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How to put numbers to array in while loop in java?

I am trying to add two binary numbers and then get their sum in binary system. I got their sum in decimal and now I am trying to turn it into binary. But there is problem that when I take their sum (in decimal) and divide by 2 and find remainders(in while loop), I need to put remainders into array in order print its reverse. However, there is an error in array part. Do you have any suggestions with my code? Thanks in advance.
Here is my code:
import java.util.Scanner;
public class ex1 {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
int n = scan.nextInt();
int m = scan.nextInt();
int k = dec1(n)+dec2(m);
int i=0,c;
int[] arr= {};
while(k>0) {
c = k % 2;
k = k / 2;
arr[i++]=c; //The problem is here. It shows some //error
}
while (i >= 0) {
System.out.print(arr[i--]);
}
}
public static int dec1(int n) {
int a,i=0;
int dec1 = 0;
while(n>0) {
a=n%10;
n=n/10;
dec1= dec1 + (int) (a * Math.pow(2, i));
i++;
}
return dec1;
}
public static int dec2(int m) {
int b,j=0;
int dec2 = 0;
while(m>0) {
b=m%10;
m=m/10;
dec2= dec2 + (int) (b * Math.pow(2, j));
j++;
}
return dec2;
}
}

Here:
int[] arr= {};
creates an empty array. Arrays don't grow dynamically in Java. So any attempt to access any index of arr will result in an ArrayIndexOutOfBounds exception. Because empty arrays have no "index in bounds" at all.
So:
first ask the user for the count of numbers he wants to enter
then go like: int[] arr = new int[targetCountProvidedByUser];
The "more" real answer would be to use List<Integer> numbersFromUsers = new ArrayList<>(); as such Collection classes allow for dynamic adding/removing of elements. But for a Java newbie, you better learn how to deal with arrays first.

Why are you using two different methods to do the same conversion? All you need is one.
You could have done this in the main method.
int k = dec1(n)+dec1(m);
Instead of using Math.pow which returns a double and needs to be cast, another alternative is the following:
int dec = 0;
int mult = 1;
int bin = 10110110; // 128 + 48 + 6 = 182.
while (bin > 0) {
// get the right most bit
int bit = (bin % 10);
// validate
if (bit < 0 || bit > 1) {
throw new IllegalArgumentException("Not a binary number");
}
// Sum up each product, multiplied by a running power of 2.
// this is required since bits are taken from the right.
dec = dec + mult * bit;
bin /= 10;
mult *= 2; // next power of 2
}
System.out.println(dec); // prints 182
An alternative to that is to use a String to represent the binary number and take the bits from the left (high order position).
String bin1 = "10110110";
int dec1 = 0;
// Iterate over the characters, left to right (high to low)
for (char b : bin1.toCharArray()) {
// convert to a integer by subtracting off character '0'.
int bit = b - '0';
// validate
if (bit < 0 || bit > 1) {
throw new IllegalArgumentException("Not a binary number");
}
// going left to right, first multiply by 2 and then add the bit
// Each time thru, the sum will be multiplied by 2 which shifts everything left
// one bit.
dec1 = dec1 * 2 + bit;
}
System.out.println(dec1); // prints 182
One possible way to display the result in binary is to use a StringBuilder and simply insert the converted bits to characters.
public static String toBin(int dec) {
StringBuilder sb = new StringBuilder();
while (dec > 0) {
// by inserting at 0, the bits end up in
// correct order. Adding '0' to the low order
// bit of dec converts to a character.
sb.insert(0, (char) ((dec & 1) + '0'));
// shift right for next bit to convert.
dec >>= 1;
}
return sb.toString();
}

Java. Extracting integers from bits in a byte array not fitting the byte boundary

