Summing the indices of repeating numbers

[learner999]

Hello everyone,

I am a beginner of C++ and doing some execercises. I am blocked in a point.
I have a list like this
s=[2 2 3 3 3 3 1 1 1 5 1 4 2]
I need to sum of the indices of the same numbers in this list. let me explain with an example; I assign the variable X for all repeating numbers so what I want for example for 2
X0+X1+X12
for example for 1
X6+X7+X8+X10

Since I am newly learner , I really stucked at this point, I will be very happy , if you can help me

[seeplus]

#include <iostream>
#include <iterator>

int main()
{
	constexpr int list[] {2, 2, 3, 3, 3, 3, 1, 1, 1, 5, 1, 4, 2};

	int num;

	std::cout << "Please enter number to sum: ";
	std::cin >> num;

	int sum {0};

	for (int l = 0; l < std::size(list); ++l)
		if (list[l] == num)
			sum += l;

	std::cout << "Sum of indexes is: " << sum << std::endl;
}

This will sum the indexes (starting at 0) for those elements that equal the entered number.

[lastchance]

#include <iostream>
#include <vector>
using namespace std;


int matchNums( const vector<int> &V, int target )
{
   int total = 0;
   for ( int i = 0; i < V.size(); i++ ) if ( V[i] == target ) total += i;
   return total;
}


int main()
{
   vector<int> s = { 2, 2, 3, 3, 3, 3, 1, 1, 1, 5, 1, 4, 2 };
   int n = 2;
   cout << "Sum for target " << n << " is " << matchNums( s, n );
}

Sum for target 2 is 13

You are using an awful lot of Python-like terminology. "Lists" don't have indices in C++.

You also won't be able to distinguish between "not found" and an element only appearing in the first position (index 0) of your container.

[learner999]

Thank you so much @seeplus

[learner999]

Thank you so much @lastchance;

[TomCPP]

IMHO using std::accumulate in this case makes better solution.

    const int table[] = {1,4,1,1,6,1,7,1};
    int n {1};
    cout << accumulate( cbegin(table) , cend(table) , 0 , [&]( auto& a, auto& b ){ return b==n?a+(&b-&*cbegin(table)):a; } ) << endl;

[seeplus]

The requirement is to sum the indexes, not the values themselves.

[lastchance]

#include <iostream>
#include <vector>
#include <numeric>
using namespace std;

int main()
{
   vector<int> s = { 2, 2, 3, 3, 3, 3, 1, 1, 1, 5, 1, 4, 2 };
   int n = 2, i = 0;
   cout << "Sum for target " << n << " is " << accumulate( s.begin(), s.end(), 0, [&]( int a, int b ){ return a+(i++)*(b==n); } ) << '\n';
}

Sum for target 2 is 13