How can I sort a vector containing pair<int,int> elements? Sorting is done as per the compare function

Question

typedef pair<int,int>ii;
vector<ii>vii;

sort(vii.begin(),vii.end(),comp);

 ii comp(ii a,ii b){
   if(a.first>b.first)
   return a;
   else if(a.first==b.first){
    if(a.second>b.second)
    return a;
    else
    return b;
   }
   else{
    return b;
   }
 }

//This way it is throwing a compilation error. Can you guide how to sort this vector as per //the conditions given in the compare function.

Solution

Presumably you want to sort them lexicographically, in increasing order. You can do this:

std::sort(vii.begin(), vii.end(), std::greater<std::pair<int,int>>());

The comparison functor is a binary predicate, and must return a boolean, and implement strict weak ordering. std::greater<std::pair<int,int>> does that for you.

Comments

[Alex Reinking]

It might be worth mentioning that the compare function you supply to sort must return bool, not the larger element.

[juanchopanza]

@AlexReinking Correct, I was in the process of adding that.

[Amit Kumar]

it is throwing an error. In function 'int main()':| error: '>>' should be '> >' within a nested template argument list|

[juanchopanza]

@AmitKumar You can replace >> by > >, or compile in C++11 mode. I was assuming a C++11 compiler.