Explain what this code is doing

Question

I'm a newbie in java. I was going through some tutorials and came across this code I was not able to understand the code. Please explain what it means.

class Randoms
{
    public static void main(String[] args)
    {
        Random rand = new Random();
        int freq[] = new int[7];
        for(int roll = 1; roll < 10; roll++)
        {            
             (++freq[1 + rand.nextInt(6)]);
        }
...

Answer 1

Line by line:

Random rand = new Random(); create new instance of the Random object, this is responsible for the creation of random numbers.

int[] freq = new int[7]; create a new int array that can store 7 elements, with indices from 0...6. It is worth noting that in Java, the ints stored in the array are initialized to 0. (This is not true for all languages, an example being C, as in C the int arrays initially store memory junk data, and must be explicitly initialized to zero).

for(int roll = 1; roll < 10; roll++) this rolls 9 times (because 1...9, but it's better practice to go from 0)

(++freq[1 + rand.nextInt(6)]); this line is something that you shouldn't ever do in this sort of fashion, because it's a monstrosity as you can see.

Do something like this:

    for(int roll = 0; roll < 9; roll++)
    {            
         int randomNumber = rand.nextInt(6); //number between 0...5
         int index = 1 + randomNumber; //number between 1...6
         freq[index]++; //increment the number specified by the index by 1
                        //nearly equivalent to freq[index] += 1;
    }

So basically it randomizes the number of 9 dice throws, and stores the dice throw count (or so it calls it, frequency) in the array.

Thus, it's simulating 9 dice throws (numbers from 1...6), and each time it "rolls" a particular number, it increases the number stored in the array at that specific location.

So in the end, if you say:

    for(int i = 1; i <= 6; i++)
    {
        System.out.println("Thrown " + freq[i] + " times of number " + i);
    }

Then it will be clearly visible what's happened.

Answer 2

(++freq[1 + rand.nextInt(6)]);   // this line of code.

The above line of code is pre-incrementing the value of freq[] array at the specified position,i.e., 1+rand.nextInt(6) --- referred value is ++freq[some-position to be evaluated] which we will evaluate below.

This rand.nextInt(6) will generate an integer number lesser than 6 and greater than 0,as it is a pre-defined method of Random Class ,randomly.We can't predict it. And,then say number generated is 4. SO, 1+rand.nextInt(6)=5.

Hence,your code would simplify to (++freq[1 + rand.nextInt(6)]) OR `(++freq[5]).

So,simplification of this code will be equivalent to a number which equals 1 more than 6th element of array freq[].

  // as freq[5] is the 6th element of the array freq[].

Also,there are some other points which SIR David Wallace suggested me to include which I would like to explain a bit more.It goes below :-

1. ++a here ++ is called pre-increment operator and it increases the value of a by 1. There also exists an altered reverse version of it.

2. a++ here this ++ is called post-increment operator and it also increases the value of a by 1.But,WAIT,you might have thought that there aren't differences,but there are.

For the differences potion,I'd like to suggest to have a reading of What is the difference between pre-increment and post-increment in the cycle (for/while)?, though it is questioned in sense of C++,the same is in Java too!

Answer 3

// Create a new Random Object, this helps you generate random numbers
Random rand = new Random();
// create a integer array with 7 integers
int freq[] = new int[7];
// loop 9 times
for(int roll = 1; roll < 10; roll++)
{
    // rand.nextInt(6) generates a number between 0 and 5 (<6). add one to it
    // ++ adds one to the integer in the array that is at the index of 1-6.
    (++freq[1 + rand.nextInt(6)]);
}

Some strange things about this code:

• Roll loop starts at 1 then goes to 10 so at first glance it would seem to loop 10 times but actually runs 9 times.
• The ++ inside the loop would generally be located on the right and could lead to some confusion among newer programmers.
• freq[1 + rand.nextInt(6)] causes freq[0] to never be used.

Answer 4

At first a new object of the Random-Class and an array with 7 elements are created. Each element of the Array has the value 0. Inside the for-loop you randomly pick element 2 to 7 of the Array and increase its current value by 1. This is done 9 times.

Your code will never pick the first element of the Array which has the index 0.

I would rewrite the code to make it more clear:

Random rand = new Random();
int freq[] = new int[6];
int randomIndex = 0;
for(int roll = 0; roll < 9; ++roll)
{
    randomIndex = rand.nextInt(6);
    freq[randomIndex] = freq[randomIndex] + 1;
}

This code has not been tested, but it should basicly do the same.