My paired vectors are not getting sorted in C++

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I have made paired vectors and I want to sort one vector according to the values of other vector, but when I run my program the vectors aren't getting sorted. They just remain the same. And I think it is because of the pair that I have made I have actually just copied the code to perform paired sorts from internet and replaced the arrays with vectors.

// Sort an array according to other using pair in STL.

// Function to sort vector b according to the order defined by vector a

void pairsort(vector<int> a, vector<int> b, int n)
{    
    pair<int, int> pairt[n];

// Storing the respective array

// elements in pairs.

    for (int i = 0; i < n; i++)

    {
        pairt[i].first = a[i];

        pairt[i].second = b[i];
    }

// Sorting the pair array.

sort(pairt, pairt + n);

// Modifying original arrays

for (int i = 0; i < n; i++)

{
    a[i] = pairt[i].first;

    b[i] = pairt[i].second;
}
}

// Driver function

int main()
{

vector<int> a{60, 100, 120};
vector<int> c{3, 2, 4};


int n = sizeof(c) / sizeof(c[0]);
pairsort(c, a, n);
}
std-pair std vector sorting c++
1 Answers

Copies of the original vectors are passed to the arguments vector<int> a and vector<int> b. Modifying the copies will not affect what are passed in caller.

Add & after the types to make them references to reflect changes in the function to the caller.

Also Variable-Length Array like pair<int, int> pairt[n]; is not in the standard C++. You should use std::vector<pair<int, int> > pairt(n); instead.

void pairsort(vector<int>& a, vector<int>& b, int n) // make a and be references
{    
    std::vector<pair<int, int> > pairt(n); // use std::vector instead of non-standard VLA

After making this change, this usage of sort is wrong with std::vector:

sort(pairt, pairt + n);

It should be:

sort(pairt.begin(), pairt.end());

One more point is that sizeof(c) / sizeof(c[0]) in the main() function is not a correct way to retrieve the number of elements in the vector. It should be replaced with c.size().

user_40623540
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