Why does printf print an integer as a double? [duplicate]

Question

printf("%f", 1.0); //prints 1.0

but

printf("%f", 1);  // prints 0.0

How did the conversion happen?

Answer 1

printf("%f", 1); causes undefined behavior, because double is expected, but you passed an int. There is no explanation of why it prints 0.0, because the behavior is undefined.