# Functions in C

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In C, a function is equivalent to a subroutine or function in Fortran, or a procedure or function in Pascal. A function provides a convenient way to encapsulate some computation, which can then be used without worrying about its implementation. With properly designed functions, it is possible to ignore how a job is done; knowing what is done is sufficient. C makes the sue of functions easy, convinient and efficient; you will often see a short function defined and called only once, just because it clarifies some piece of code.

So far we have used only functions like printf, getchar and putchar that have been provided for us; now it's time to write a few of our own. Since C has no exponentiation operator like the ** of Fortran, let us illustrate the mechanics of function definition by writing a function power(m,n) to raise an integer m to a positive integer power n. That is, the value of power(2,5) is 32. This function is not a practical exponentiation routine, since it handles only positive powers of small integers, but it's good enough for illustration.(The standard library contains a function pow(x,y) that computes xy.)

Here is the function power and a main program to exercise it, so you can see the whole structure at once.

```   #include <stdio.h>

int power(int m, int n);

/* test power function */
main()
{
int i;

for (i = 0; i < 10; ++i)
printf("%d %d %d\n", i, power(2,i), power(-3,i));
return 0;
}

/* power:  raise base to n-th power; n >= 0 */
int power(int base, int n)
{
int i,  p;

p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
```
A function definition has this form:
```return-type function-name(parameter declarations, if any)
{
declarations
statements
}
```
Function definitions can appear in any order, and in one source file or several, although no function can be split between files. If the source program appears in several files, you may have to say more to compile and load it than if it all appears in one, but that is an operating system matter, not a language attribute. For the moment, we will assume that both functions are in the same file, so whatever you have learned about running C programs will still work.

The function power is called twice by main, in the line

```   printf("%d %d %d\n", i, power(2,i), power(-3,i));
```
Each call passes two arguments to power, which each time returns an integer to be formatted and printed. In an expression, power(2,i) is an integer just as 2 and i are. (Not all functions produce an integer value;)

The first line of power itself,

```    int power(int base, int n)
```
declares the parameter types and names, and the type of the result that the function returns. The names used by power for its parameters are local to power, and are not visible to any other function: other routines can use the same names without conflict. This is also true of the variables i and p: the i in power is unrelated to the i in main.

We will generally use parameter for a variable named in the parenthesized list in a function. The terms formal argument and actual argument are sometimes used for the same distinction.

The value that power computes is returned to main by the return: statement. Any expression may follow return:

```   return expression;
```
A function need not return a value; a return statement with no expression causes control, but no useful value, to be returned to the caller, as does ``falling off the end'' of a function by reaching the terminating right brace. And the calling function can ignore a value returned by a function.

You may have noticed that there is a return statement at the end of main. Since main is a function like any other, it may return a value to its caller, which is in effect the environment in which the program was executed. Typically, a return value of zero implies normal termination; non-zero values signal unusual or erroneous termination conditions. In the interests of simplicity, we have omitted return statements from our main functions up to this point, but we will include them hereafter, as a reminder that programs should return status to their environment.

The declaration

```    int power(int base, int n);
```
just before main says that power is a function that expects two int arguments and returns an int. This declaration, which is called a function prototype, has to agree with the definition and uses of power. It is an error if the definition of a function or any uses of it do not agree with its prototype.

parameter names need not agree. Indeed, parameter names are optional in a function prototype, so for the prototype we could have written

```    int power(int, int);
```
Well-chosen names are good documentation however, so we will often use them.

A note of history: the biggest change between ANSI C and earlier versions is how functions are declared and defined. In the original definition of C, the power function would have been written like this:

```   /* power:  raise base to n-th power; n >= 0 */
/*         (old-style version) */
power(base, n)
int base, n;
{
int i, p;

p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
```
The parameters are named between the parentheses, and their types are declared before opening the left brace; undeclared parameters are taken as int. (The body of the function is the same as before.)

The declaration of power at the beginning of the program would have looked like this:

```    int power();
```
No parameter list was permitted, so the compiler could not readily check that power was being called correctly. Indeed, since by default power would have been assumed to return an int, the entire declaration might well have been omitted.

The new syntax of function prototypes makes it much easier for a compiler to detect errors in the number of arguments or their types. The old style of declaration and definition still works in ANSI C, at least for a transition period, but we strongly recommend that you use the new form when you have a compiler that supports it.

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