Suppose the list of pairs <(1, 3),(3, 2), (2, 4)> and i want to result as
<(1, 3), (2, 4), (3, 2)>. How can i implement it using some STL in C++?
Also sorting with respect to second element of the pair?
like: <(3, 2), (1, 3), (2, 4)> !
STL Pairs already have the less than (and other relational) operator defined for them. The behavior is such that a < b iff a.first < b.first || a.first == b.first && a.second < b.second
That definition is compatible with your task, therefore simply firing up a sort() or stable_sort() should work.
#include <vector>
#include <pair>
#include <algorithm>
using namespace std;
...
vector<pair<int, int>> a;
sort(a.begin(), a.end());
2 Likes
thank you for your reply.
may you explain how this sort is working and what if I wish to sort with respect to y value of (x, y)!
The solution provided by @knb_dtu is accurate for the question you asked.
But still, if you feel any problem in understanding or implementing like that, you can have a look on this piece of code.
Define a struct in c named point (or anything you wish)
How to define ?
struct point{
int x,y;
}
After that, make a array of elements using line like this : struct point a[10000];
-> according to your use…
Now its turn to sort the data for you, here is the code :
void sort(struct point* data, int n) {
int j,i;
for(i=0;i<n;i++)
{
for(j=i+1;j<n;j++)
{
if(data[i].x>data[j].x)
{
struct data temp = data[i];
data[i] = data[j];
data[j] = temp;
}
}
}
}
Just wrote using an O(n^2) sort algo, you can easily understand, and then work on even implementing merge sort/quick sort using this struct which would be simple if you get the concept. 
another alternative approach would be using std::set as the following link illustrates
what if I wish to sort with respect to y value of (x, y)?
using namespace std;
// compare function to sort pairs with respect to first element.
bool cmpfunc(const pair<int,int> &u,const pair<int,int> &v)
{
if(u.first<v.first)
return true;
return false;
}
// compare function to sort pairs with respect ot second element.
/*
bool cmpfunc (const pair<int,int> &u , const pair<int,int> &v)
{
if(u.second<v.second)
return true;
return false;
}
*/
int main()
{
//initialize vector of pairs
vector<pair<int,int> v;
sort(v.begin(),v.end(),cmpfunc());
return 0;
}
You can solve it using
[sort][1] function in STL C++.
When you will sort a pair of vectors using sort() then it will automatically sort wrt to first element of pair i.e x.
To sort with respect to y , trick is to pass a compare() function as third parameter in function and work accordingly
bool compare(pair p,pair q){
//to solve w.r.t to y
return p.second < q.second;
}
Here is complete [implementation][2].
[1]: http://www.cplusplus.com/reference/algorithm/sort/
[2]: http://ideone.com/fdV1pg