convert letters to uppercase in c 2b 2b

1char choice; 
2// it will instantly transform it to upper case without the need
3// to convert it to int first
4choice = (char)toupper(choice);
5

1str[i] = toupper(str[i]);

1#include <iostream>
2#include <string>
3using namespace std;
4
5int main()
6{
7    char letter;
8
9    cout << "You will be asked to enter a character.";
10    cout << "\nIf it is a lowercase character, it will be converted to uppercase.";
11    cout << "\n\nEnter a character. Press . to stop: ";
12
13    cin >> letter;
14
15    if(islower(letter))
16    {
17        letter = isupper(letter);
18        cout << letter;
19    }
20
21    while(letter != '.')
22    {
23        cout << "\n\nEnter a character. Press . to stop: ";
24        cin >> letter;
25
26        if(islower(letter))
27        {
28            letter = toupper(letter);
29            cout << letter;
30        }
31    }
32
33    return 0;
34}
35