Due Date

  • Due: Tuesday, September 17, before 11:59 PM

This, and all subsequent labs, are to be done with a partner. Your assigned partner for this lab is: Lab 2 Partners

1. Lab 2 Goals:

  • Practice with C programming basics: declaring variables, types, arrays, and functions.

  • Writing C code and functions that uses statically declared arrays.

  • Practice writing and using C functions. Pass by value: basic types and array parameters.

  • C I/O: scanf, printf, and using a simplified file reading library.

  • More practice with top-down design.

  • Using a Makefile.

  • Use git to clone a repository and share code with your partner.

2. Working with Partners and Partner Etiquette

For this lab, and for all subsequent lab assignments in this class, you will work with a partner. The instructors will assign partners for the first several lab assignments. And for these, you will be assigned a different partner for each lab assignment, with the goal of giving you experience working with, and meeting, several other students in the class. Later in the semester we will give you an opportunity to choose your partner on some lab assignments, or you may continue to choose to be assigned a partner by us on these labs.

During Wednesday lab, we will talk through our expectations for working with partners, and some tips for effectively working with a partner on CS31 lab assignments. Please continue to refer to this information as you and your partner work on lab assignments together:

CS31 Partner Etiquette and Expectations.

3. Lab Description

You will implement program that sorts floating point numbers using a sorting algorithm of your choice in C. Your program will read a collection of unsorted floats from a file, store those floats in an array, provide some information about the floats to the user, sort them from smallest magnitude to largest magnitude (i.e., ascending order), and print them out in sorted form to the user.

4. Lab 2 Starting Point Code

4.1. Getting Your Lab Repo

Both you and your partner should clone your Lab2 repo into your cs31/labs subdirectory:

  1. get your Lab2 ssh-URL from the CS31 git org. The repository to clone is named Lab2-user1-user2, where user1 and user2 are the user names of you and your Lab 2 partner.

  2. cd into your cs31/labs subdirectory:

    $ cd ~/cs31/labs
$ pwd

  3. clone your repo

    $ git clone [your_Lab2_repo_url]

There are more detailed instructions about getting your lab repo from the "Getting Lab Starting Point Code" section of the Using Git for CS31 Labs page.

4.2. Test sharing changes to your joint repo

As you and your partner work on your joint lab solution, you will want to push and pull changes from/to your local repos to/from the master repo.

As your start each lab assignment, it is good to first test out that you have both successfully cloned your shared repo, and that you can successfully share code by pushing a small change and pulling a small change made by your partner. We will practice this together in Wednesday lab. You can also follow the directions in the "Sharing Code with your Lab Partner" section of the Using Git for CS31 Labs page.

4.3. Starting Point Files

Cloning the repository will give you the following starting point code files (note: the exact number of floatsX.txt files in your repo may differ from this example listing):

$ ls
 Makefile       README.md     floats2.tx     floatsbig.txt  readfile.h
 floats.txt     floats3.txt   readfile.c     sorter.c

These files are:

  1. Makefile: A Makefile simplifies the process of compiling your program. We’ll look at these in more detail later in the course. You’re welcome to look over this one, but you shouldn’t need to edit it for this lab. If you are interested, take a look at Section 8 for more information about make and makefiles.

  2. readfile.h and readfile.c:These files contain a library for reading from files. You should make calls to functions in this library in your program to simplify file I/O. The instructions for using this library are explained below. Do not modify any code in these two files.

  3. sorter.c: the file into which you will add your C solution and comments. The starting point code includes a helper function for you:

    1. get_filename_from_commandline: takes the storage space for a string and the command line arguments and initializes the string storage space with the filename from the command line argument. You should not change this function, but feel free to change where it gets called.

  4. floats.txt, floats2.txt, and floats3.txt: example input files to your program. These are provided for your convenience to help you test your program. You will ultimately want to create new test files to more extensively test your program before you consider it finished.

