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

Conditions like if(freq == X && freq == X+1) are always false.

You probably meant to use || (OR)

Answer 2

Your code has a lot of repetitions of the same problem:

if(freq == 28 && freq == 29) { ... }

You are telling Java to follow a condition when freq equals to 28 AND freq equals to 29. It's impossible. You will want to use the OR operator:

if(freq == 28 || freq == 29) { ... }

What's happening now is that when freq equals to any number inside those mistaken conditions, nothing will be added to your string and it will become smaller.

Answer 3

You seem to have made some typos. On one occasion you write

if(freq == 49 && freq == 50){ name += "j";

which is in fact never true.

Answer 4

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)

---

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.

---

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)

Answer 5

Here is my solution:

import java.util.Random;

Random gen = new Random(474587); //put in random seed
int min = 6;
int max = 10;

// we want 20 random strings 
for(int i=0; i < 20; i++){
 int len = min+gen.nextInt(max-min+1);
 StringBuilder s = new StringBuilder(len);
 while(s.length() < len){
  //97 is ASCII for character 'a', and 26 is number of alphabets
  s.append((char)(97+gen.nextInt(26)));     
 }

System.out.println(s.toString());
}

---

Sample of output:

zqwloh
jefcso
spcnhxyyk
tzlobaukn
keyxkn
cllhsxybz
ieaudei
bolfzqlxrl
scpfcbztyh
thkfrybffe
nbspabxjh

Answer 6

Just for reference and completeness, here's an "easy" (but less efficient) solution assuming that presence of numbers in the string isn't a big issue:

private static final Random random = new Random();

public static String generateRandomString() {
    return new BigInteger((4 + random.nextInt(3)) * 8, random).toString(36);
}

This generates a random string matching [a-z0-9] of length 6~10 (inclusive).

Answer 7

I bet that you no longer need an answer, but since I've never answered a stack overflow question before, I though this would be a good warm-up problem.

One thing that everyone else seemed to leave out is the frequency aspect of your code. The following code will create 10 random words of length 6 to 10 according to the frequency you wanted:

import java.util.Random;


public class Stuff {
public static void main(String[] args) {

    Random rand = new Random();
    int[] freqs = new int[] {6,8,11,15,25,27,29,33,48,50,55,60,62,70,75,77,78,84,86,88,93,94,96,97,99,100};

    int numWords = 10;
    for(int i = 0; i<numWords; i++)
    {
        String word = "";
        int numLetters = 6 + rand.nextInt(5);
        for(int j = 0; j<numLetters; j++)
        {
            int freq = rand.nextInt(100) + 1;
            int index = 0;
            while(freqs[index] < freq) index++;
            word = word + (char)(97 + index );              
        }
        System.out.println(word);
    }
}

}

Now, my question for you is can you tell me how this works?

JB