Loop user input until conditions met

Question

I need to ask the user to input a number to be used as the start of a range, and then input another number that is the end of the range. The start number has to be 0 or grea

Answer 1

Easy with do-while:

Scanner keyboard = new Scanner(System.in);
int startr, endr;
boolean good = false;
do
{
  System.out.println("Enter the Starting Number of the Range: ");
  startr = keyboard.nextInt();
  if(startr % 10 == 0 && startr >= 0)
    good = true;
  else
    System.out.println("Numbers is not divisible by 10");
}
while (!good);

good = false;
do
{
    System.out.println("Enter the Ending Number of the Range: ");
    endr = keyboard.nextInt();
    if(endr % 10 == 0 && endr <= 1000)
      good = true;
    else
      System.out.println("Numbers is not divisible by 10");
}
while (!good);

// do stuff

Answer 2

You need to use a while, something like:

while conditionsMet is false
    // gather input and verify
    if user input valid then
        conditionsMet = true;
end loop

should do it.

Answer 3

The all-purpose procedure is:

1. Read the input in an infinite loop.
2. Use a break; statement to exit the loop when the conditions are met.

Example:

Scanner keyboard = new Scanner(System.in);
int startr, endr;

for (;;) {
    System.out.println("Enter the starting number of the range: ");
    startr = keyboard.nextInt();
    if (startr >= 0 && startr % 10 == 0) break;
    System.out.println("Number must be >= 0 and divisible by 10.");
}

for (;;) {
    System.out.println("Enter the ending number of the range: ");
    endr = keyboard.nextInt();
    if (endr <= 1000 && endr % 10 == 0) break;
    System.out.println("Number must be <= 1000 and divisible by 10.");
}

If after invalid input you want to display just the error message without repeating the initial prompt message, move the initial prompt message just above/outside the loop.

If you do not have need for the separate error message, you can re-arrange the code to use a do-while loop to check the conditions, which is just a little shorter:

Scanner keyboard = new Scanner(System.in);
int startr, endr;

do {
    System.out.println("Enter the starting number of the range.");
    System.out.println("Number must be >= 0 and divisible by 10: ");
    startr = keyboard.nextInt();
} while (!(startr >= 0 && startr % 10 == 0));

do {
    System.out.println("Enter the ending number of the range.");
    System.out.println("Number must be <= 1000 and divisible by 10: ");
    endr = keyboard.nextInt();
} while (!(endr <= 1000 && endr % 10 == 0));