Homework 6: C Introduction 1

Create a new file called hello.c in VS Code. Then, copy and paste the content from this linked hello.c file into that file. This program (unsurprisingly) is supposed to print "Hello, world!" to the screen. The top line, /* Hello World in C */, is a comment: /* starts the comment and */ ends the comment. (Like in Java, you can also use // for comments within a function.)

Next, compile the program to a binary executable by running the following in the terminal:

clang -o hello hello.c

This command should produce a file named hello. You should be able to see it in files list in VS Code, and also if you type ls in the terminal. Try running it at the command prompt:

./hello

If this worked, you should see "Hello, world!" output to the terminal.

EXERCISE 1: Change the program to print

Hello, CS251!

Run the program again (after recompiling it). When complete, you can also run ./test-m, which will run the automated M tests.

There are three sections of tests. If all went well, the HelloTests should pass. You'll know that happened by the fact that you don't see any errors or failures for HelloTests, and the last couple lines of output should look something like:

Ran 3 tests in 0.183s

FAILED (errors=2)

Your number of seconds will differ (mine was 0.183 seconds), but it should say it ran 3 tests and only 2 of them had errors. If it says FAILED (failures=1, errors=2) that means that your printing in hello.c isn't quite right.

The first non-commented line of hello.c is: #include <stdio.h>. This directs the C compiler (or technically, a subprocess called the C preprocessor) to include a header file named stdio.h that lists the signatures of standard IO functions, such as printf.

The documentation for most of the built-in C functions can be found in the UNIX manual ("man") pages. Take a look at the documentation for printf in particular. It tells you which header file to include to get access to that function (in this case, stdio.h), as well as documenting the interface and behavior of the function of interest. In this case, the manual page also includes several similarly-named functions such as sprintf and fprintf. Read through the first few paragraphs the man page for printf, and skim through the rest so you get a sense of what these pages look like. There are examples at the end, which is often the most useful part.

printf is similar to Python's string formatting operator, in that it accepts a string that includes format specifiers to print integers, floats, etc. To see more about how printf and other operations work, create a file name printing.c in your directory, and then copy in the contents of this printing.c file. It contains a number of different examples of print formats. To compile and run your code, go back and look at how we did it for the hello.c program, and change accordingly.

Here are some common C types and their printf formatting specifications:

EXERCISE 2: Add code to printing.c that subtracts the value 15 from 87 and prints the result, together with an appropriate message. Check the PrintingTests section of the results from running ./test-m to see if you've got it working correctly.

You can get user input using the scanf function (the manual page is here):

int i;
scanf("%i", &i);
printf("You entered: %i\n", i);

The first argument to scanf is a string containing formatting specification(s) for the type of input expected. In this case we expect an int.

The second argument should look pretty weird to you. How is scanf able to modify the value of i? What's that ampersand symbol? The answer is that we're actually passing a pointer to i's location, not i itself - so the value of i can be modified from within scanf. There will be much more on pointers later.

EXERCISE 3: Write a program temperature.c that asks the user for a temperature in Fahrenheit and prints the temperature converted to Celsius. Assume that the input is handled as a float, instead of as an int. The relevant formula is:

temp_c = (temp_f - 32) * 5/9

For example, here's what a sample run of temperature.c might now look like:

What is the temperature in degrees Fahrenheit? 42.5
42.500000 degrees Fahrenheit is 5.833333 degrees Celsius.

(If you're interested: there are variants on the formatting specifiers that limit the number of zeroes. Read up on that further if you like.)

Check the TemperatureTests section after running ./test-m to see if you have the output correct.

C has basically the same syntax for if statements, for loops, while loops and do/while loops as Java. However, there is a critical and very subtle difference with if statements that you should be aware of. C doesn't have boolean variables, exactly. It just has integers, so false is 0 and true is any non-zero value. So the following abomination is legal C code:

int x = 1;
int y = 0;
if (x - y) {
    printf("You entered two different numbers!\n");
} else {
    printf("You entered the same number!\n");
}

If you want to make your code more clear, you can use the library stdbool.h as follows:

#include <stdbool.h>

int main() {
    bool x = true;
}

However, this is just syntactic sugar, which is a fancy phrase which means that we haven't changed the underlying language, we've just changed the syntax a little bit. With the above, x is really just an int, and true is really just a 1. Why does this matter? Try running the following BUGGY code and see what happens. (You can create a new file for this called buggy.c for it.)

#include <stdio.h>

int main() {
    int x = 3;
    if (x = 5) {
        printf("x must be 5, even though I assigned it to 3.\n");
    }
}

Happily, clang does try to warn you when you do this.

EXERCISE 4: What happens if the user enters an impossible temperature in Exercise 3? Absolute zero, which is the coldest temperature anything can possibly be, is -459.67 Fahrenheit. Modify temperature.c so that if a temperature lower than that is input, it displays the text "Invalid temperature." This test is in the E tests. Run ./test-e to see if you have this part correct.