Hacking Loops

While/Until Loops
Another elementary programming concept is the while control structure,
which is a type of loop. A programmer will often want to execute a set of
instructions more than once. A program can accomplish this task through
looping, but it requires a set of conditions that tells it when to stop looping,
lest it continue into infinity. A while loop says to execute the following set of
instructions in a loop while a condition is true. A simple program for a hungry
mouse could look something like this:
While (you are hungry)
{
Find some food;
Eat the food;
}
The set of two instructions following the while statement will be repeated
while the mouse is still hungry. The amount of food the mouse finds each
time could range from a tiny crumb to an entire loaf of bread. Similarly, the
number of times the set of instructions in the while statement is executed
changes depending on how much food the mouse finds.
Another variation on the while loop is an until loop, a syntax that is
available in the programming language Perl (C doesn’t use this syntax). An
until loop is simply a while loop with the conditional statement inverted. The
same mouse program using an until loop would be:
Until (you are not hungry)
{
Find some food;
Eat the food;
}
Logically, any until-like statement can be converted into a while loop.
The driving directions from before contained the statement Continue on
Main Street until you see a church on your right. This can easily be changed into a
standard while loop by simply inverting the condition.
While (there is not a church on the right)
Drive down Main Street;

For Loops
Another looping control structure is the for loop. This is generally used when
a programmer wants to loop for a certain number of iterations. The driving
direction Drive straight down Destination Road for 5 miles could be converted to
a for loop that looks something like this:
For (5 iterations)
Drive straight for 1 mile;
In reality, a for loop is just a while loop with a counter. The same statement
can be written as such:
Set the counter to 0;
While (the counter is less than 5)
P rogramming 11
{
Drive straight for 1 mile;
Add 1 to the counter;
}
The C-like pseudo-code syntax of a for loop makes this even more
apparent:
For (i=0; i<5; i++)
Drive straight for 1 mile;
In this case, the counter is called i, and the for statement is broken up
into three sections, separated by semicolons. The first section declares the
counter and sets it to its initial value, in this case 0. The second section is like
a while statement using the counter: While the counter meets this condition,
keep looping. The third and final section describes what action should be
taken on the counter during each iteration. In this case, i++ is a shorthand
way of saying, Add 1 to the counter called i.
Using all of the control structures, the driving directions from page 6
can be converted into a C-like pseudo-code that looks something like this:
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 on 15th Street;
Turn left on Pine Street;
Turn right on 16th Street;
}
Else
Turn right on 16th Street;
Turn left on Destination Road;
For (i=0; i<5; i++)
Drive straight for 1 mile;
Stop at 743 Destination Road;