using lower_bound or upper_bound on Structure in C++ - c++

Question

#include <iostream>
#include <algorithm>
using namespace std;
struct arr
{
int a;int b;
}a[1000];
bool comp(arr &lhs, arr &rhs)
{ return lhs.a < rhs.a ; }
int main()
{
int n,i ;
sort(a,a+n,comp);
int ind= lower_bound(a,a+n,x,comp)-a;
return 0;
}
Error message :
/usr/include/c++/4.9/bits/predefined_ops.h: In instantiation of 'bool
__gnu_cxx::__ops::_Iter_comp_val<_Compare>::operator()(_Iterator, _Value&) [with _Iterator = arr*; _Value = const int; _Compare = bool ()(arr&, arr&)]': /usr/include/c++/4.9/bits/stl_algobase.h:965:30:
required from '_ForwardIterator std::__lower_bound(_ForwardIterator,
_ForwardIterator, const _Tp&, _Compare) [with _ForwardIterator = arr; _Tp = int; _Compare = __gnu_cxx::__ops::_Iter_comp_val]' /usr/include/c++/4.9/bits/stl_algo.h:2036:46: required from
'_FIter std::lower_bound(_FIter, _FIter, const _Tp&, _Compare) [with
_FIter = arr*; _Tp = int; _Compare = bool ()(arr&, arr&)]' prog.cpp:28:38: required from here
/usr/include/c++/4.9/bits/predefined_ops.h:141:37: error: invalid
initialization of reference of type 'arr&' from expression of type
'const int' { return bool(_M_comp(__it, __val)); }
^
I wish to use lower_bound over the struct to search for the value x that equals to a[i].a ? I have constructed the comparator function accordingly but getting a long error message of which I am not able to make anything.
What changes are required for the function to run.

Answer 1

lower_bound returns an iterator, not an index of the element found. You'll need to use std::distance to retrieve the index, but usually, the iterator is what you need/want for further processing.
Also note an index is usually returned as an std::size_t, not an int as you seem to assume.

Answer 2

As pointed in previous answers, lower_bound returns an iterator.
The type of x in lower_bound is not clear, x should also be the same type as that of the type in the container and compare function.
In this case, the type of x should be arr.