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Lets look at an example program:
#include<stdio.h>#include<conio.h>int mult ( int x, int y );
int main(){
int x;int y;
printf("Please input two numbers to be multiplied: ");scanf("%d%d",&x,&y);
printf("The product of your two numbers is %d\n", mult ( x, y )) ;return 0;
getch();}
int mult ( int x, int y ){
return x * y;}
The parameters of a function are ordinary local variables. The program creates them, and initializes them with the values of the corresponding arguments, when a function call occurs. Their scope is the function block. A function can change the value of a parameter without affecting the value of the argument in the context of the function call. In the following listing, the factorial( ) function, which computes the factorial of a whole number, modifies its parameter n in the process.
//factorial( ) calculates n!, the factorial of a non-negative number n.
// For n>0, n! is the product of all integers from 1 to n inclusive.
// 0! equals 1.// Argument: A whole number, with type unsigned int.
// Return value: The factorial of the argument, with type long double.long double factorial(register unsigned int n)
{long double f = 1;
while ( n>1 )
f *= n--;return f;
}
Although the factorial of an integer is always an integer, the function uses the type long double in order to accommodate very large results. As the above listing illustrates, you can use the storage class specifier register in declaring function parameters. The register specifier is a request to the compiler to make a variable as quickly accessible as possible. No other storage class specifiers are permitted on function parameters.
If you need to pass an array as an argument to a function, you would generally declare the corresponding parameter in the following form:
type name[ ]
Because array names are automatically converted to pointers when you use them as function arguments, this statement is equivalent to the declaration:
type *name
When you use the array notation in declaring function parameters, any constant expression between the brackets ([ ]) is ignored. In the function block, the parameter name is a pointer variable, and can be modified. Thus the function addArray() in the following listing modifies its first two parameters as it adds pairs of elements in two arrays.
// addArray( ) adds each element of the second array to the
// corresponding element of the first (i.e., "array1 += array2", so to speak).// Arguments: Two arrays of float and their common length.
// Return value: None.void addArray( register float a1[ ], register const float a2[ ], int len )
{register float *end = a1 + len;
for ( ; a1<end; ++a1, ++a2 )
*a1 += *a2;}
An equivalent definition of the addArray() function, using a different notation for the array parameters, would be:
void addArray( register float *a1, register const float *a2, int len )
{ /* Function body as earlier. */ }
An advantage of declaring the parameters with brackets ([ ]) is that human readers immediately recognize that the function treats the arguments as pointers to an array, and not just to an individual float variable. But the array-style notation also has two peculiarities in parameter declarations :
Here is an example that combines both of these possibilities:
int func( long array[const static 5] ){ /* ... */ }
In the function defined here, the parameter array is a constant pointer to long, and so cannot be modified. It points to the first of at least five array elements.
In the following listing, the maximum( ) function's third parameter is a two-dimensional array of variable dimensions.
// The function maximum( ) obtains the greatest value in a
// two-dimensional matrix of double values.
// Arguments: The number of rows, the number of columns, and the matrix.// Return value: The value of the greatest element.
double maximum( int nrows, int ncols, double matrix[nrows][ncols] ){
double max = matrix[0][0];for ( int r = 0; r<nrows; ++r )
for ( int c = 0; c<ncols; ++c )
if ( max<matrix[r][c])
max = matrix[r][c];return max;
}
The parameter matrix is a pointer to an array with ncols elements.
Since C passes arguments to functions by value, there is no direct way for the called function to alter a variable in the calling function. For instance, a sorting routine might exchange two out-of-order arguments with a function called swap. It is not enough to write
swap(a, b);
where the swap function is defined as
void swap(int x, int y) /* WRONG */
{int temp;
temp = x;x = y;
y = temp;}
Because of call by value, swap can't affect the arguments a and b in the routine that called it. The function above swaps copies of a and b.
The way to obtain the desired effect is for the calling program to pass pointers to the values to be changed:
swap(&a,&b);
Since the operator&produces the address of a variable,&a is a pointer to a. In swap itself, the parameters are declared as pointers, and the operands are accessed indirectly through them.
void swap(int *px, int *py) /* interchange *px and *py */
{int temp;
temp = *px;*px = *py;
*py = temp;}
{
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