Exercise 7: Variables and Types

You should be getting a grasp of how a simple C program is structured, so let's do the next simplest thing and make some variables of different types:

View Source file ex7.c Only

#include <stdio.h>

int main(int argc, char *argv[])
{
    int distance = 100;
    float power = 2.345f;
    double super_power = 56789.4532;
    char initial = 'A';
    char first_name[] = "Zed";
    char last_name[] = "Shaw";

    printf("You are %d miles away.\n", distance);
    printf("You have %f levels of power.\n", power);
    printf("You have %f awesome super powers.\n", super_power);
    printf("I have an initial %c.\n", initial);
    printf("I have a first name %s.\n", first_name);
    printf("I have a last name %s.\n", last_name);
    printf("My whole name is %s %c. %s.\n",
            first_name, initial, last_name);

    int bugs = 100;
    double bug_rate = 1.2;

    printf("You have %d bugs at the imaginary rate of %f.\n",
            bugs, bug_rate);

    long universe_of_defects = 1L * 1024L * 1024L * 1024L;
    printf("The entire universe has %ld bugs.\n", universe_of_defects);

    double expected_bugs = bugs * bug_rate;
    printf("You are expected to have %f bugs.\n", expected_bugs);

    double part_of_universe = expected_bugs / universe_of_defects;
    printf("That is only a %e portion of the universe.\n",
            part_of_universe);

    // this makes no sense, just a demo of something weird
    char nul_byte = '\0';
    int care_percentage = bugs * nul_byte;
    printf("Which means you should care %d%%.\n", care_percentage);

    return 0;
}

In this program, we're declaring variables of different types and then printing them using different printf format strings. I can break it down as follows:

• ex7.c:1-4: The usual start of a C program.
• ex7.c:5-7: Declare an int, float, and double for some fake calculations.
• ex7.c:8-10: Declare some character data. First a single character, then two character array strings.
• ex7.c:12-19: Use printf to print out each of these declared variables.
• ex7.c:21-22: Declare an integer and a double variable to show you don't have to declare variables at the top of a function.
• ex7.c:24-25: Print those variables using printf again.
• ex7.c:27-28: Calculate a really large number using a long integer type. Notice the use of L notation to specify long integer constants (1L, 1024L).
• ex7.c:30-35: Print out some calculations and math using the variables we have.
• ex7.c:37-40: This is actually not good form, but demonstrate that C will let you use char variables in math expressions by multiplying a char by an int, then print it.
• ex7.c:42-43: The end of the main function.

This source file demonstrates how some math works with different types of variables. At the end of the program, it also demonstrates something you see in C but not in many other languages. To C, a character is just an integer. It's a really small integer, but that's all it is. This means you can do math on them, and a lot of software does just that, -for good or bad.

This last bit is your first glance at how C gives you direct access to the machine. We'll be exploring that more in later exercises.

What You Should See

As usual, here's what you should see for the output:

View Source file ex7.sh-session Only

$ make ex7
cc -Wall -g ex7.c   -o ex7
$ ./ex7
You are 100 miles away.
You have 2.345000 levels of power.
You have 56789.453200 awesome super powers.
I have an initial A.
I have a first name Zed.
I have a last name Shaw.
My whole name is Zed A. Shaw.
You have 100 bugs at the imaginary rate of 1.200000.
The entire universe has 1073741824 bugs.
You are expected to have 120.000000 bugs.
That is only a 1.117587e-07 portion of the universe.
Which means you should care 0%.

How to Break It

Again, go through this and try to break the printf by passing in the wrong arguments. See what happens if you try to print out the nul_byte variable along with %s versus %c. When you break it, run it under the debugger to see what it says about what you did.

Extra Credit

• Make the number you assign to universe_of_defects various sizes until you get a warning from the compiler.
• What do these really huge numbers actually print out?
• Change long to unsigned long and try to find the number that makes it too big.
• Go search online to find out what unsigned does.
• Try to explain to yourself (before I do in the next exercise) why you can multiply a char and an int.