Tenth Line - Solution & Explanation
Problem Statement
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
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.
Approach Overview
Problem Overview: The task is simple: read a file named file.txt and output only the tenth line. If the file has fewer than ten lines, nothing should be printed. The challenge mainly tests familiarity with shell text-processing utilities and basic file iteration.
Approach 1: Line-by-Line Reading (O(n) time, O(1) space)
This approach reads the file sequentially and keeps track of the current line number. You iterate through each line, increment a counter, and print the line when the counter reaches 10. The key insight is that you never need to store the entire file in memory—only the current line and the counter. This makes the method memory efficient and practical even for very large files. Languages like Python, Java, C++, or JavaScript typically implement this with a loop over file input streams or buffered readers. The scan stops as soon as the tenth line is found, which keeps the logic simple and efficient for general file processing tasks.
Approach 2: Using Shell Commands (O(n) time, O(1) space)
Shell utilities are optimized for line-based text processing. Tools like sed, awk, and combinations of head and tail can extract a specific line directly from the command line. For example, sed -n '10p' file.txt prints only the tenth line, while awk 'NR==10' checks the current record number and outputs the matching line. Another option is head -n 10 file.txt | tail -n 1, which first truncates the file to the first ten lines and then selects the last of them. These utilities internally iterate through the file once, giving an overall time complexity of O(n) and constant memory usage. This approach is common in command-line scripting and log analysis.
Recommended for interviews: Interviewers usually expect familiarity with standard shell tools like sed or awk. Showing the line-by-line iteration demonstrates understanding of the underlying logic, but the shell command solution shows practical command-line proficiency. Both run in linear time with constant memory, but the shell approach is typically considered the most idiomatic for this problem.
Approach 1: Line-by-Line Reading
The line-by-line reading approach involves iterating through each line in the file until the 10th line is reached. If the file has less than 10 lines, the script should handle this scenario gracefully, typically by not outputting anything or by printing a message that indicates the file is too short.
The C solution opens the file and reads it line-by-line using a buffer. It keeps a counter to check whether the currently read line is the 10th line. If the 10th line is not reached before the end of the file, the program simply closes the file and exits.
Complexity
Time Complexity: O(n) where n is the number of lines in the file (since each line is read once).
Space Complexity: O(1) as only fixed memory is used for reading each line.
Approach 2: Using Shell Commands
Some programming languages allow shell commands to be executed directly. This approach uses shell utilities to find the 10th line. This is often the most concise solution, leveraging built-in shell tools, if the environment allows it.
In this Python code, the os.system function is used to execute a shell command. The command uses sed to print just the 10th line from the file. This is efficient and leverages shell capabilities.
Code
Python
JavaScript
Complexity
Time Complexity: O(n) due to the reading operation.
Space Complexity: O(1) as it's handled by the shell tool, not Python itself.
Approach 3: sed
Code
Shell
Complexity Comparison
| Approach | Complexity |
|---|---|
| Line-by-Line Reading | Time Complexity: O(n) where n is the number of lines in the file (since each line is read once). |
| Using Shell Commands | Time Complexity: O(n) due to the reading operation. |
| sed | — |
Detailed Complexity Analysis
| Approach | Time | Space | When to Use |
|---|---|---|---|
| Line-by-Line Reading | O(n) | O(1) | When implementing the logic in general-purpose languages like Python, Java, or C++ |
| Shell Commands (sed / awk / head+tail) | O(n) | O(1) | Best for shell scripting, quick command-line processing, and typical interview expectations for shell problems |
Video Solution
Leetcode - Shell Scripting - 195 - Tenth Line • The Polyglot Programmer • 1,100 views views
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