Help for creating a random String

Question

I need to create a random string which should be between the length of 6 to 10 but it sometimes generates only about the length of 3 to 5. Here's my code. Can anyone would be able to find out the problem? :(

            int lengthOfName = (int)(Math.random() * 4) + 6;
        String name = "";
        /* randomly choosing a name*/
        for (int j = 0; j <= lengthOfName; j++) {
            int freq = (int)(Math.random() * 100) + 1;
            if(freq <= 6){
                name += "a";
            }if(freq == 7 && freq == 8){
                name += "b";
            }if(freq >= 9 && freq <= 11){
                name += "c";
            }if(freq >= 12 && freq <= 15){
                name += "d";
            }if(freq >= 16 && freq <= 25){
                name += "e";                        
            }if(freq == 26 && freq == 27){
                name += "f";
            }if(freq == 28 && freq == 29){
                name += "g";
            }if(freq >= 30 && freq <= 33){
                name += "h";
            }if(freq >= 34 && freq <= 48){
                name += "i";
            }if(freq == 49 && freq == 50){
                name += "j";
            }if(freq >= 51 && freq <= 55){
                name += "k";
            }if(freq >= 56 && freq <= 60){
                name += "l";
            }if(freq == 61 && freq == 62){
                name += "m";
            }if(freq >= 63 && freq <= 70){
                name += "n";
            }if(freq >= 71 && freq <= 75){
                name += "o";
            }if(freq == 76 && freq == 77){
                name += "p";
            }if(freq == 78){
                name += "q";
            }if(freq >= 79 && freq <= 84){
                name += "r";
            }if(freq == 85 && freq == 86){
                name += "s";
            }if(freq == 87 && freq == 88){
                name += "t";
            }if(freq >= 89 && freq <= 93){
                name += "u";
            }if(freq == 94){
                name += "v";
            }if(freq == 95 && freq == 96){
                name += "w";
            }if(freq == 97){
                name += "x";
            }if(freq == 98 && freq == 99){
                name += "y";
            }if(freq == 100){
                name += "z";
            }
        }

Answer 1

I'm sorry but the code is too poorly written to be salvageable. I recommend something like this.

    Random r = new Random(); // just create one and keep it around
    String alphabet = "abcdefghijklmnopqrstuvwxyz";

    final int N = 10;
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < N; i++) {
        sb.append(alphabet.charAt(r.nextInt(alphabet.length())));
    }
    String randomName = sb.toString();

    System.out.println(randomName);

Key points are:

• Use java.util.Random, specifically nextInt(int n) to get a random int in a given range
  • No need for funky formulas
• When building a string in a loop, use StringBuilder.
• Use an alphabet string, and charAt to index its letters.

API links

• java.util.Random
  • int nextInt(int n)
    • Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive)
• StringBuilder - A mutable sequence of characters.

Related questions

• String vs StringBuilder (C#)
• String, StringBuffer and StringBuilder (Java)

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Problems with the original code

There are plenty, unfortunately.

• String += in a loop yields very poor performance for longer strings
• for (int j = 0; j <= lengthOfName; j++) is an off-by-one-error
• freq == 7 && freq == 8 is a logical contradiction
• It's just unnecessarily verbose!
  • Warning signs should go off whenever you write something like that

I highly recommend doing lots of small but simple exercises to learn Java basics. codingbat.com is great; it has hundreds of these, they're automatically graded so you'll know when your solution works as expected or not. It has sections on logic, strings, arrays, etc.

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On uneven letter distribution

The simplest solution is to just have duplicates in the alphabet:

• String alphabet = "aab"; will have probability for a twice as much as b
• You can generate the alphabet programmatically from a frequency table
  • I'll leave this as an exercise (or you can ask another question if you need it)