C++ functions that process arrays need to be informed about

• The kind of data in the array
• The location of the beginning of the array
• And the number of elements in the array

The traditional C/C++ approach to functions that process arrays

• Is to pass a pointer to the start of the array as one argument
  • And to pass the size of the array as a second argument
    • (The pointer tells the function both where to find the array and the kind of data in it.)
      • That gives the function the information it needs to find all the data

There is another approach to giving a function the information it needs: to specify a range of elements

• This can be done by passing two pointers
• One identifying the start of the array and one identifying the end of the array
• The C++ Standard Template Library, for example, generalizes the range approach
• The STL approach uses the concept of “one past the end” to indicate an extent
• That is, in the case of an array, the argument identifying the end of the array would be a pointer to the location just after the last element
• For example, suppose you have this declaration (A-1):
• Then the two pointers elboud and elboud + 20 define the range
• First, elboud, being the name of the array, points to the first element
• The expression elboud + 19 points to the last element (that is, elboud[19]), so elboud + 20 points to one past the end of the array
• Passing a range to a function tells it
• Which elements to process
• (A-2) use two pointers to specify a range

Example of (A-1)

double elbuod[20];

Example of (A-2)

// arrfun4.cpp -- functions with an array range
#include <iostream>
const int ArSize = 8;
int sum_arr(const int * begin, const int * end);
int main()
{
	using namespace std;
	int cookies[ArSize] = {1,2,4,8,16,32,64,128};
// some systems require preceding int with static to
// enable array initialization
 
	int sum = sum_arr(cookies, cookies + ArSize);
	cout << "Total cookies eaten: " << sum << endl;
	sum = sum_arr(cookies, cookies + 3); // first 3 elements
	cout << "First three eaters ate " << sum << " cookies.\n";
	sum = sum_arr(cookies + 4, cookies + 8); // last 4 elements
	cout << "Last four eaters ate " << sum << " cookies.\n";
	return 0;
}
 
// return the sum of an integer array
int sum_arr(const int * begin, const int * end)
{
	const int * pt;
	int total = 0;
	for (pt = begin; pt != end; pt++)
		total = total + *pt;
	return total;
}

Here’s the output of the program in (A-2):

Program Notes

In (A-2), notice the for loop in the sum_array() function (A-3)

• It sets pt to point to the first element to be processed (the one pointed to by begin) and adds *pt (the value of the element) to total
  • Then the loop updates pt by incrementing it, causing it to point to the next element
    • The process continues as long as pt != end
      • When pt finally equals end, it’s pointing to the location following the last element of the range, so the loop halts

Example of (A-3)

for (pt = begin; pt != end; pt++)
	total = total + *pt;

Second, notice how the different function calls specify different ranges within the array (A-4)

• The pointer value cookies + ArSize points to the location following the last element
• (The array has ArSize elements, so cookies[ArSize - 1] is the last element, and its address is cookies + ArSize – 1.)
• So the range cookies, cookies + ArSize specifies the entire array
• Similarly, cookies, cookies + 3 specifies the first three elements, and so on
• Note, by the way, that the rules for pointer subtraction imply that
• In sum_arr(), the expression end - begin is an integer value equal to the number of elements in the range

Example of (A-4)

int sum = sum_arr(cookies, cookies + ArSize);
...
sum = sum_arr(cookies, cookies + 3); // first 3 elements
...
sum = sum_arr(cookies + 4, cookies + 8); // last 4 elements