Language: C++

#include<bits/stdc++.h>
using namespace std;

using ll=long long;
const int inf=100000000;

int in(){
  int a;
  cin >> a;
  return a;
}//int型関数の受け取り

void forin(int* x,int y){
  for (int i=0;i<y;i++){
    cin >> x[i];
  }
}//(x,y) int型配列x[y]の受け取り

int intpow(int x,int y){
  double a=x,b=y;
  return pow(a,b);
}//(x,y) xをy乗

int intfabs(int x){
  double a=x;
  return fabs(a);
}//(x) xの絶対値

int fact(int x){
  if (x == 1) return 1;
  return x*fact(x-1);
}//(x) x!

int fib(int x){
  if (x <= 2) return x;
  int a[x];
  a[0]=1;
  a[1]=2;
  for (int i=2;i<x;i++){
    a[i]=a[i-1]+a[i-2];
  }
  return a[x-1];
}//(x) フィボナッチ数列のx番目

int div1(ll x){
  return x%1000000007;
}

main(){
  int n=in(),k=in();
  string s;
  cin >> s;
  transform(s.at(k),s.at(k),::tolower);
  cout << s << endl;
}

Compiler output:
a.cc:49:1: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
   49 | main(){
      | ^~~~
a.cc: In function 'int main()':
a.cc:53:12: error: no matching function for call to 'transform(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&, __gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&, int (&)(int) noexcept)'
   53 |   transform(s.at(k),s.at(k),::tolower);
      |   ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:61,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algo.h:4209:5: note: candidate: 'template<class _IIter, class _OIter, class _UnaryOperation> _OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation)'
 4209 |     transform(_InputIterator __first, _InputIterator __last,
      |     ^~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4209:5: note:   candidate expects 4 arguments, 3 provided
/usr/include/c++/14/bits/stl_algo.h:4247:5: note: candidate: 'template<class _IIter1, class _IIter2, class _OIter, class _BinaryOperation> _OIter std::transform(_IIter1, _IIter1, _IIter2, _OIter, _BinaryOperation)'
 4247 |     transform(_InputIterator1 __first1, _InputIterator1 __last1,
      |     ^~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4247:5: note:   candidate expects 5 arguments, 3 provided
In file included from /usr/include/c++/14/algorithm:86:
/usr/include/c++/14/pstl/glue_algorithm_defs.h:156:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryOperation> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::transform(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _UnaryOperation)'
  156 | transform(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result,
      | ^~~~~~~~~
/usr/include/c++/14/pstl/glue_algorithm_defs.h:156:1: note:   candidate expects 5 arguments, 3 provided
/usr/include/c++/14/pstl/glue_algorithm_defs.h:162:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator, class _BinaryOperation> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::transform(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator, _BinaryOperation)'
  162 | transform(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
      | ^~~~~~~~~
/usr/include/c++/14/pstl/glue_algorithm_defs.h:162:1: note:   candidate expects 6 arguments, 3 provided