4.4. Compiling and Running

To compile your program run make, which will use the Makefile to compile the binary executable, sorter, from sorter.c and from linking in the readfile library. Running make lists the gcc compiler output, which you should examine for errors and warnings. The starting point code has one warning telling you that the variable values is declared but not used in the program:

$ make
gcc -g -Wall -c readfile.c
gcc -g -Wall -o sorter sorter.c  readfile.o
sorter.c: In function 'main':
sorter.c:23:9: warning: unused variable 'values' [-Wunused-variable]
    float values[ARRAYSIZE];

To run the sorter executable binary, you need to include the name of the input file as a command line argument. The input file containing the floating point values for your program to sort, one value per line (see the floats.txt files included with the starting point). When run, your program will read in these values from the file (using the provided library), sort them, and print out the sorted result back to the user. Here is an example of how to run the program:

$ ./sorter floats.txt

4.5. File Format

The input file format consists of several lines of an ASCII text file. A properly formatted file will contain a short header, consisting of a single line with one integer and two floats on it. These numbers represent the total number of floats in the file (i.e., the number of subsequent lines), as well as the minimum and maximum to-be-sorted float value in the file.

For example, here is the header of a valid input file:

4 0.0 9.0

This header indicates that the file contains a total of 4 floats that need to be sorted. The smallest float value in the file will be 0.0, and the largest will be 9.0. You may not need to know the minimum and maximum values to successfully sort the values, but you will need to inform the user about the range of values being sorted.

Included with the starting point code are a few sample test files you can use to test your code. Every line after the first line in the file will contain a single floating point number. These are the float values that must be sorted. For example, a file containing the header we just saw might look like this (after the first line):

  0.0
  2.1
  9.0
  5.3

4.6. File I/O

For this assignment we’ll use functions from the provided readfile library (in the files readfile.c and readfile.h). You should not change any code in these files. The readfile.h file contains function prototypes for the readfile library. There are function comments in this file that describe each function and a high-level comment describes how to use the library.

Here are the general rules for how to use these functions:

  1. Open a file by calling: open_file(), passing in the name of the file to open as a string. The return value of open_file() tells you whether or not the file was opened successfully. It returns 0 if the file is successfully opened, and -1 if the file cannot be opened. You should always check the return value of this function and respond appropriately!

  2. Call the read_int(), read_string(), read_float() functions to read values from the file into your program’s variables, where the name of the function you call determines the resulting type of the value that gets filled in. Like open_file(), these functions return 0 on success, and you should always check their return value. If you’ve reached the end of the file, they will return -1.

    1. These functions take arguments much like scanf does. They need to know the memory location of where to put the value read in.

      For example (NOTE: this is not the sequence of function calls you need to make in your lab solution, but are just examples of how to call the three different read functions in this library…​in fact, you don’t need to use read_string for this lab assignment):

      int x;
float f;
char s[20];
...
/* These functions return 0 on success or -1 if read fails or
 * if there is nothing left to read (end-of-file has been reached). */
ret = read_float(&f);
ret = read_int(&x)
ret = read_string(s)

  3. Close the file when you’re done with it: close_file().

If you are curious, the implementation of these functions is in readfile.c. You can open this file in vim and look to see how it uses the the C FILE * interface and fscanf functions for reading. We will use this interface directly later in the semester. For now, we’re hiding it under a layer of abstraction!

5. Lab Requirements

For full credit, your solution should meet the following requirements:

  • When run, your program’s output should look like the output shown below from a run of a working program. This example doesn’t show every possible run or error handling, but should give you some idea of what a correct program run looks like. To make my job of grading easier, please make your output match the example as closely as possible (to print just a new line use: printf("\n");)

    $ ./sorter floats.txt
Reading file named floats.txt
The file contains 10 floating point numbers, ranging from 0.00 to 9.00
The unsorted values are: 9.00 8.00 4.00 5.00 7.00 3.00 2.00 1.00 6.00 0.00
The sorted values are: 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

  • As in the example above, your program should print the following four items in a human-readable way:

    1. The name of the file.

    2. The header contents of the file (number of values, max/min values).

    3. The values in their original, unsorted order.

    4. The values in sorted order, from smallest to largest.

  • You should store the set of floating point values only once (e.g., keep only one array, don’t copy all the values to a second array.) You may assume that there will never be more than 100 values to sort, and you can use any sorting algorithm you like, it doesn’t need to be fancy. You should not attempt to use merge sort, as we haven’t yet discussed all the C functionality that you’ll need to implement it.

