Java decimal to binary int array

Question

For this exercise I'm making I want a decimal < 4096 to be written in binary form in an int array.

So for example, 4 would be {0,0,0,0,0,0,0,0,0,1,0,0} . I need this for (nearly) all integers up to 4096, so I've written this piece of code:

for(int k=0; k<4096; k++){
    int[] myNumber =  { (k / 2048) % 2, (k / 1024) % 2, (k / 512) % 2, (k / 256) % 2, (k / 128) % 2, (k / 64) % 2, (k / 32) % 2, (k / 16) % 2, (k / 8) % 2, (k / 4) % 2, (k / 2) % 2, (k / 1) % 2 }
    /* Some processing */
}

This looks kind of ugly, so that's why I'm curious to find out if there is a more elegant way of achieving this?

For the interested reader:
I chose for the array approach of storing the binary numbers, because I need to perform some shifting and addition modulo 2. I'm using an LFSR, and this is my implementation for that:

public class LFSR {

    private int[] polynomial;

    public LFSR(int[] polynomial) {
        this.polynomial = polynomial;
    }

    public int[] shiftLeft(int[] input) {
        int[] result = new int[input.length];

        int out = input[0];
        result[input.length - 1] = out;
        for (int i = input.length - 1; i > 0; i--) {
            result[i - 1] = (input[i] + polynomial[i - 1] * out) % 2;
        }

        return result;
    }

}

Any suggestions?

Answer 1

Some pseudo code:

While (int i = 0; i < 12; i++) { 
   bitarray[i] = numericalValue & 0x1;
   numericalValue = numericalValue >> 1;
}

So, shifting right by one bit is division by 2, and ANDing with 1 always leaves you only with the lowest bit which is what you want.

Answer 2

public int[] toBin (int num)
{
int[] ret = new int[8];
for (int i = 7, p = 0; i>=0; i--, p++)
{
ret[i] = (num/2**p) % 2;
}
return ret;
}

Answer 3

One quick suggestion would be to switch to a byte array instead of an int array, simply to save space, as they will only be 'bits'.

With regards to improving the elegance of your solution, it is perhaps easier to use subcomputations:

int[] intToBinaryArray(int dec){

int[] res = int[12]
for(int i =0; i < 12; i++)
    bitarray[i] = numericalValue & 0x1; //grab first bit only
    dec  /= 2;
}

return res;
}

Answer 4

"Elegance" is in the eye of the beholder, but I'd start by creating a method to avoid repetition and improve clarity:

int[] myNumber =  { getBit(k, 12), getBit(k, 11), ... };

I personally feel this is the "most elegant" way to get a particular bit:

int getBit(int v, int i)
{
    return v >> i & 1;
}

Then you have to decide whether you want to keep repeating yourself with the calls to getBit or whether you'd rather just use a single while or for loop to populate the whole array. You'd think it'd be quicker the way you've got it written, but there's good chance the JIT compiler automatically unrolls your loop for you if you use a loop like Jochen suggests.

Since this particular operation is entirely self contained, you might even want to create a special method for it:

int[] getBits(int v, int num)
{
    int[] arr = new int[num];
    for(int i=0; i<num; i++) {
        arr[i] = getBit(v, num - i - 1);
    }
    return arr;
}

That makes it easier to unit test, and you can reuse it in a variety of situations.

Answer 5

String s = Integer.toBinaryString(int value);

Now convert the String to int[]

int[] intArray = new int[s.length()];
for (int i = 0; i < s.length(); i++) {
intArray[i] = Character.digit(s.charAt(i), 10);
}

Answer 6

This is somewhat shorter ;)

    int[] bits = new int[13];
    String bin = Integer.toBinaryString(8192 + value);
    for(int i = 1; i < bin.length(); i++) {
        bits[i-1] = bin.charAt(i)-'0';
    }

Answer 7

You could use the bit shift operator together withbthe bitwise AND operator as follows. Note bitCount - i - 1 is needed to get the high bit first.

    final int bitCount =12; // Increase to support greater than 4095
    int[] number = new int[bitCount];
    for( int i = 0; i < bitCount; i++ )
    {
      number[i] = ( k >>> ( bitCount - i - 1 ) ) & 1;
    }