Hacking Functions

Functions
Sometimes there will be a set of instructions the programmer knows he will
need several times. These instructions can be grouped into a smaller subprogram
called a function. In other languages, functions are known as subroutines
or procedures. For example, the action of turning a car actually
consists of many smaller instructions: Turn on the appropriate blinker, slow
down, check for oncoming traffic, turn the steering wheel in the appropriate
direction, and so on. The driving directions from the beginning of this chapter
require quite a few turns; however, listing every little instruction for every
turn would be tedious (and less readable). You can pass variables as arguments
to a function in order to modify the way the function operates. In this case,
the function is passed the direction of the turn.
Function Turn(variable_direction)
{
Activate the variable_direction blinker;
Slow down;
Check for oncoming traffic;
while(there is oncoming traffic)
{
Stop;
Watch for oncoming traffic;
}
Turn the steering wheel to the variable_direction;
while(turn is not complete)
{
if(speed < 5 mph)
Accelerate;
}
Turn the steering wheel back to the original position;
Turn off the variable_direction blinker;
}
This function describes all the instructions needed to make a turn. When
a program that knows about this function needs to turn, it can just call this
function. When the function is called, the instructions found within it are
executed with the arguments passed to it; afterward, execution returns to
where it was in the program, after the function call. Either left or right can
be passed into this function, which causes the function to turn in that
direction.
By default in C, functions can return a value to a caller. For those
familiar with functions in mathematics, this makes perfect sense. Imagine a
function that calculates the factorial of a number—naturally, it returns the
result.
In C, functions aren’t labeled with a “function” keyword; instead, they
are declared by the data type of the variable they are returning. This format
looks very similar to variable declaration. If a function is meant to return an
P rogramming 17
integer (perhaps a function that calculates the factorial of some number x),
the function could look like this:
int factorial(int x)
{
int i;
for(i=1; i < x; i++)
x *= i;
return x;
}
This function is declared as an integer because it multiplies every value
from 1 to x and returns the result, which is an integer. The return statement
at the end of the function passes back the contents of the variable x and ends
the function. This factorial function can then be used like an integer variable
in the main part of any program that knows about it.
int a=5, b;
b = factorial(a);
At the end of this short program, the variable b will contain 120, since
the factorial function will be called with the argument of 5 and will return 120.
Also in C, the compiler must “know” about functions before it can use
them. This can be done by simply writing the entire function before using it
later in the program or by using function prototypes. A function prototype is
simply a way to tell the compiler to expect a function with this name, this
return data type, and these data types as its functional arguments. The actual
function can be located near the end of the program, but it can be used anywhere
else, since the compiler already knows about it. An example of a function
prototype for the factorial() function would look something like this:
int factorial(int);
Usually, function prototypes are located near the beginning of a program.
There’s no need to actually define any variable names in the prototype, since
this is done in the actual function. The only thing the compiler cares about is
the function’s name, its return data type, and the data types of its functional
arguments.
If a function doesn’t have any value to return, it should be declared as void,
as is the case with the turn() function I used as an example earlier. However,
the turn() function doesn’t yet capture all the functionality that our driving
directions need. Every turn in the directions has both a direction and a street
name. This means that a turning function should have two variables: the
direction to turn and the street to turn on to. This complicates the function
of turning, since the proper street must be located before the turn can be
made. A more complete turning function using proper C-like syntax is listed
below in pseudo-code.
18 0x200
void turn(variable_direction, target_street_name)
{
Look for a street sign;
current_intersection_name = read street sign name;
while(current_intersection_name != target_street_name)
{
Look for another street sign;
current_intersection_name = read street sign name;
}
Activate the variable_direction blinker;
Slow down;
Check for oncoming traffic;
while(there is oncoming traffic)
{
Stop;
Watch for oncoming traffic;
}
Turn the steering wheel to the variable_direction;
while(turn is not complete)
{
if(speed < 5 mph)
Accelerate;
}
Turn the steering wheel right back to the original position;
Turn off the variable_direction blinker;
}
This function includes a section that searches for the proper intersection
by looking for street signs, reading the name on each street sign, and storing
that name in a variable called current_intersection_name. It will continue to
look for and read street signs until the target street is found; at that point, the
remaining turning instructions will be executed. The pseudo-code driving
instructions can now be changed to use this turning function.
Begin going East on Main Street;
while (there is not a church on the right)
Drive down Main Street;
if (street is blocked)
{
Turn(right, 15th Street);
Turn(left, Pine Street);
Turn(right, 16th Street);
}
else
Turn(right, 16th Street);
Turn(left, Destination Road);
for (i=0; i<5; i++)
Drive straight for 1 mile;
Stop at 743 Destination Road;
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Functions aren’t commonly used in pseudo-code, since pseudo-code is
mostly used as a way for programmers to sketch out program concepts before
writing compilable code. Since pseudo-code doesn’t actually have to work,
full functions don’t need to be written out—simply jotting down Do some
complex stuff here will suffice. But in a programming language like C, functions
are used heavily. Most of the real usefulness of C comes from collections of
existing functions called libraries.