Language: C++

#include<iostream>
#include<string>
#include <algorithm>
using namespace std;

int main()
{
	int i;
	int count[26]={0};
	string ss;

	getline(cin,ss);

	transform (ss.begin(), ss.end(), ss.begin(), tolower);

	for(i=0;i<ss.size();i++)
	{
		if(((int)ss[i] >=97)&&((int)ss[i]<=122))
		{
			count[((int)ss[i]-97)]++;
		}
	}
	
	cout << "a : " << count[0] << endl ; 
	cout << "b : " << count[1] << endl ;
	cout << "c : " << count[2] << endl ;
	cout << "d : " << count[3] << endl ;
	cout << "e : " << count[4] << endl ;
	cout << "f : " << count[5] << endl ;
	cout << "g : " << count[6] << endl ;
	cout << "h : " << count[7] << endl ;
	cout << "i : " << count[8] << endl ;
	cout << "j : " << count[9] << endl ;
	cout << "k : " << count[10] << endl ;
	cout << "l : " << count[11] << endl ;
	cout << "m : " << count[12] << endl ;
	cout << "n : " << count[13] << endl ;
	cout << "o : " << count[14] << endl ;
	cout << "p : " << count[15] << endl ;
	cout << "q : " << count[16] << endl ;
	cout << "r : " << count[17] << endl ;
	cout << "s : " << count[18] << endl ;
	cout << "t : " << count[19] << endl ;
	cout << "u : " << count[20] << endl ;
	cout << "v : " << count[21] << endl ;
	cout << "w : " << count[22] << endl ;
	cout << "x : " << count[23] << endl ;
	cout << "y : " << count[24] << endl ;
	cout << "z : " << count[25] << endl ;

	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:14:19: error: no matching function for call to 'transform(std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, <unresolved overloaded function type>)'
   14 |         transform (ss.begin(), ss.end(), ss.begin(), tolower);
      |         ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:61,
                 from a.cc:3:
/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:   template argument deduction/substitution failed:
a.cc:14:19: note:   couldn't deduce template parameter '_UnaryOperation'
   14 |         transform (ss.begin(), ss.end(), ss.begin(), tolower);
      |         ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/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, 4 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, 4 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, 4 provided