Language: C++

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/tree_policy.hpp> // Including tree_order_statistics_node_update

using namespace std;
using namespace __gnu_pbds;

//find_by_order gives kth-largest element
//order_of_key gives the number of items in the set that are strictly smaller!!

// OT.find_by_order(k-1)
// 
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;

#define db(x) cerr << #x << "=" << x << endl
#define db2(x, y) cerr << #x << "=" << x << "," << #y << "=" << y << endl
#define db3(x, y, z) cerr << #x << "=" << x << "," << #y << "=" << y << "," << #z << "=" << z << endl
#define db4(x, y, z, w) cerr << #x << "=" << x << "," << #y << "=" << y << "," << #z << "=" << z << "," << #w << "=" << w << endl
#define ll long long
#define pb push_back
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define mp make_pair
#define X first
#define Y second
#define sz(x) (int)((x).size())
#define pii pair<int,int>
#define MOD (ll)(998244353)
#define rep(i, n)    for(int i = 0; i < (n); ++i)
#define repA(i, a, n)  for(int i = a; i <= (n); ++i)
#define repD(i, a, n)  for(int i = a; i >= (n); --i)
#define trav(a, x) for(auto& a : x)
typedef vector<int> vi;
#define inf (ll)(1e18)
#define double long double
#define int long long

//////////////////////
// const int N = 316330;


int32_t main() {

    ios::sync_with_stdio(false);
    cin.tie(0); 
    cout.tie(0);

    int n,k;
    string st;
    cin >> n >> st >> k;
    k--;
    rep(i,st) {
        if(st[i] != st[k])
            st[i] = st[k];
    }
    cout << st << '\n';
}

Compiler output:
a.cc: In function 'int32_t main()':
a.cc:29:39: error: no match for 'operator<' (operand types are 'long long int' and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                       ^ ~~~
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181,
                 from a.cc:1:
/usr/include/c++/14/bits/regex.h:1143:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1143 |     operator<(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1143:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/bits/regex.h:1224:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const sub_match<_BiIter>&)'
 1224 |     operator<(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1224:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/bits/regex.h:1317:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(const sub_match<_BiIter>&, __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)'
 1317 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1317:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/bits/regex.h:1391:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1391 |     operator<(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1391:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/bits/regex.h:1485:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1485 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1485:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/bits/regex.h:1560:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1560 |     operator<(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1560:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/bits/regex.h:1660:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1660 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1660:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/bits/stl_pair.h:1045:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1045 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1045:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'const std::pair<_T1, _T2>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:448:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  448 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:448:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  493 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1694 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1760 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/string_view:673:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  673 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:673:5: note:   template argument deduction/substitution failed:
a.cc:29:43: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'long long int'
   29 | #define rep(i, n)    for(int i = 0; i < (n); ++i)
      |                                           ^
a.cc:52:5: note: in expansion of macro 'rep'
   52 |     rep(i,st) {
      |     ^~~
/usr/include/c++/14/string_view:680:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  680 |     operator