What is the faster way to conver BigInteger to array of int?

Question

I have BigInt

BigInteger i = new BigInteger("5876934265987526278534978564378568734564937563487564327564376534875483753475");

I need to convert it to [] int. How can I do this fast?

My method is very slow

private static int[] convertDigitsToIntArray(java.math.BigInteger x) {
    String s = x.toString();
    int[] result = new int[s.length()];
    for (int i = 0; i < s.length(); i++) {
        result[i] = Integer.parseInt(String.valueOf(s.charAt(i)));
    }
    return result;
}

You must use `Notepad++` or `EditPadPro` and use a macro.. You don't need programming here I think.. you need regular record macro and run it. — SSpoke, May 10 '14 at 15:55
Call `toString()` and then convert each character to an `int`? The `toString` is likely to be fairly well optimised. — Alan Stokes, May 10 '14 at 15:55
what would your result int[] be if the bigint is `new BigInteger("-123")` — Kent, May 10 '14 at 15:58
To convert a char to a digit use `Character.digit(s.charAt(i), 10)` - that will be much faster than what you have. — Alan Stokes, May 10 '14 at 16:10
@user It's never going to get a lot faster. Converting to decimal is inherently complex (lots of division). The final step of converting characters to digits is trivial by comparison. — Alan Stokes, May 10 '14 at 16:13
It would be polite to credit @Nielarshi when you paste his answer in your question. — Alan Stokes, May 10 '14 at 16:16
If you really need fast conversion to decimal, maybe you should do your computations in decimal instead of binary - see e.g. http://stackoverflow.com/a/1694615/212870. But I don't know of an existing implementation of that in Java. — Alan Stokes, May 10 '14 at 16:27

score 4 · Answer 1 · answered May 10 '14 at 16:02

4

Convert it to a string and then iterate the characters:

int[] getArr(BigInteger num)
{
    String str = num.toString();
    int[] arr = new int[str.length()];
    for (int i=0; i<arr.length; i++)
         arr[i] = str.charAt(i)-'0';
    return arr;
}

answered May 10 '14 at 16:02

barak manos

29,648
10
62
114

score 1 · Answer 2 · answered May 10 '14 at 15:59

1

May be this will help...

public static void main(String[] args) {
      BigInteger i = new BigInteger("5876934265987526278534978564378568734564937563487564327564376534875483753475");
      String iStr = i.toString();
      int[] intArray = new int[iStr.length()];
      for(int j=0; j<iStr.length(); j++) {
          intArray[j] = Integer.parseInt(String.valueOf(iStr.charAt(j)));
      }
  }

answered May 10 '14 at 15:59

Nielarshi

1,126
7
11

Its too slow solution. – Finkelson May 10 '14 at 16:00
2

Define "too slow". Have you measured it? What are the tresholds you have to reach? – Jeroen Vannevel May 10 '14 at 16:02
I think this as fast as it could be it uses very little code just substring every character, you can use divide by `1000000, 100000, 10000, 1000, 100, 10` but this will be slower – SSpoke May 10 '14 at 16:04
1

What's the difference between this code and the code in the question? – Simon Forsberg May 10 '14 at 16:09
4

that was put after my ans was added.. nothing else.. ;) – Nielarshi May 10 '14 at 16:10
ya..i agree..when u have a very large bigint..its slow...hope someone puts optimised code.. – Nielarshi May 10 '14 at 16:13

score 0 · Answer 3 · answered May 10 '14 at 16:04

0

Try using this:

private static int[] convertDigitsToIntArray(java.math.BigInteger x) {
    String s = x.toString();
    int[] result = new int[s.length()];
    for (int i = 0; i < s.length(); i++) {
        result[i] = s.charAt(i) - '0';
    }
    return result;
}

answered May 10 '14 at 16:04

Valdrinium

1,398
1
13
28

1

Java is pass by value, so assigning a new reference to `int[] result` inside the method won't work. – Luiggi Mendoza May 10 '14 at 16:05

score 0 · Answer 4 · answered May 10 '14 at 16:35

You can convert the BigInteger to a String, get the char array of that string and iterate over that:

java.math.BigInteger source = new java.math.BigInteger("5876934265987526278534978564378568734564937563487564327564376534875483753475");

char[] array = source.toString().toCharArray();
int[] result = new int[array.length];
for(int i=0; i<array.length; i++)
{
    result[i] = array[i] - '0';
}

score 0 · Answer 5 · answered Sep 20 '21 at 12:20

private static int[] convertDigitsToIntArray(java.math.BigInteger x) {
    String s = x.toString();
    int[] result = new int[s.length()];
    for (int i = 0; i < s.length(); i++) {
        // Change your code to subtract the char '0'
        result[i] = s.charAt(i) - '0';
    }
    return result;
}

What is the faster way to conver BigInteger to array of int?

5 Answers5