How do i repeat an if statement until it's false in Java

Question

So i have this code that is supposed to generate random words of length the user decides with these conditions:

[1] Every other letter is a random vowel (letter + vowel + letter + vowel...etc)
[2] Two vowels cannot meet together (ie: bout, caravaa)

I got over the first part condition, and the second condition needs some edit. i used an if statement to check if letter1 or letter2 happen to be vowels, and if true generate generate different letters. Problem is those newly generated letters have a chance of being vowels. How do i keep repeating the if statement until it's false? (Letter1 and Letter2 are not vowels)

import java.util.Random;

public class NickName_1
{
     public static void main(String[] args)
{
    java.util.Scanner scanner = new java.util.Scanner(System.in);

    int letterNum = 6;
    Random rn = new Random();
    char letter1 = (char) (rn.nextInt(26) + 'a');
    char letter2 = (char) (rn.nextInt(26) + 'a');
    char[] vowels = { 'a', 'e', 'i', 'o', 'u' };
    char vowel;


    while (true)
    {
        //Ask user for word's length and store it
        System.out.println();
        System.out.print("Please enter the length of the word in letters: ");
        letterNum = scanner.nextInt();

        vowel = vowels[(int)(Math.random() * 5)];

        for (int i = 0; i < (letterNum / 3); i++)
        {
            letter1 = (char) (rn.nextInt(26) + 'a');
            letter2 = (char) (rn.nextInt(26) + 'a');
            vowel = vowels[(int)(Math.random() * 5)];

            // if letter1 or letter2 equal a vowel then regenerate letters
            if (letter1 == 'a' || letter1 == 'u' || letter1 == 'i' || letter1 == 'e' || letter1 == 'o' || 
                letter2 == 'a' || letter2 == 'u' || letter2 == 'i' || letter2 == 'e' || letter2 == 'o')

            {
                letter1 = (char) (rn.nextInt(26) + 'a');
                letter2 = (char) (rn.nextInt(26) + 'a');
            }
                System.out.print(letter1 + "" + vowel + "" + letter2);
        }
    }
}
}

Answer 1

You have mentioned about two criteria. But the code does not fulfill them even replacing if with while.

I have modified your code so that it could fulfill your criteria. I want to point out the major things below:

1. You are trying to print 3 letters at a time. That means input should be multiple of 3, otherwise the program would fail. I would suggest you to print a letter at a time.

2. Your need is letter + vowel + letter + vowel..., but you are printing letter + vowel + letter + letter + vowel + letter.... Rather I recommend keep a boolean variable and change it in every turn, check whether it is the turn of vowel or consonant, then do the corresponding work in their own code blocks.

3. It is a bad practice to end up a program with exception. Thats why I have added a try-catch and wrapped the input in it.

4. To make your code more convenient, Carcigenicate suggested a good way in this comment -- "Make a function or set and check for membership". I would suggest you to make another array for consonants as you made for vowels and pick a random one.

Here is the modified code in ideone. Below is your version of the code:

import java.util.Random;
/* Name of the class has to be "Main" only if the class is public. */
class NickName_1
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        java.util.Scanner scanner = new java.util.Scanner(System.in);

        int letterNum = 6;
        Random rn = new Random();
        char letter = (char) (rn.nextInt(26) + 'a');
        boolean shouldCons = true;
        char[] vowels = { 'a', 'e', 'i', 'o', 'u' };

        while (true)
        {
            //Ask user for word's length and store it
            System.out.println();
            System.out.print("Please enter the length of the word in letters: ");
            try{
                letterNum = scanner.nextInt();
            }catch(Exception e){
                break;
            }
    
            for (int i = 0; i < letterNum; i++, shouldCons^=true)
            {
                if(shouldCons) {
                    letter = (char) (rn.nextInt(26) + 'a');
                    while (letter == 'a' || letter == 'u' || letter == 'i' || letter == 'e' || letter == 'o')
                    {
                        letter = (char) (rn.nextInt(26) + 'a');
                    }
                }
                else letter = vowels[(int)(Math.random() * 5)];
              
                System.out.print(letter);
            }
        }
    }
}

---

Answer 2

Just replace your if statement with a while statement.

while (letter1 == 'a' || letter1 == 'u' || letter1 == 'i' || letter1 == 'e' || letter1 == 'o' || 
       letter2 == 'a' || letter2 == 'u' || letter2 == 'i' || letter2 == 'e' || letter2 == 'o')
{
    letter1 = (char) (rn.nextInt(26) + 'a');
    letter2 = (char) (rn.nextInt(26) + 'a');
} 

Update:

Looking at your code, it would be much better to check each of the letters separately, otherwise letter1 could be getting consonants but because letter 2 is not it continues changing both of them.

while (letter1 == 'a' || letter1 == 'u' || letter1 == 'i' || letter1 == 'e' || letter1 == 'o')
{
    letter1 = (char) (rn.nextInt(26) + 'a');
}
while (letter2 == 'a' || letter2 == 'u' || letter2 == 'i' || letter2 == 'e' || letter2 == 'o')
{
    letter2 = (char) (rn.nextInt(26) + 'a');
}