Thursday, June 4, 2015

Write a function that subtract two integer streams


You are given two streams 's_a' and 's_b' of digits. Each stream represents an integer 'a' and 'b' from its less significant digit to the most significant digit.
For example, integer 2048 is represented in the stream as 8, 4, 0, 2. 

Write a function that subtract two integers 'a - b' and returns the result as a string. You are not allowed to buffer entire 'a' or 'b' into a single string, i.e. you may access only a single digit per stream at time (imagine that 'a' and 'b' are stored in huge files). You may assume that 'a>=b'.

#include <stdio.h>

/* Subtracts two single digits, prints the reminder and returns 1(carry) if a<b */
int subtract(char c_a, char c_b)
{
  /* Convert the char to interger */
  int i_a = c_a - '0';
  int i_b = c_b - '0';
  int ret = 0;

  /* If a<b, increase 'a' by 10 and return 1(carry) to subtract it from 'a' of the next set */
  if (i_a < i_b) {
    i_a += 10;
    ret = 1;
  }
  
  printf ("%d", i_a - i_b);

  return ret;
}


int main(int agrc, char **argv)
{
  /* Get the two operand streams from input arguments */
  char *s_a = argv[1];
  char *s_b = argv[2];

  int carry_one = 0;
  
  /* Pass one char at a time from each input arguments to function subtract */
  while(*s_b != '\0'){
    /* Subtract carry_one from first operand before sending it to function subtract */
    carry_one = subtract(*s_a - carry_one, *s_b);
    s_a++;
    s_b++;
  }

  /* Print the remining digits from the first operand */
  while(*s_a != '\0'){
    printf("%c", *s_a - carry_one);
    s_a++;
    carry_one = 0;
  }
  printf("\n");
  
  return 0;
}
Output:
  $ ./str_subtract 00051 1005 | rev
  09999
  
  $ ./str_subtract 5 3 | rev
  2
  
  $ ./str_subtract 1234 1233 | rev
  1000