1.1.2 Function Body

The rest of the function definition is called the function body. Like every function body, this one starts with ‘{’, ends with ‘}’, and contains zero or more statements and declarations. Statements specify actions to take, whereas declarations define names of variables, functions, and so on. Each statement and each declaration ends with a semicolon (‘;’).

Statements and declarations often contain expressions; an expression is a construct whose execution produces a value of some data type, but may also take actions through “side effects” that alter subsequent execution. A statement, by contrast, does not have a value; it affects further execution of the program only through the actions it takes.

This function body contains no declarations, and just one statement, but that one is a complex statement in that it contains nested statements. This function uses two kinds of statements:

return

The return statement makes the function return immediately. It looks like this:

return value;

Its meaning is to compute the expression value and exit the function, making it return whatever value that expression produced. For instance,

return 1;

returns the integer 1 from the function, and

return fib (n - 1) + fib (n - 2);

returns a value computed by performing two function calls as specified and adding their results.

if…else

The if…else statement is a conditional. Each time it executes, it chooses one of its two substatements to execute and ignores the other. It looks like this:

if (condition)
  if-true-statement
else
  if-false-statement

Its meaning is to compute the expression condition and, if it’s “true,” execute if-true-statement. Otherwise, execute if-false-statement. See if-else Statement.

Inside the if…else statement, condition is simply an expression. It’s considered “true” if its value is nonzero. (A comparison operation, such as n <= 2, produces the value 1 if it’s “true” and 0 if it’s “false.” See Numeric Comparisons.) Thus,

if (n <= 2)
  return 1;
else
  return fib (n - 1) + fib (n - 2);

first tests whether the value of n is less than or equal to 2. If so, the expression n <= 2 has the value 1. So execution continues with the statement

return 1;

Otherwise, execution continues with this statement:

return fib (n - 1) + fib (n - 2);

Each of these statements ends the execution of the function and provides a value for it to return. See return Statement.

Calculating fib using ordinary integers in C works only for n < 47, because the value of fib (47) is too large to fit in type int. The addition operation that tries to add fib (46) and fib (45) cannot deliver the correct result. This occurrence is called integer overflow.

Overflow can manifest itself in various ways, but one thing that can’t possibly happen is to produce the correct value, since that can’t fit in the space for the value. See Integer Overflow.

See Functions, for a full explanation about functions.