toupper() function returns the uppercase of the parameter if its a letter, otherwise returns the same parameter without any change. It converts string to uppercase characters. The letter to convert may differ based on the locale settings, the default letters would be from A-Z.

    int toupper( int ch);

   #include <iostream.h>
   #include <ctype.h>
   int main()
     {
       int n;
       char c;
       char str[]="aaaaa3434bbbb";
       cout << "String after using toupper is::";
       for ( n=0 ; str[n]!='\0' ; n++)
         {
            c=str[n];
            putchar (toupper(c));
                         
         }
       return 0;
     }

    String after using toupper is::AAAAA343BBBB

In the above example, "toupper()" function is used to convert the string to uppercase characters, but the integers are displayed as it is.