Problem to Solve

Recall from lecture that we saw a few algorithms for sorting a sequence of numbers: selection sort, bubble sort, and merge sort.

  • Selection sort iterates through the unsorted portions of a list, selecting the smallest element each time and moving it to its correct location.
  • Bubble sort compares pairs of adjacent values one at a time and swaps them if they are in the incorrect order. This continues until the list is sorted.
  • Merge sort recursively divides the list into two repeatedly and then merges the smaller lists back into a larger one in the correct order.

In this problem, you’ll analyze three (compiled!) sorting programs to determine which algorithms they use. In a file called answers.txt in a folder called sort, record your answers, along with an explanation for each program, by filling in the blanks marked TODO.

Distribution Code

For this problem, you’ll need some “distribution code”—that is, code written by CS50’s staff. Provided to you are three already-compiled C programs, sort1, sort2, and sort3, as well as several .txt files for input and another file, answers.txt, in which to write your answers. Each of sort1, sort2, and sort3 implements a different sorting algorithm: selection sort, bubble sort, or merge sort (though not necessarily in that order!). Your task is to determine which sorting algorithm is used by each file. Start by downloading these files.

Download distribution files

Open VS Code.

Start by clicking inside your terminal window, then execute cd by itself. You should find that its “prompt” resembles the below.


Click inside of that terminal window and then execute


followed by Enter in order to download a ZIP called in your codespace. Take care not to overlook the space between wget and the following URL, or any other character for that matter!

Now execute


to create a folder called sort. You no longer need the ZIP file, so you can execute


and respond with “y” followed by Enter at the prompt to remove the ZIP file you downloaded.


Explore the .txt files
  • Multiple .txt files are provided to you. These files contain n lines of values, either reversed, shuffled, or sorted.
    • For example, reversed10000.txt contains 10000 lines of numbers that are reversed from 10000, while random100000.txt contains 100000 lines of numbers that are in random order.
  • The different types of .txt files may help you determine which sort is which. Consider how each algorithm performs with an already sorted list. How about a reversed list? Or shuffled list? It may help to work through a smaller list of each type and walk through each sorting process.
Time each sort with different inputs
  • To run the sorts on the text files, in the terminal, run ./[program_name] [text_file.txt]. Make sure you have made use of cd to move into the sort directory!
    • For example, to sort reversed10000.txt with sort1, run ./sort1 reversed10000.txt.
  • You may find it helpful to time your sorts. To do so, run time ./[sort_file] [text_file.txt].
    • For example, you could run time ./sort1 reversed10000.txt to run sort1 on 10,000 reversed numbers. At the end of your terminal’s output, you can look at the real time to see how much time actually elapsed while running the program.


Not sure how to solve?

How to Test


check50 cs50/problems/2024/spring/sort

How to Submit

  1. Download your answers.txt file by control-clicking or right-clicking on the file in your codespace’s file browser and choosing Download.
  2. Go to CS50’s Gradescope page.
  3. Click Problem Set 3: Sort.
  4. Drag and drop your answers.txt file to the area that says Drag & Drop. Be sure it has that exact filename! If you upload a file with a different name, the autograder likely will fail when trying to run it. Ensuring you have uploaded files with the correct filename is your responsibility!
  5. Click Upload.

You should see a message that says “Problem Set 3: Sort submitted successfully!”