sol1.c File Reference

Problem 8 solution More...

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

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Functions
long long int	get_product (FILE *fp, long start_pos, int num_digits)
	Compute the product of two numbers in a file. More...
int	main (int argc, char *argv[])
	Main function. More...

Detailed Description

Problem 8 solution

Author

Krishna Vedala

Function Documentation

get_product()

long long int get_product	(	FILE *	fp,
		long	start_pos,
		int	num_digits
	)

Compute the product of two numbers in a file.

Parameters

[in]	fp	pointer to file that is already open
[in]	start_pos	line number of the first numer
[in]	num_digits	number of digits on the line to multiply

Returns

expected product

{
    char ch = ' '; /* temporary variable to store character read from file */
    unsigned char num = 0;  /* temporary variable to store digit read */
    long long int prod = 1; /* product accumulator */
    int count =
        0; /* we use this variable to count number of bytes of file read */
 
    /* accumulate product for num_digits */
    for (int i = 0; i < num_digits; i++, count++)
    {
        /* get character from file */
        ch = getc(fp);
 
        /* the ASCII codes of digits is between 0x30 and 0x39.
         * any character not in this range implies an invalid character
         */
        if (ch < 0x30 || ch > 0x39)
        {
            if (ch == EOF)
                return 0;
            i--;
            continue;
        }
 
        num = ch - 0x30; /* convert character digit to number */
        if (num == 0)
        {
            /* If number is zero, we can skip the next 'num_digits'
             * because this '0' will repeat in the next 'num_digit'
             * multiplications. Hence, we also do not update the file position
             */
            /* NOTE: this is not needed but helps get results faster :) */
            return 0;
        }
 
        prod *= num; /* accumulate product */
    }
 
    /* set file position to the next starting character + 1 */
    fseek(fp, -count + 1, SEEK_CUR);
 
    return prod;
}

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main()

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

Main function.

{
    int position = 0;
    int num_digits = 4;
    long long int prod, max_prod = 0;
 
    /* if second command-line argument is ge=iven,
     * use it as the number of digits to compute
     * successive product for
     */
    if (argc == 2)
        num_digits = atoi(argv[1]);
 
    /* open file to read digits from */
    FILE *fp = fopen("digits.txt", "rt");
    if (!fp)
    {
        perror("Unable to open file");
        return -1;
    }
 
    /* loop through all digits in the file */
    do
    {
        /* get product of 'num_digits' from current position in file */
        prod = get_product(fp, ftell(fp), num_digits);
 
        if (prod > max_prod)
        {
            max_prod = prod;
            position = ftell(fp) - 1;
        }
    } while (!feof(fp)); /* loop till end of file is reached */
 
    printf("Maximum product: %lld\t Location: %d^th position\n\t", max_prod,
           position);
    fseek(fp, position,
          SEEK_SET); /* move cursor to identified position in file */
    /* loop through all digits */
    for (; num_digits > 0; num_digits--)
    {
        char ch = getc(fp); /* get character */
        /* skip invalid character */
        if (ch < 0x30 || ch > 0x39)
            continue;
        if (num_digits > 1)
            printf("%c x ", ch);
        else
            printf("%c = %lld\n", ch, max_prod);
    }
 
    fclose(fp); /* close file */
 
    return 0;
}

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