  • Your program should exit gracefully if you detect an error that’s not your code’s fault (for example, if it was given the name of an invalid input file). Be sure to check those return values!

  • It should be evident that you applied top-down design when constructing your submission. In addition to main(), you should have at least two non-trivial functions, each with a specific role. (See the Implementation Recommendations section below for suggestions.)

  • Your code should be commented, modular, robust, and use meaningful variable and function names. Each function should include a brief description of its behavior.

    Look at the C code style guide for examples of complete comments, tips on how not to wrap lines, good indenting styles and suggestions for meaningful variable and function names in your program.

  • Your program should not use global variables. If a function needs a value from its caller, then that value should be passed in as a parameter. If a function needs to store variables, then those should be declared as local variables inside the function. If you don’t know what a global variable is, no problem, you won’t make the mistake of using one!

  • Your code should compile cleanly with no errors or warnings and should not crash unexpectedly during execution.

  • Code you submit should have my "TODO" comments removed (these are my notes to you, as you implement them remove the "TODO").

Implementation Guidelines

The following is the suggested way to implement the lab and fulfill the requirements. If you intend to significantly deviate from this design, please let me know so that I can do a basic sanity check of your proposed changes.

  • Store the floating point values in an array of C float types.

  • Write a function that knows how to print the contents of an array of floating point values. You’ll use this twice: once to print the original unsorted values, and later, to print the sorted values.

  • Write a sort function that takes in an array of floating point values (and the size of the array), sorting the array values such that the original array is in sorted order after the call completes.

    You will likely find it useful for your sort function to call two other helper functions that you define: a swap function (to swap the contents of an array, given two indices), and a check function (to determine whether or not your objective has been achieved). Feel free to add your own helper functions too!

6. Tips

  • Before even starting to write code, use top-down design to break your program into manageable functionality.

  • Test your code in small increments. It’s much easier to localize a bug when you’ve only changed a few lines.

  • Many of the functions that you’ll write to deal with arrays will need to know the size (length) of the array. Unlike Python, C does NOT keep that information around (there is no len() function). If a function needs to know the array length, you should make that a parameter to the function.

  • When printing floating point values, use formatting directives to make the output more readable by limiting the number of digits that get printed after the decimal point. For example, using %.2f in the format string for printf() limits the printed float to two digits after the decimal.

  • For values that will never change in your program, you can compile in constants using #defines. There are some example with the starting point code.

  • Use CNTRL-C to kill a running program stuck in an infinite loop.

  • To debug programs with command line options in gdb, include the command line arguments when you start the program with the gdb run command:

$ gdb ./sorter
(gdb) break main
(gdb) run floats.txt           # run with command line args

  • Post on Piazza, attend ninja sessions, and/or come to office hours if you have questions!

7. Submitting

Please remove any debugging output prior to submitting.

To submit your code, commit your changes locally using git add and git commit. Then run git push while in your lab directory. Only one partner needs to run the final git push, but make sure both partners have pulled and merged each others changes.

Also, it is good practice to run make clean before doing a git add and commit: you do not want to add to the repo any files that are built by gcc (e.g. executable files). Included in your lab git repo is a .gitignore file telling git to ignore these files, so you likely won’t add these types of files by accident. However, if you have other gcc generated binaries in your repo, please be careful about this.

Here are the commands to submit your solution in the sorter.c file (from one of you or your partner’s cs31/labs/Lab2-partner1-partner2 directory:

$ make clean
$ git add sorter.c
$ git commit -m "lab 2 completed and well commented"
$ git push

If you have difficulty pushing your changes, see the "Troubleshooting" section and "can’t push" sections at the end of the Using Git for CS31 Labs page. And for more information and help with using git, see the git help page.

8. Handy References