sort function is giving unexpected results

[sdream]

i have structure data as :

typedef struct 
    {
    	int num,index;
    }data;

and my sort function as :

bool comp(const data &i , const data &j)
    {
    		return (i.num < j.num);
    }

and here is my main function :

        int t,n,p,tcase,i,j;
    	vector<data> a;
    	vector<int > b ;
    
    		cin >> n;
    					
    		a.resize(n);
    		b.resize(n);
    		
    		j=0;
    		
    		for(i=0 ; i<2*n ; i++)
    		{
    			if(i%2)
    			{
    				b.emplace_back();
    				cin >> b[j];
    				j++;
    			}
    			else
    			{
    				a.emplace_back();
    				cin >> a[j].num;
    				a[j].index = j;
    				
    			}
    		}
    	
    		for(i=0 ; i<n ; i++)
    		{
    			cout << a[i].num << endl;
    		}
    		
    		cout << endl;
    		
    		sort(a.begin() , a.end() , comp);
    
    		for(i=0 ; i<n ; i++)
    		{
    		
    			cout << a[i].num << endl;
    		}
    						
    		a.erase(a.begin() , a.end());
    		b.erase(b.begin() , b.end());		
    		tcase++;

So when i run my program with following input :
4
15
25
30
35
45
50
10
20

then my expected answer should be :
10
15
30
45
But my program is giving :
0
0
0
0

Where am i doing wrong ?

[Peter87]

On line 7-8 you resize the vectors so that they contain n zero value. Then in the loop you add n additional values so vectors will have a size of 2*n.

After a has been sorted it will contain the values: 0 0 0 0 10 15 30 45

[sdream]

thanx @Peter87 . I am new to c++ STL container , thts y i took it differently.