Programming in C

136 Chapter 8 Working with Functions
pass a negative argument to the squareRoot function. Although this approach might
seem reasonable, it does have its drawbacks. Eventually, you would develop a program
that used the squareRoot function but which forgot to check the argument before calling
the function. If a negative number were then passed to the function, the program
would go into an infinite loop as described and would have to be aborted.
A much wiser and safer solution to the problem is to place the onus of checking the
value of the argument on the squareRoot function itself. In that way, the function is
“protected” from any program that used it. A reasonable approach to take is to check the
value of the argument x inside the function and then (optionally) display a message if the
argument is negative.The function can then immediately return without performing its
calculations. As an indication to the calling routine that the squareRoot function did not
work as expected, a value not normally returned by the function could be returned. 1
The following is a modified squareRoot function, which tests the value of its argument
and which also includes a prototype declaration for the absoluteValue function as
described in the previous section.
/* Function to compute the square root of a number.
If a negative argument is passed, then a message
is displayed and -1.0 is returned. */
float squareRoot (float x)
{
const float epsilon = .00001;
float guess = 1.0;
float absoluteValue (float x);
if ( x < 0 )
{
printf ("Negative argument to squareRoot.\n");
return -1.0;
}
while ( absoluteValue (guess * guess - x) >= epsilon )
guess = ( x / guess + guess ) / 2.0;
return guess;
}
If a negative argument is passed to the preceding function, an appropriate message is displayed,
and the value –1.0 is immediately returned to the calling routine. If the argument
is not negative, calculation of the square root proceeds as previously described.
1.The square root routine in the standard C library is called sqrt and it returns a domain error if a
negative argument is supplied.The actual value that is returned is implementation-defined. On
some systems, if you try to display such a value, it displays as nan, which means not a number.