Write a function to multiply two arbitrarily large integers.

Question

lang c · Accepted Answer

BINT mult (BINT a, BINT b) {
    BINT sign = 1;
    if (a  0) {
        BINT a16 = a & 0xFF;
        while (b > 0) {
            BINT b16 = b & 0xFF;
            BINT tr = a16 * b16;
            r += tr >= 16;
            sn += 16;            
        }
        a >>= 16;
        sn += 16;        
    }

   return sign < 0 ? -r : r;

}

BigJ · Answer

void Add(char str1[], char str2[], int offset) {
  int carry = 0;
  int len2 = strlen(str2);
  int i;
  for (i = 0; i < len2; i++) {
    int digit1 = TO_INT(str1[i + offset]);
    int digit2 = TO_INT(str2[i]);
    int result = digit1 + digit2 + carry;
    str1[i + offset] = TO_CHAR(result % 10);
    carry = result / 10;
  }
  while (carry != 0) {
    int digit = TO_INT(str1[i]);
    int result = digit + carry;
    str1[i + offset] = TO_CHAR(result % 10);
    carry = result / 10;
  }
}

char *Multiply(char str1[], char str2[]) {
  int len1 = strlen(str1);
  int len2 = strlen(str2);
  int product_len = len1 + len2;

  char *product = new char[product_len + 1];
  for (int i = 0; i < product_len; i++)
    product[i] = '0';
  product[product_len] = '\0';

  int partial_len = len1 + 1;
  char partial[partial_len + 1];
  for (int i = 0; i < partial_len; i++)
    partial[i] = '0'; 
  partial[partial_len] = '\0';

  for (int i = 0; i < len2; i++) {
    int digit2 = TO_INT(str2[i]);
    int carry = 0;
    int j;
    for (j = 0; j < len1; j++) {
      int digit1 = TO_INT(str1[j]);
      int result = digit1 * digit2 + carry;
      partial[j] = TO_CHAR(result % 10);
      carry = result / 10;
    }
    partial[j] = TO_CHAR(carry);

    Add(product, partial, i);
  }
  return product;
}

BigJ · Answer

#define TO_INT(char) ((char) - '0')
#define TO_CHAR(int) ((int) + '0')

lang c · Answer

The solution I posted assumed that it is 32 bit computer. but if you are not sure about this, you have to use string to do multiplication.

Premchand Jayamohan · Answer

The below function 'multiple', can multiply integers (represented as strings)

void strrev(string& s)
{
  for(int i=0;i s2.size())? s1.size() : s2.size();
  for(int i=0;i= i+1 )? s1.at(idx)-'0' : 0 ; 
    unsigned int x2 = (s2.size() >= i+1 )?  s2.at(idx)-'0' : 0 ;
    unsigned int x = carryOver + x1 + x2;
    //cout=0;--i){
    char c = s1.at(i);
    //cout<<"ABC:"<

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