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There are 3 marbles randomly generated and i'll compare them if they are same,different or one is different. My code is below and my question is above... can u help me out ?

public static void marb(){
    int a[],b[];
    int num=0;

    a=new int[3];
    b=new int[3];

    a[0]=1;
    a[1]=2;
    a[2]=3;

    **num=(int)Math.random();** //num is always assigned  1

    for(int x=0;x<3;x++)
    {
        num=(int)Math.random();

        b[x]=a[num];

        System.out.println(""+x+". marble:"+b[x]);      

    }

    int x=0;

    if(b[x]==b[x+1] && b[x+1]==b[x+2])
        System.out.println("ALL SAME");
    else if(b[x]!=b[x+1] && b[x]!=b[x+2] && b[x+1]!=b[x+2])
        System.out.println("ALL DIFFERENT");
    else
        System.out.println("One is different");

    }
Oliver Charlesworth
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user310000
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8 Answers8

12

Math.random() returns a double between 0 and 1. Casting to int will always truncate to 0.

If you want a random integer use the Random.nextInt(range) method.

sverre
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3

Are you sure that (int) Math.random() always returns 1? If you told us that it always returns 0, I would believe that immediately.

Math.random() returns a double in the range [0;1). The conversion from double to int truncates the number, to it is always 0.

Roland Illig
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2

Math.random() returns a double >= 0 and < 1. If you want a random number between 0 and x, use (int)(Math.random() * x).

Gabriel Negut
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    No, you should rather get your own random number generator and then use `rnd.nextInt(x)`. Otherwise not all outcomes are equally distributed. – Roland Illig Jun 04 '11 at 12:04
2

Create an instance of the random number generator

Random rand = new Random();

which also seeds the generator.

Then call

int myrandnum = rand.nextInt();

Subsequent calls to the nextInt method will generate a different number.

jonsca
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0

The correct way to use Math.Random() is to multiple it by the uppermost number that you want, then close the parenthesis and cast it to an int (if needed)

int randomNumber = (int)(Math.random()*max)

Its the placement of the parenthesis that you are off on. Math.random is a great function to use when you have an upper bounds for the range of numbers that you want. There is another stack overflow answer on getting it to work with a lower bounds as well, for a range of numbers from a min to a max.

Click here: Math.random() explained

Community
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Charlene
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My Math.random() method always return with 1

Well, this isn't true- in that case it always returns 0, which is used as num but what happens next:

b[x]=a[num];

you call a[0] three times as b[0], b[1] and b[2] and since you hardcoded 

a[0]=1;

you'll get only that value. In given example you can try

num=(int)(Math.random()*3);

as it simply generates random int from 0 to 2 or even use 

num=(int)(Math.random()*a.length);
dnk
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0

Because Math.random returns a value between 0 and 1 (either double or float I guess). Casting it to an int it loses the value after the decimal point and I guess casting rounds up to 1.

rich
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-1

I'm not a Java programmer but I have some idea of it.
Math.random() will produce random numbers between 0 and 1. Casting to int means you want either 1 or 2.
Since you are getting only 1, Math.random() just happens to be producing numbers between 0.5 and 1, that's all.
I also know that if you run the program again and again, it will produce the same set of pseudorandom values (except maybe you recompile).

afaolek
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