C :: Explain Use Of Question Mark

how the '?' works at the end of the "int luhn" function ( sum += odd ? digit : m[digit]; ) and at the end of main ( printf( "%s
", luhn( buf )? "Valid" : "Invalid" ); ).

I know it's determining what results to report, but the usage is not clear to me. I guess what I'm asking, since I'm not familiar with the use of '?' here, is what the if/else equivalent of these two lines would be.

Code:

#include <stdio.h>
#include <string.h>
int luhn(const char* cc)
{
const int m[]= { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9};

[Code]....