This is because C language operates type conversion. Three INT type data operations get int type, after /3, only shaping. 1.0 is a floating-point number, which is obtained after sum shaping operation. Floating-point numbers are only after /3.

1 /3 = 0; (int /int = int)

1.0 / 3 = 0.3333333....(float / int = float)

I suggest you check the data type conversion rules.