Lecture07.md

Sorting of vectors of pairs

Suppose to have a vector of pairs: vector<pair<int, int>> vect; When sorting a vector of pairs, we have 4 different cases:

1. Ascending sorting in according to the first element of the pair. sort(vect.begin(), vect.end());
2. Ascending sorting in according to the second element.

// Driver function to sort the vector elements 
// by second element of pairs 
bool sortbysec(const pair<int,int> &a, 
              const pair<int,int> &b) 
{ 
    return (a.second < b.second); 
}
sort(vect.begin(), vect.end(), sortBySec);

3. Descending sorting in according to the first element.

bool inverseSorting(const pair<int, int> &a,
					const pair<int, int > &b)
{
	return (a.first > b.first);
}

// using modified sort() function to sort 
 sort(vect.begin(), vect.end(), inverseSorting);

4. Descending sorting in according to the second element.

bool sortbysecdesc(const pair<int,int> &a, 
                   const pair<int,int> &b) 
{ 
    if(p1.second > p2.second)
        return true;
    else if(p1.second == p2.second && p1.first < p2.first)
        return true;
    return false;
} 
// using modified sort() function to sort 
sort(vect.begin(), vect.end(), sortbysecdesc); 

Comparison Operators

The function sort requires that a comparison operator is defined for the data type of the element to be sorted.

User defined structus do not have a comparison operator automatically.

Comparison functions

// Sort vector of strng primarily by lenht and secondary by alphabetical order
bool comp(string a, string, b)
{
    if(a.size() != b.size()) return a.size() < b.size();
    return a < b;
}
sort(v.begin(), v.end(), comp);