I have the following array of bytes:
01010110 01110100 00100101 01001011
These bytes are broken into two groups to encode seven integers. I know that the first group consists of 3 values 4 bits each (0101 0110 0111) that represent numbers 5,6,7. The second group consists of 4 values 5 bits each (01000 01001 01010 01011), which represent integers 8,9,10, and 11.
To extract the integers, I am currently using the following approach. Convert the array into a binary string:
public static String byteArrayToBinaryString(byte[] byteArray)
{
String[] arrayOfStrings = new String[byteArray.length];
for(int i=0; i<byteArray.length; i++)
{
arrayOfStrings[i] = byteToBinaryString(byteArray[i]);
}
String bitsetString = "";
for(String testArrayStringElement : arrayOfStrings)
{
bitsetString += testArrayStringElement;
}
return bitsetString;
}
// Taken from here: http://helpdesk.objects.com.au/java/converting-large-byte-array-to-binary-string
public static String byteToBinaryString(byte byteIn)
{
StringBuilder sb = new StringBuilder("00000000");
for (int bit = 0; bit < 8; bit++)
{
if (((byteIn >> bit) & 1) > 0)
{
sb.setCharAt(7 - bit, '1');
}
}
return sb.toString();
}
Then, I split the binary string into 2 substrings: 12 characters and 20 characters. Then I split each substring into new substrings, each of which has length that equals the number of bits. Then I convert each sub-substring into an integer.
It works but a byte array representing thousands of integers takes 30 seconds to a minute to extract.
I am a bit at a loss here. How do I do this using bitwise operators?
Thanks a lot!

I assume you have an understanding of the basic bit operations and how to express them in Java.
Use a pencil to draw a synthetic picture of the problem
byte 0 byte 1 byte 2 byte 3
01010110 01110100 00100101 01001011
\__/\__/ \__/\______/\___/\______/\___/
a b c d e f g
To extract a, b and c we need to do the following
a b c
byte 0 byte 0 byte 1
01010110 01010110 01110100
\. \. |||||||| \. \.
'\ '\ XXXX|||| '\ '\
0.. 0101 0.. 0110 0.. 0111
Shift And Shift
In Java
int a = byteArray[0] >>> 4, b = byteArray[0] & 0xf, c = byteArray[1] >>> 4;
The other values d, e, f and g are computed similarly but some of them require to read two bytes from the array (d and f actually).
d e
byte 1 byte 2 byte 2
01110100 00100101 00100101
||||\\\\ | |\\\\\
XXXX \\\\ | X \\\\\
\\\\| \\\\\
0.. 01000 01001
To compute d we need to isolate the least four bits of byte 1 with byteArray[1] & 0xf then make space for the bit from byte 2 with (byteArray[1] & 0xf) << 1, extract that bit with byteArray[1] >>> 7 and finally merge together the result.
int d = (byteArray[1] & 0xf) << 1 | byteArray[2] >>> 7;
int e = (byteArray[2] & 0x7c) >>> 2;
int f = (byteArray[2] & 0x3) << 3 | byteArray[3] >>> 5;
int g = byteArray[3] & 0x1f;
When you are comfortable with handling bits operations you may consider generalizing the function that extract the integers.
I made function int extract(byte[] bits, int[] sizes, int[] res), that given an array of bytes bits, an array of sizes sizes, where the even indices hold the size of the integers to extract in bits and the odd indices the number of integers to extract, and an output array res large enough to hold all the integers in output, extracts from bits all the integers expressed by sizes.
It returns the number of integers extracted.
For example the original problem can be solved as
int res[] = new int[8];
byte bits[] = new byte[]{0x56, 0x74, 0x25, 0x4b};
//Extract 3 integers of 4 bits and 4 integers of 5 bits
int ints = BitsExtractor.extract(bits, new int[]{4, 3, 5, 4}, res);
public class BitsExtractor
{
public static int extract(byte[] bits, int[] sizes, int[] res)
{
int currentByte = 0; //Index into the bits array
int intProduced = 0; //Number of ints produced so far
int bitsLeftInByte = 8; //How many bits left in the current byte
int howManyInts = 0; //Number of integers to extract
//Scan the sizes array two items at a time
for (int currentSize = 0; currentSize < sizes.length - 1; currentSize += 2)
{
//Size, in bits, of the integers to extract
int intSize = sizes[currentSize];
howManyInts += sizes[currentSize+1];
int temp = 0; //Temporary value of an integer
int sizeLeft = intSize; //How many bits left to extract
//Do until we have enough integer or we exhaust the bits array
while (intProduced < howManyInts && currentByte <= bits.length)
{
//How many bit we can extract from the current byte
int bitSize = Math.min(sizeLeft, bitsLeftInByte); //sizeLeft <= bitsLeftInByte ? sizeLeft : bitsLeftInByte;
//The value to mask out the number of bit extracted from
//The current byte (e.g. for 3 it is 7)
int byteMask = (1 << bitSize) - 1;
//Extract the new bits (Note that we extract starting from the
//RIGHT so we need to consider the bits left in the byte)
int newBits = (bits[currentByte] >>> (bitsLeftInByte - bitSize)) & byteMask;
//Create the new temporary value of the current integer by
//inserting the bits in the lowest positions
temp = temp << bitSize | newBits;
//"Remove" the bits processed from the byte
bitsLeftInByte -= bitSize;
//Is the byte has been exhausted, move to the next
if (bitsLeftInByte == 0)
{
bitsLeftInByte = 8;
currentByte++;
}
//"Remove" the bits processed from the size
sizeLeft -= bitSize;
//If we have extracted all the bits, save the integer
if (sizeLeft == 0)
{
res[intProduced++] = temp;
temp = 0;
sizeLeft = intSize;
}
}
}
return intProduced;
}
}

