How to convert an arbitrary large integer from base 10 to base 16?

Question

The program requires an input of an arbitrary large unsigned integer which is expressed as one string in base 10. The outputs is another string that expresses the integer in bas

Answer 1

The real challenging part is the "arbitrary large" unsigned integer.

Have you tried using GNU MP Bignum library?

Answer 2

Here is the above-mentioned algorithm implemented in javascript:

function addDecValue(hexArray, value) {
  let carryover = value;
  for (let i = (hexArray.length - 1); i >= 0; i--) {
    let rawDigit = ((hexArray[i] || 0) * 10) + carryover;
    hexArray[i] = rawDigit % 16;
    carryover = Math.floor(rawDigit / 16);
  }
}

function toHexArray(decimalString) {
  let hexArray = new Array(decimalString.length);
  for (let i = 0; i < decimalString.length; i++) {
    addDecValue(hexArray, Number(decimalString.charAt(i)));
  }
  return hexArray;
}

function toHexString(hexArray) {
  const hexDigits = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'];
  let result = '';
  for (let i = 0; i < hexArray.length; i++) {
    if (result === '' && hexArray[i] === 0) continue;
    result += hexDigits[hexArray[i]];
  }
  return result
}

toHexString(toHexArray('1234567890987654321234567890987654321234567890987654321'));

Answer 3

Python:

>>> from string import upper
>>> input = "1234567890987654321234567890987654321234567890987654321"
>>> output = upper(hex(int(input)))[2:-1]
>>> print output
CE3B5A137DD015278E09864703E4FF9952FF6B62C1CB1

Answer 4

I have written an article which describes a simple solution in Python which can be used to transfrom a series of numbers from and to arbitrary number bases. I've originally implemented the solution in C, and I didn't want a dependency to an external library. I think you should be able to rewrite the very easy Python code in C or whatever you like.

Here is the Python code:

import math
import string

def incNumberByValue(digits, base, value):
   # The initial overflow is the 'value' to add to the number.
   overflow = value
   # Traverse list of digits in reverse order.
   for i in reversed(xrange(len(digits))):
      # If there is no overflow we can stop overflow propagation to next higher digit(s).
      if not overflow:
         return
      sum = digits[i] + overflow
      digits[i] = sum % base
      overflow = sum / base

def multNumberByValue(digits, base, value):
   overflow = 0
   # Traverse list of digits in reverse order.
   for i in reversed(xrange(len(digits))):
      tmp = (digits[i] * value) + overflow
      digits[i] = tmp % base
      overflow = tmp / base

def convertNumber(srcDigits, srcBase, destDigits, destBase):
   for srcDigit in srcDigits:
      multNumberByValue(destDigits, destBase, srcBase)
      incNumberByValue(destDigits, destBase, srcDigit)

def withoutLeadingZeros(digits):
   for i in xrange(len(digits)):
      if digits[i] != 0:
         break
   return digits[i:]

def convertNumberExt(srcDigits, srcBase, destBase):
   # Generate a list of zero's which is long enough to hold the destination number.
   destDigits = [0] * int(math.ceil(len(srcDigits)*math.log(srcBase)/math.log(destBase)))
   # Do conversion.
   convertNumber(srcDigits, srcBase, destDigits, destBase)
   # Return result (without leading zeros).
   return withoutLeadingZeros(destDigits)


# Example: Convert base 10 to base 16
base10 = [int(c) for c in '1234567890987654321234567890987654321234567890987654321']
base16 = convertNumberExt(base10, 10, 16)
# Output list of base 16 digits as HEX string.
hexDigits = '0123456789ABCDEF'
string.join((hexDigits[n] for n in base16), '')

Answer 5

Here's a BigInt library:

http://www.codeproject.com/KB/cs/BigInt.aspx?msg=3038072#xx3038072xx

No idea if it works, but it's the first one I found with Google. It appears to have functions to parse and format big integers, so they may support different bases too.

Edit: Ahh, you're using C, my mistake. But you may be able to pick up ideas from the code, or someone using .NET may have the same question, so I'll leave this here.

Answer 6

1. Allocate an array of integers, number of elements is equal to the length of the input string. Initialize the array to all 0s.

   This array of integers will store values in base 16.

2. Add the decimal digits from the input string to the end of the array. Mulitply existing values by 10 add carryover, store new value in array, new carryover value is newvalue div 16.

   carryover = digit;
   for (i = (nElements-1); i >= 0; i--)
   {
       newVal = array[index] * 10) + carryover;
       array[index] = newval % 16;
       carryover = newval / 16;
   }
   

3. print array, start at 0th entry and skip leading 0s.

---

Here's some code that will work. No doubt there are probably a few optimizations that could be made. But this should suffice as a quick and dirty solution:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "sys/types.h"

char HexChar [16] = { '0', '1', '2', '3', '4', '5', '6', '7',
                      '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };

static int * initHexArray (char * pDecStr, int * pnElements);

static void addDecValue (int * pMyArray, int nElements, int value);
static void printHexArray (int * pHexArray, int nElements);

static void
addDecValue (int * pHexArray, int nElements, int value)
{
    int carryover = value;
    int tmp = 0;
    int i;

    /* start at the bottom of the array and work towards the top
     *
     * multiply the existing array value by 10, then add new value.
     * carry over remainder as you work back towards the top of the array
     */
    for (i = (nElements-1); (i >= 0); i--)
    {
        tmp = (pHexArray[i] * 10) + carryover;
        pHexArray[i] = tmp % 16;
        carryover = tmp / 16;
    }
}

static int *
initHexArray (char * pDecStr, int * pnElements)
{
    int * pArray = NULL;
    int lenDecStr = strlen (pDecStr);
    int i;

    /* allocate an array of integer values to store intermediate results
     * only need as many as the input string as going from base 10 to
     * base 16 will never result in a larger number of digits, but for values
     * less than "16" will use the same number
     */

    pArray = (int *) calloc (lenDecStr,  sizeof (int));

    for (i = 0; i < lenDecStr; i++)
    {
        addDecValue (pArray, lenDecStr, pDecStr[i] - '0');
    }

    *pnElements = lenDecStr;

    return (pArray);
}

static void
printHexArray (int * pHexArray, int nElements)
{
    int start = 0;
    int i;

    /* skip all the leading 0s */
    while ((pHexArray[start] == 0) && (start < (nElements-1)))
    {
        start++;
    }

    for (i = start; i < nElements; i++)
    {
        printf ("%c", HexChar[pHexArray[i]]);
    }

    printf ("\n");
}

int
main (int argc, char * argv[])
{
    int i;
    int * pMyArray = NULL;
    int nElements;

    if (argc < 2)
    {
        printf ("Usage: %s decimalString\n", argv[0]);
        return (-1);
    }

    pMyArray = initHexArray (argv[1], &nElements);

    printHexArray (pMyArray, nElements);

    if (pMyArray != NULL)
        free (pMyArray);

    return (0);
}