Hey! 1) Yes in general when you are working on one-file only program you set your main at the end, it's typical in C/C++ and in fact a lot of languages. If you're on a bigger program your main will be in a separate file. 2) We set main function at the end to avoid declare prototype in the code, here you add preprocessor treatment and it's useless, if main is at the end it does know what are your functions. A more correct way to declare prototype is to declare them in a header file and not in the cpp file 3) I don't, naming things in very hard but you have to follow one principle in software engineering, it's: separation of concerns. Each function must have one thing to do and not a lot of things. That means your code must be clean and cut! (I'm not a native english speaker)

Đăng Tú I would recommand you something like this: # include <iostream> # include <cstdio> using namespace std ; void printParity ( int n) { string number[ 9 ] = { "one" , "two" , "three" , "four" , "five" , "six" , "seven" , "eight" , "nine" }; if (n >= 1 && n <= 9 ) cout << number[n - 1 ] << endl ; else if (n > 9 ) cout << ((n % 2 == 1 ) ? "odd" : "even" ) << endl ; } void checkNumbers () { int minN, maxN; cin >> minN >> maxN; for ( int i = minN; i <= maxN; i++) { printParity(i); } } int main () { checkNumbers(); return 0 ; } I don't know if this code compile but some recommendations about code clarity Avoid using global declaration like for your number array, you just use it in your checkN function (same for the declaration of min and max N) Try to not do some business logic in your main, keep it clean it's better A hard thing in software engineering is naming, you should use explicit functions names! Declare your functions BEFORE the main to avoid that kind of declaration void checkN(int);