Well I did the first group , the second can be done in similar fashion
public static void main(String args[]) {
//an example 32 bits like your example
byte[] bytes = new byte[4];
bytes[0] = 31;//0001 1111
bytes[1] = 54;//0011 0110
bytes[2] = 67;
bytes[3] = 19;
//System.out.println(bytes[0]);
int x = 0;
int j = -1; // the byte number
int k = 0; // the bit number in that byte
int n = 0; // the place of the bit in the integer we are trying to read
for (int i = 0; i < 32; i++) {
if (i < 12) { //first group
if (i % 8 == 0) {
j++;
k = 0;
}
if (i % 4 == 0) {
x = 0;
n = 0;
}
byte bit = (byte) ((bytes[j] & (1 << (7 - k))) >> (7 - k));
System.out.println("j is :" + j + " k is :" + k + " " + bit);
x = x | bit << (3 - n);
if ((i + 1) % 4 == 0) {
System.out.println(x);
}
k++;
n++;
} else {
}
}
}
It's a bit tricky because you are trying to encode an integer on less than what java allocates (8 bits). So I had to take each bit and "construct" the int from them
To get each bit
byte bit = (byte) ((bytes[j] & (1 << (7 - k))) >> (7 - k));
this takes the byte we are at and does And operation. For example I want the 3rd bit of the 1st byte, I do
bytes[0] & 1 << (7 - 3)
but this gives me an integer encoded over 8 bits, so I still have to shift it to get that single bit with >> (7 - 3)
Then I just Or it with x (the int we are trying to decode). All while putting it at the right position with << (3 - n) . 3 because your integer is encoded over 4 bits
Try running the code and reading the output.
I am honestly not sure if this is the best way, but I believe it's at least faster than dealing with Strings

Right (LSB) radix -- how to handle the 2's complement?

I am attempting to implement a right/LSB radix sort for integers, and once it's working, I will try to parallelize it. My sequential code works well for unsigned numbers, but once I throw negative integers at it, it doesn't "see" the signed bit and ends up with positive (sorted) integers from 0 to n mixed with negative (again, sorted) integers from -n to -1.
Here's my code:
public class SeqRadix {
static void radix2(int[] a) {
// 2 digit radixSort: a[]
int max = a[0];
int numBit = 2;
int n = a.length;
// a) find max value in a[]
for (int i = 1; i < n; i++){
if (a[i] > max) {
max = a[i];
}
}
while (max >= (1 << numBit)){
numBit++; // digits in max
}
// decide num of bits in bit1 and bit2
int bit1 = numBit / 2, bit2 = numBit - bit1;
int[] b = new int[n];
radixSort(a, b, bit1, 0); // first digit from a[] to b[]
radixSort(b, a, bit2, bit1);// second digit, back from b[] to a[]
} // end
/**
* Sort a[] on one digit ; number of bits = maskLen, shiftet up �shift�
* bits
*/
static void radixSort(int[] a, int[] b, int maskLen, int shift) {
int acumVal = 0, j, n = a.length;
int mask = (1 << maskLen) - 1;
int[] count = new int[mask + 1];
// b) count=the frequency of each radix value in a
for (int i = 0; i < n; i++) {
count[(a[i] >> shift) & mask]++;
}
// c) Add up in 'count' - accumulated values
for (int i = 0; i <= mask; i++) {
j = count[i];
count[i] = acumVal;
acumVal += j;
}
// c) move numbers in sorted order a to b
for (int i = 0; i < n; i++) {
b[count[(a[i] >> shift) & mask]++] = a[i];
}
}// end radixSort
}// end SekvensiellRadix
As it first sorts on the LSBs, then more and more significant bits, that last 2's complement/signed bit doesn't get caught it seems. Thanks in advance!

