Questions about array

Question

Here is my code and it works perfectly fine.

import java.util.Random;
class apples
{
    public static void main(String args[])
    {
         Random rand = new Random();
         int frequency[] = new int[7];

         for(int roll = 1;roll < 1000;roll++){
             ++frequency[1+rand.nextInt(6)];
         }

         System.out.println("Face\tFrequency");

         for(int face = 1;face < frequency.length;face++){
             System.out.println(face + "\t" + frequency[face]);
         }
    }
}

I do not understand this line of code

++frequency[1+rand.nextInt(6)];

When I removed the "++" operator, it couldn't be compiled. I know that it will add 1 to the randon numbers generated from 0 to 5 but why there is a "++" in front of frequency ? Why is it neccessary to put the "++" operator there ?

Likely it was just a warning that the statement does nothing; not a compiler error. If you're able to run the program, then it was not an error, but a warning. — AndyG
– AndyG, Commented Dec 9, 2013 at 23:34

Jon Newmuis · Accepted Answer · 2013-12-10 00:16:12Z

3

The ++ operator is incrementing the frequency at the specified index. In this case, it's the same as saying:

for(int roll = 1;roll < 1000;roll++){
  int index = 1+rand.nextInt(6);
  frequency[index] = frequency[index] + 1;
}

Removing the ++ operator, you're left with:

for(int roll = 1;roll < 1000;roll++){
  frequency[1+rand.nextInt(6)];
}

The line frequency[1+rand.nextInt(6)] makes no sense; it is not an operation, it does not do anything.

EDIT:

Perhaps a better illustration: let x be the value looked up in the frequency array. Then the original is equivalent to:

for(int roll = 1;roll < 1000;roll++){
  int x = frequency[1+rand.nextInt(6)];
  ++x; // Equivalent to "x = x + 1"
}

Whereas if you remove the increment operator, your resulting loop would be:

for(int roll = 1;roll < 1000;roll++){
  int x = frequency[1+rand.nextInt(6)];
  x; // ...what?
}

edited Dec 10, 2013 at 0:16

answered Dec 9, 2013 at 23:32

Jon Newmuis

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aaaa bbbb · Accepted Answer · 2013-12-09 23:32:14Z

0

The statement without the ++ looks at the contents of the array, but does not do anything with the value. Adding the ++ operator tells it to increment the value found there.

You probably are thinking that frequency[1+rand.nextInt(6)] is adding a value to the array. It is not. It is looking up a location in memory from a random location.

answered Dec 9, 2013 at 23:32

aaaa bbbb

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gerrytan · Accepted Answer · 2013-12-09 23:32:51Z

0

++ is a shorthand for

int i = 1+rand.nextInt(6);
frequency[i] = frequency[i] + 1;

Removing it causes no increment expression is executed.

answered Dec 9, 2013 at 23:32

gerrytan

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Khalid Abu El-Soud · Accepted Answer · 2013-12-09 23:46:55Z

0

that line is equivalent to this line of code

int index = 1+rand.nextInt(6);
frequency[index] = frequency[index]+1 ;

so if you removed '++' you make the statement incomplete like the following:

 int x= 5;
 x;

so it gives you error

edited Dec 9, 2013 at 23:46

answered Dec 9, 2013 at 23:35

Khalid Abu El-Soud

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