how to convert string into lowercase in c++ code example

Example 1: c++ string functions lowercase

transform(su.begin(), su.end(), su.begin(), ::toupper);
transform(sl.begin(), sl.end(), sl.begin(), ::tolower);

Example 2: function to write a string in loercase in c++

/* tolower example */
#include <stdio.h>
#include <ctype.h>
int main ()
{
  int i=0;
  char str[]="Test String.\n";
  char c;
  while (str[i])
  {
    c=str[i];
    putchar (tolower(c));
    i++;
  }
  return 0;
}