Modulo in a conditional expression - c++

Question

I'm very new to programming and am starting out with Bjourne's book: Programming principles and practice c++ 2nd edition. Exercise 8 chapter 3 he asks for:
"Write a program to test an integer value to determine if it is odd or even...Hint: See the remainder (modulo)operator in §3.4."
I can do that with something like:
int main() {
int n;
cout << "Enter an integer: ";
cin >> n;
if ( n%2 == 0) {
cout << n << " is even.";
}
else {
cout << n << " is odd.";
}
return 0;
}
But he gives in his website his own solution:
int main()
{
int val = 0;
cout << "Please enter an integer: ";
cin >> val;;
if (!cin) error("something went bad with the read");
string res = "even";
if (val%2) res = "odd";
cout << "The value " << val << " is an " << res << " number\n";
keep_window_open();
}
catch (runtime_error e) {
cout << e.what() << '\n';
keep_window_open("~");
}
/*
Note the technique of picking a default value for the result ("even") and changing it only
if needed.
The alternative would be to use a conditional expression and write
string res = (val%2) ? "even" : "odd";
What is
string res = "even";
if (val%2) res = "odd";
and
string res = (val%2) ? "even" : "odd";
actually doing? I haven't seen him explaining those before in the book. Also, the last code, It gives me "odd" result when I type an even value and gives an "even " result when I type and odd one. what is going on? Sorry for the long post, hope I could explain what I need...

Answer 1

The ? : is the ternary operator.
if (val%2) res = "odd";
is just a rather terse version of
if (val%2) {
res = "odd";
}
And note that if(...) actually doesn't care whether the value is "true" or "false." It just checks for zero or nonzero. So it's equivalent to
if( val%2 != 0)
The second command line : string res = (val%2) ? "even" : "odd"; similary is a short way for writing:
string res;
if(val%2 != 0){
res = "even";
}
else{
res = "odd";
}
The syntax for those kind of commands is condition ? value_if_true : value_if_false

Answer 2

Adding to the previous answers, you have to note the boolean (or "true" values) are 0s and 1s, (0 being false and 1 being true in boolean algebra)
So, when
string res = (val % 2) ? "even" : "odd";
Note that when you give and odd value it will always return number 1 which is "true", and viceversa for even numbers.
You would have to turn those around for the program to work.

Answer 3

He just instead of writing multiple brackets, does not use them at all
string res = "even"; //default value
if (val%2) res = "odd"; //in case it is odd, value changes
//output or threat in some way value.
instead
string res = (val%2) ? "even" : "odd";
Simply a short way of writing the same if/else you wrote before.