在我的do-while循环之前获取输入，但是无法正确响应该输入

Question

I was trying to make a simple input and do-while program where I asked for the code to keep repeating if the user didn't input 3.14 but for some reason it isn't running properly. It shows no errors when it I typed it.

Scanner num = new Scanner(System.in);
double pi = num.nextDouble();
do {
    System.out.println("What is pi?");
    pi = num.nextDouble();
}
while( pi != 3.14);
System.out.println("Yup pi = 3.14");

Answer 1

You are asking for input before the loop without notifying the user, take out the first scanner next. like so

Scanner num = new Scanner(System.in);
double pi = 0.0;
do {
    System.out.println("What is pi?");
    pi = num.nextDouble();
}
while( pi != 3.14);

System.out.println("Yup pi = 3.14");

Answer 2

You are reading pi in twice, one on line 2 and once on line 5. You only need to declare pi in line 2 and your code will work (see below). Because the body of a do-while loop will always run once you only need one line to ask. You would need to have two lines if you had used only a basic while loop.

Scanner num = new Scanner(System.in);
double pi;
do {
    System.out.println("What is pi?");
    pi = num.nextDouble();
} while(pi != 3.14);

System.out.println("Yup pi = 3.14");

Answer 3

This is probably related to the fact that comparing floats exactly is bad. Instead, use something like this:

if(Math.abs(pi - (double) 3.14) < epsilon)

Where epsilon is a number which regulates the precision (Something like 0.001 should be enough in this case). See here for more details: What's wrong with using == to compare floats in Java?