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======Exercise 7: More Variables, Some Math======

   Let's get familiar with more things you can do with variables by
   declaring various ints, floats, chars, and doubles. We'll then use
   these in various math expressions so you get introduced to C's basic
   math.
#include <stdio.h>

int main(int argc, char *argv[])
{
    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;
}

   Here's what's going on in this little bit of nonsense:

   ex7.c:1-4
          The usual start of a C program.

   ex7.c:5-6
          Declare an int and double for some fake bug data.

   ex7.c:8-9
          Print out those two, so nothing new here.

   ex7.c:11
          Declare a huge number using a new type long for storing big
          numbers.

   ex7.c:12-13
          Print out that number using %ld which adds a modifier to the
          usual %d. Adding 'l' (the letter ell) means "print this as a
          long decimal".

   ex7.c:15-17
          Just more math and printing.

   ex7.c:19-21
          Craft up a depiction of your bug rate compared to the bugs in
          the universe, which is a completely inaccurate calculation. It's
          so small though that we have to use %e to print it in scientific
          notation.

   ex7.c:24
          Make a character, with a special syntax '\0' which creates a
          'nul byte' character. This is effectively the number 0.

   ex7.c:25
          Multiply bugs by this character, which produces 0 for how much
          you should care. This demonstrates an ugly hack you find
          sometimes.

   ex7.c:26-27
          Print that out, and notice I've got a %% (two percent chars) so
          I can print a '%' (percent) character.

   ex7.c:28-30
          The end of the main function.

   This bit of source is entirely just an exercise, and demonstrates how
   some math works. At the end, 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:
$ make ex7
cc -Wall -g    ex7.c   -o ex7
$ ./ex7
======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 breaking the printf by passing in the
   wrong arguments. See what happens when you try to print out that
   nul_byte variable too with %s vs. %c. When you break it, run it under
   Valgrind to see what it says about your breaking attempts.

======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
       that one 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.


   Copyright (C) 2010 Zed. A. Shaw

   Credits