empty string, null, or raise an error?
Should the returned content of the tenth line include the trailing newline character, or should it be trimmed?
If the tenth line exists but is an empty line (i.e., just a newline character), what should be returned?
How are lines delimited in the file? Can I assume a standard newline character (\n), or should I handle other line endings like carriage returns (\r\n)?

Brute Force Solution

Approach

Here's how the algorithm would work step-by-step:

Start reading the file from the very first line.
Keep a mental counter, beginning with the number one for the first line.
After reading the first line, move to the second line and change your counter to two.
Continue this process, reading one line and then increasing your counter by one each time.
After reading each line, check if your counter has reached the number ten.
The moment your counter hits ten, the line you just read is the one you are looking for.
Present that specific line as the solution and you can stop.

Code Implementation

def get_tenth_line(file_path):
    # Initialize a counter to track which line number we are currently processing.

    current_line_number = 0
    with open(file_path, 'r') as file_to_read:
        # Iterating line-by-line is memory-efficient for large files, matching the step-by-step read.

        for line_content in file_to_read:
            current_line_number += 1

            # This check is the core of the algorithm, stopping exactly when we've read the target line.

            if current_line_number == 10:
                return line_content

    # This handles the edge case where the file has fewer than ten lines, returning an empty string.

    return ""

Big(O) Analysis

Time Complexity

Space Complexity

Optimal Solution

Approach

Here's how the algorithm would work step-by-step:

Start at the very beginning of the file.
Read the first line and count it as line one.
Read the next line and update the count to two.
Continue reading the file one line at a time, increasing your count for each new line you see.
When the count reaches ten, you have found the line you're looking for.
Display that tenth line.
Immediately stop processing the file, as there is no need to read any further.

Code Implementation

line_counter = 0

with open('file.txt', 'r') as file_to_read:
    # Reading line-by-line is memory-efficient, avoiding loading the entire file at once.

    for current_line in file_to_read:
        line_counter += 1

        # This check is performed for every line to identify exactly when we reach the tenth one.

        if line_counter == 10:
            print(current_line.rstrip())

            # Processing stops immediately after the target line is found for optimal performance.

            break

Big(O) Analysis

Time Complexity

Space Complexity

Edge Cases

Case	How to Handle
File has fewer than ten lines	The program should produce no output or an empty string as the requested line does not exist.
File is extremely large and does not fit in memory	The solution must read the file line-by-line instead of loading the entire content at once to avoid memory exhaustion.
The tenth line itself is an empty line	The program must correctly output the empty line as valid content rather than treating it as the end of file.
File does not exist or user lacks read permissions	The program should handle the resulting I/O error gracefully, for instance by printing a message to standard error.
File uses different line endings (e.g., Windows CRLF)	The line-reading logic should correctly interpret any standard line ending format as a line separator.
The tenth line has no trailing newline character	The solution must return the line's content even if it is the last line and lacks a terminating newline character.
Input path is a directory, not a regular file	The file open operation will fail, and this specific error should be caught and handled cleanly without crashing.
File contains non-textual (binary) data	Reading a binary file as text may result in a decoding error or produce meaningless output.

File has fewer than ten lines

How to Handle:

The program should produce no output or an empty string as the requested line does not exist.

File is extremely large and does not fit in memory

How to Handle:

The solution must read the file line-by-line instead of loading the entire content at once to avoid memory exhaustion.

The tenth line itself is an empty line

How to Handle:

The program must correctly output the empty line as valid content rather than treating it as the end of file.

File does not exist or user lacks read permissions

How to Handle:

The program should handle the resulting I/O error gracefully, for instance by printing a message to standard error.

File uses different line endings (e.g., Windows CRLF)

How to Handle:

The line-reading logic should correctly interpret any standard line ending format as a line separator.

The tenth line has no trailing newline character

How to Handle:

The solution must return the line's content even if it is the last line and lacks a terminating newline character.

Input path is a directory, not a regular file

How to Handle:

The file open operation will fail, and this specific error should be caught and handled cleanly without crashing.

File contains non-textual (binary) data

How to Handle:

Reading a binary file as text may result in a decoding error or produce meaningless output.