java中循环if和else语句

Question

I'm working on this program that asks for model numbers of cars infinitely until the person inputs 0 to break the loop. When i run it and input a number it just infinitely loops either your car is defective or it is not defective until it crashes. I'm pretty stuck right now any help would be greatly appreciated.

Scanner input = new Scanner(System.in);

System.out.print("Enter a model number or 0 to quit: ");
modelNum = input.nextInt();

while (modelNum != 0) {

    if (modelNum >= 189 && modelNum <= 195) {
        System.out.println("Your car is defective it must be repaired");
    } else if (modelNum == 189 || modelNum == 221) {
        System.out.println("Your car is defective it must be repaired");
    } else if (modelNum == 780) {
        System.out.println("Your car is defective it must be repaired");
    } else if (modelNum == 119 || modelNum == 179) {
        System.out.println("Your car is defective it must be repaired");

    } else {
        System.out.println("Your car is not defective");
    }
    if (modelNum == 0) {
        System.out.println("end");
        break;
    }
}

Answer 1

It's because you never ask the user for another input. You should do so before the end of the loop.

Answer 2

Include the this part into your loop:

Scanner input = new Scanner(System.in);    

   System.out.print("Enter a model number or 0 to quit: ");
   modelNum = input.nextInt(); 

Answer 3

You have to ask for a new value to be evaluated:

while (modelNum != 0) {
    // if conditions
    modelNum = input.nextInt();
}

Also note that:

if (modelNum == 0) {
    System.out.println("end");
    break;
}

won't be necessary because if the last value is 0 the condition in the while loop will be false and won't loop again.

Last thing: why you have all those if-else-if when they all do the same thing (print "Your car is defective it must be repaired"). This will be enough:

while (modelNum != 0) {
    if ((modelNum >= 189 && modelNum <= 195) || modelNum == 221 || modelNum == 780 || modelNum == 119 || modelNum == 179) {
        System.out.println("Your car is defective it must be repaired");
    } else {
        System.out.println("Your car is not defective");
    }
    modelNum = input.nextInt();
}

Answer 4

如果输入 0，循环将中断，因此最后一个 if 语句将永远不会运行。

Answer 5

This loop just tells you if the car is defective or not depending on the model number, but you never tell the program to exit the loop if the car is defective. To do so you have to put break statements into each if statement of the loop.

Moreover this statement is useless:

if(modelNum == 0) { System.out.println("end"); break;

since if u enter 0 the loop won't start.