Going with the Flow
So far, all the programs we've seen simply compute answers to
In fact, they all engage in the same computations regardless
of what input is given.
One of the most powerful features of a computer, however, is
its ability to make decisions.
That is to change it's course of action according to the
results of some computations.
(Of course, we may also make decisions based on the input
directly; we're just doing a trivial computation and looking
at that result.)
Throughout Part 3, we'll be looking at decision making
in programs and at their expression in C.
Flow of Control
In order to better understand this type of decision making
we want to take a look at the concept of flow of control.
If we have a code segment like:
x = y + z;
y = z * 2;
we say that the control flows from one statement directly
to the next.
This is illustrated in the following
Such straight-line or linear control flow will be the
driving force in a program unless we introduce some
mechanisms to break in that flow.
Two-Way Control Branching
The first alternative mode of control flow that we'll look
at is the two-way decision.
The flow chart for this mode of control looks like:
The idea here is that we first test the condition in
If the condition is true, then control flows down the left
branch of the diagram.
If it is false, then control flows down the right branch
of the chart.
Either way, the control flow joins back up at the bottom
of the diagram and continues downward.