Functions on Call
When we use functions in C, a very specific sequence of events
takes place:

Each of the
arguments
are evaluated.
That is, we perform any calculations that are necessary to get a single
value for that argument.

The function is
called
with the arguments
passed
to
it.
Calling a function temporarily
transfers the control of the program to that function.
By passing of arguments we mean that the values which were determined for the
arguments in Step 1, are made available to function being called.

The function executes the statements in it's
body
.

The called function then
returns
to the calling function.
(We will often refer to the called function as the
callee
and the
calling function as the
caller
.)
The callee may compute a
return value
which is then sent back
to the caller.
By way of example, let us look at the call to the function
sqrt()
:
sqrt(square(side_a) + square(side_b))
that we saw in
Part 21
.
Here the function
sqrt()
(which computes square roots) has one
argument, the expression:
square(side_a) + square(side_b)
When we call
sqrt()
, we first evaluate this expression.
In order to evaluate it, we must make two function calls and add the
results.
For the first of these calls, we pass the value of the variable
side_a
to the function
square()
.
Suppose that the value of
side_a
is 4.0.
Then after we've called
square()
on
side_a
but
before calling it on
side_b
it's as if the expression were
16.0 + square(side_b)
Then we call square on
side_b
.
If
side_b
is 3.0, then we now have the expression:
16.0 + 9.0
which evaluates to 25.0 which is the value passed to
sqrt()
.
It then returns with the value 5.0.
So our return statement is as if we had typed:
return 25.0;
and 25.0 becomes the value of the call:
pythag(adjacent, opposite)
where
adjacent
is 4.0 and
opposite
is 3.0.