What you need to do is invert the comparison operation on the sign bit. For every other bit, 0 < 1, but for the sign bit we use 1 < 0. As you sort bits 0 through 30 (for 32-bit integers, obviously), you sort the magnitude of that integer. Not absolutely, because there's that one-shift, but relative to all other integers of the same sign, which is all we need.
So, if we have the numbers {5, -1, 3, 2, -3, -8} (signed 4-bit for simplicity):
0101 = 5
1111 = -1
0011 = 3
0010 = 2
1101 = -3
1000 = -8
After sorting up to bit 2 we have:
1 000 = -8
0 010 = 2
0 011 = 3
0 101 = 5
1 101 = -3
1 111 = -1
Notice that each of the negative numbers are sorted in increasing order relative to the other negative numbers. (Likewise for the positives.) Now, for the comparison of the sign bit we say 1 < 0. That will move all of the negative numbers to the front of the list and, since we're using a stable sorting mechanism, all of the negative numbers stay in the same position relative to each other. (Again, likewise for the positives.)
In the end we have our list sorted in ascending order:
1000 = -8
1101 = -3
1111 = -1
0010 = 2
0011 = 3
0101 = 5

You simply need to convert all int values to unsigned by inverting bit 31 when making decisions on them:
unsigned = signed ^ 0x80000000;
Alternately, if you find that more convenient to implement, reverse the result of all decisions made based on bit 31.
Edit: That already starts when you try to find the max value - the way you search for the max you will never find a value with bit 31 set as max. You need to use unsigned comparison (which java doesn't support by language means) there as well, so flip bit 31 before comparing:
// a) find max value in a[]
for (int i = 1; i < n; i++){
if ((a[i]^0x80000000) > (max^0x80000000)) {
max = a[i];
}
}

represent an integer using binary in java language

Here is the problem:
You're given 2 32-bit numbers, N & M, and two bit positions, i & j. write a method to set all bits between i and j in N equal to M (e.g. M becomes a substring of N at locating at i
and starting at j)
For example:
input:
int N = 10000000000, M = 10101, i = 2, j = 6;
output:
int N = 10001010100
My solution:
step 1: compose one mask to clear sets from i to j in N
mask= ( ( ( ((1<<(31-j))-1) << (j-i+1) ) + 1 ) << i ) - 1
for the example, we have
mask= 11...10000011
step 2:
(N & mask) | (M<<i)
Question:
what is the convenient data type to implement the algorithm? for example
we have int n = 0x100000 in C, so that we can apply bitwise operators on n.
in Java, we have BitSet class, it has clear, set method, but doesnt support
left/right shift operator; if we use int, it supports left/right shift, but
doesnt have binary representation (I am not talking about binary string representation)
what is the best way to implement this?
Code in java (after reading all comments):
int x = Integer.parseInt("10000000000",2);
int x = Integer.parseInt("10101",2);
int i = 2, j = 6;
public static int F(int x, int y, int i, int j){
int mask = (-1<<(j+1)) | (-1>>>(32-i));
return (mask & x ) | (y<<i);
}

the bit-wise operators |, &, ^ and ~ and the hex literal (0x1010) are all available in java
32 bit numbers are ints if that constraint remains int will be a valid data type
btw
mask = (-1<<j)|(-1>>>(32-i));
is a slightly clearer construction of the mask

Java's int has all the operations you need. I did not totally understand your question (too tired now), so I'll not give you a complete answer, just some hints. (I'll revise it later, if needed.)
Here are j ones in a row: (1 << j)-1.
Here are j ones in a row, followed by i zeros: ((1 << j) - 1) << i.
Here is a bitmask which masks out j positions in the middle of x: x & ~(((1 << j) - 1) << i).
Try these with Integer.toBinaryString() to see the results. (They might also give strange results for negative or too big values.)

