Computing index using Pointers Returned by STL Functions in C++

In this tutorial we will learn how to compute index using pointers which returned by STL functions in C++. Many built-in C++ functions return pointers to memory places that provide an address for the requested number but have no relationship to the computed value's actual index in a container. To find the maximum element in a code, for example, we use std::max element(), which returns the memory location rather than the index of the needed element. Because we sometimes need the index corresponding to that address, the following example explains how we can do so.

C++ Program:

// C++ code displaying max element's return value ()
#include <bits/stdc++.h>
using namespace std;


int main ()
{
	// initialization of vector
	vector<int> vi1 = { 2, 4, 5, 7, 3 };


	// printing maximum element
	cout << "The maximum element is : "
		<< ( *max_element ( vi1.begin (), vi1.end () ) );
	// printing the position in memory
	cout << "\nThe position of the maximum element is : ";
	printf ( "%d", max_element ( vi1.begin (), vi1.end () ) );
}

Output:

The maximum element is : 7
The position of the maximum element is : 202059452

The output in the preceding output does not appear to be correct, hence there is a need for methods to compute the exact index of the position of the requested element in the supplied container. These are some examples:

• Subtracting the first iterator
• Using std::distance()

• Subtracting the First Iterator: In this method, we simply subtract the container's beginning pointer, in the case of vectors, the "begin()" iterator that corresponds to the address of the container's first element; by subtracting this, we may obtain the exact index of the needed value with respect to the container.

C++ Program:

// CPP code displaying how to print the exact position after subtracting the first iterator.
#include <bits/stdc++.h>
using namespace std;


int main ()
{
	// initialization vector
	vector<int> vi1 = { 2, 4, 5, 7, 3 };


	// printing the maximum element
	cout << "The max element is : "
		<< ( *max_element ( vi1.begin (), vi1.end () ) );


	// printing the position
	cout << "\nThe position of maximum element is : "
		<< max_element ( vi1.begin (), vi1.end () ) - vi1.begin ();
	;
	return 0;
}

Output:

The max element is : 7
The position of maximum element is : 3

• Using std::distance() : Another option for computing the desired position is to use distance(), which bypasses the first iterator as the first parameter and the intended point as the second.

C++ Program:

// C++ code showing how to output a precise position using distance ()
#include <bits/stdc++.h>
using namespace std;


int main ()
{
	// initialization of vector
	vector<int> vi1 = { 2, 4, 5, 7, 3 };


	// printing the maximum element
	cout << "The maximum element is : "
		<< ( *max_element ( vi1.begin (), vi1.end () ) );


	// printing the position
	cout << "\nThe position of maximum element is : ";
	cout << distance ( vi1.begin (),max_element ( vi1.begin (), vi1.end () ) );
	return 0;
}

Output:

The maximum element is : 7
The position of maximum element is : 3