Java For循环数据验证

Question

I am currently doing some java excersises in uni and ive been stuck on this one for about 5 hours now! I am practising For loops and have the loop ask 5 times for a number from 1 to 3. When testing, if I enter an invalid selection it carries on and includes the invalid selection as a zero, I have got an error message working when an invalid input is entered but it still carries on until the loop finishes, I know there is a way to return to the beggining of the selection but I cant figure it out. I have searched everywhere for a solution but cannot find it! I know it cant be much and I'm not back in uni for a few days so I cant ask the lecturer and I would really like to crack on to the next chapter.

Here is my code (I know its probably a bit scrappy!!), thanks, Rob

   import java.util.Scanner;

/* this is s a survey of how 5 people sweeten thier coffee */

class coffee
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);

        int person, preference, nothing, sugar, sweetner;

        String pluralone = "People dont";
        String pluraltwo = "People use";
        String pluralthree = "People use";

        person = 0;
        preference = 0;
        nothing = 0;
        sugar = 0;
        sweetner = 0;

        for (person = 1; person <= 5; person++)

        {
            System.out.println("How do you sweeten your coffee");
            System.out.println("1. I Don't");
            System.out.println("2. With Sweetner");
            System.out.println("3. With Sugar");

            preference = input.nextInt();

            if (preference != 1 && preference != 2 && preference != 3)
                System.out.println("Sorry that is not a valid option");

            else if (preference == 1)
                nothing++;

            else if (preference == 1)
                sweetner++;

            else
                sugar++;
        }

        System.out.println("Survey Report");

        System.out.println("#############");

        if (nothing < 2)

        {
            pluralone = "person doesnt";
        }

        System.out.println(nothing + "  " + " " + pluralone + " sweeten thier coffee");

        if (sweetner < 2)

        {
            pluraltwo = "person uses";
        }

        System.out.println(sweetner + "  " + pluraltwo + " " + "sweetner to sweeten thier coffee");

        if (sugar < 2)

        {
            pluralthree = "person uses";
        }

        System.out.println(sugar + "  " + pluralthree + " " + "sugar to sweeten thier coffee ");

    }
}

Answer 1

just ask for the users selection in a while loop so that it doesn't continue until a valid option has been entered, something like:

preference = input.nextInt();
while (preference != 1 && preference != 2 && preference != 3) {
    System.out.println("Sorry that is not a valid option");
    preference = input.nextInt();
}

alternatively you could decrement person in your if statement to cause another iteration of the for loop, but that's a bit hacky:

if (preference != 1 && preference != 2 && preference != 3) {
    System.out.println("Sorry that is not a valid option");
    person--;
}

Answer 2

If you change your for loop to this

for (person = 1; person <= 5; person ++)
{
    System.out.println ("How do you sweeten your coffee");
    System.out.println ("1. I Don't");
    System.out.println ("2. With Sweetner");
    System.out.println ("3. With Sugar");

    preference = input.nextInt();

    while(preference != 1 && preference != 2 && preference != 3) {
        System.out.println ("Sorry that is not a valid option");
        System.out.println ("How do you sweeten your coffee");
        System.out.println ("1. I Don't");
        System.out.println ("2. With Sweetner");
        System.out.println ("3. With Sugar");
        preference = input.nextInt();
    }

    if(preference == 1) {
        nothing ++;
    } else if(preference == 2) {
        sweetner ++;
    } else if(preference == 3) {
        sugar ++;
    }
}

This will fix it

Answer 3

Replace the if/else with an Switch/Case:

preference = input.nextInt();
switch(preference) {
  case 1: 
    nothing++;
    break;
  case 2:
    sweetner++;
    break;
  case 3:
    sugar++;
    break;
  default:
    System.out.println("Sorry, thats not a valid option! Please pick a valid option");
    preference = input.nextInt();
    person--;
    break;

}

Answer 4

I did the while loop and it works, thank you very much Dan. While loops are the next part in the book that im moving onto so its given me a head start too. Many thanks to you and everyone else who replied, a couple of the other things suggested seem a little advanced for me but I know I can always refer to the comments if I need them in the future. Thanks again, Robin.