I think you're misunderstanding how Java works. All values are represented as 'a series of bits' under the hood, ints and longs are included in that.
Based on your question, a rough solution is:
public static int applyBits(int N, int M, int i, int j) {
M = M << i; // Will truncate left-most bits if too big
// Assuming j > i
for(int loopVar = i; loopVar < j; loopVar++) {
int bitToApply = 1 << loopVar;
// Set the bit in N to 0
N = N & ~bitToApply;
// Apply the bit if M has it set.
N = (M & bitToApply) | N;
}
return N;
}
My assumptions are:
i is the right-most (least-significant) bit that is being set in N.
M's right-most bit maps to N's ith bit from the right.
That premature optimization is the root of all evil - this is O(j-i). If you used a complicated mask like you did in the question you can do it in O(1), but it won't be as readable, and readable code is 97% of the time more important than efficient code.

Storing int value of bitmask - extract 1 valued bits

I am calculating the int equivalent of a given set of bits and storing that in memory. From there, I would like to determine all 1 value bits from the original bitmask. Example:
33 --> [1,6]
97 --> [1,6,7]
Ideas for an implementation in Java?

On BitSet
Use java.util.BitSet to store, well, a set of bits.
Here's how you can convert from an int to a BitSet, based on which bits in the int is set:
static BitSet fromInt(int num) {
BitSet bs = new BitSet();
for (int k = 0; k < Integer.SIZE; k++) {
if (((num >> k) & 1) == 1) {
bs.set(k);
}
}
return bs;
}
So now you can do the following:
System.out.println(fromInt(33)); // prints "{0, 5}"
System.out.println(fromInt(97)); // prints "{0, 5, 6}"
And just for completeness, here's the reverse transformation:
static int toInt(BitSet bs) {
int num = 0;
for (int k = -1; (k = bs.nextSetBit(k + 1)) != -1; ) {
num |= (1 << k);
}
return num;
}
So composing both together, we always get back the original number:
System.out.println(toInt(fromInt(33))); // prints "33"
System.out.println(toInt(fromInt(97))); // prints "97"
On 0-based indexing
Note that this uses 0-based indexing, which is the more commonly used indexing for bits (and most everything else in Java). This is also more correct. In the following, ^ denotes exponentiation:
33 = 2^0 + 2^5 = 1 + 32 97 = 2^0 + 2^5 + 2^6 = 1 + 32 + 64
33 -> {0, 5} 97 -> {0, 5, 6}
If you insist on using 1-based indexing, however, you can use bs.set(k+1); and (1 << (k-1)) in the above snippets. I would advise strongly against this recommendation, however.
Related questions
What does the ^ operator do in Java? -- it's actually not exponentiation

For bit fiddling, java.lang.Integer has some very helpful static methods. Try this code as a starting base for your problem:
public int[] extractBitNumbers(int value) {
// determine how many ones are in value
int bitCount = Integer.bitCount(value);
// allocate storage
int[] oneBits = new int[bitCount];
int putIndex = 0;
// loop until no more bits are set
while (value != 0) {
// find the number of the lowest set bit
int bitNo = Integer.numberOfTrailingZeros(value);
// store the bit number in array
oneBits[putIndex++] = bitNo+1;
// clear the bit we just processed from the value
value &= ~(1 << bitNo);
}
return oneBits;
}

I can show you C# implementation, Java should be very similar.
int value = 33;
int index = 1;
while (value > 0)
{
if ((value % 2) == 1)
Console.WriteLine(index);
index++;
value /= 2;
}

If you want to get an array like that you'll likely need to loop the number of bits you want to check & the integer with a bit shifted 1 for each step.
Something like (pseudo):
Init array
mask = 1
for (0 to BitCount):
if Integer & mask
array[] = pos
mask << 1

A bit-crunching variation would be something like:
int[] getBits(int value) {
int bitValue = 1;
int index = 1;
int[] bits = new int[33];
while (value >= bitValue)
{
bits[index++] = (value & bitValue);
bitValue << 1; // or: bitValue *= 2;
}
return bits;
}
Note that since the bits are indexed from 1 as you requested, bits[0] is left unused.