CODEWITHSUNDEEP
crecursionmultiplication
Ace
Ace

Reputation: 379

Multiplying a large number in C with a single digit number recursively?

I'm having trouble creating a recursive program that multiplies a large number with a single digit number. I understand they're are simpler methods to doing this, but I would like to do it recursively. I provided a SSCCE in the code. The problem is that the multiplication is not occurring correctly. For numbers with more than 1 digit, the program will only multiply the last digit, instead of multiplying the entire number. 

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *getmem(int len) {
  char *ret;
  ret = malloc(len);
  if (NULL == ret) {
    printf("memory allocation failed!\n");
    exit (0);
  }
  else
    return ret;
}


void printdigs(char *digs) {
    if('0' == digs[0])  {
        if (0 == digs[1]){
            printf("%s", digs);
            return;
        }
        else printdigs(&digs[1]);
    }
  else printf("%s",digs);

}


void multi_helper(char *a, char *r, char b, int len, int carry) {
    int sum;
    if (-1 == len) {
        r[0] = (char) (carry + 48);
        return;
    }
    sum = (a[len]-48) * (b-48) +carry;
    r[len+1] = (char) ((sum % 10) + 48);
    if (sum > 9)
        carry = 1;
    else carry = 0;
    multi_helper(a,r,'0', len-1,carry);

}


char *multi(char *a, char b) {
  char *res;
    int l = strlen(a);
  res = getmem(l + 2);
    res[l+1] = 0;
  multi_helper(a, res, b, l-1,0);
  return res;
}


int main(int argc, char *argv[]) {

  char *n1 = "1000";


  printf("%s multiplied by 5 is ", n1);
  printdigs(multi(n1,"5"));
  printf("\n");


  return 0;
}

Thanks for any help.

Upvotes: 0

Views: 845

Answers (2)

xaxxon
xaxxon

Reputation: 19781

I don't really know what you're looking for (this doesn't use strings to store numbers), but this uses recursion to add a number to itself a certain number of times (multiplication) using recursion for counting how many iterations are left.

int recursive_multiply(a, b) {

  if (b==0) {
    return 0;
  }

  char sign = 1;
  if (b < 0) {
    sign = -1;
    b = -b;
  }

  return (a + recursive_multiply(a, b-1)) * sign;

}

edit: with the updates to the question, it's clear this solution doesn't directly answer the question Ace has, but it may answer questions other people may have when they search for something similar to the question, so I'm going to leave it. If anyone thinks this is wrong, comment and I'll consider removing it if warranted.

Upvotes: 2

BLUEPIXY
BLUEPIXY

Reputation: 40155

printdigs(multi(n1,"5"));

to

printdigs(multi(n1,'5'));

also need free return value of function multi

multi_helper(a,r,'0', len-1,carry);

to

multi_helper(a,r,b, len-1,carry);

Upvotes: 3

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