8.c

/*
The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.
    *see code for number*
Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?
*/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <locale.h>

#define ADJ_DIGITS 13

unsigned long long int get_product(char *arr) {
    int len = strlen(arr);
    int value;
    unsigned long long int product = 1;

    for(int i = 0; i < len; i++) {
        value = arr[i] - '0'; // quick and dirty char to int conversion
        product *= value;
    }

    return product;
}

int main(void) {
    char largenum[] = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";
    char digits[ADJ_DIGITS + 1];
    unsigned long long int product;
    unsigned long long int maxProduct = 0;
    char maxDigits[ADJ_DIGITS + 1];

    int sizelargenum = strlen(largenum);
    memset(digits, '\0', sizeof(digits));
    memset(maxDigits, '\0', sizeof(maxDigits));
    for(int i = 0; i <= sizelargenum; i++) {
        strncpy(digits, largenum + i, ADJ_DIGITS);

        product = get_product(digits);

        if (product > maxProduct) {
            maxProduct = product;
            memcpy(maxDigits, digits, strlen(digits) + 1);
        }
    }

    setlocale(LC_NUMERIC, "");
    printf("max: %'llu\n", maxProduct);
    printf("adjacent digits: %s\n", maxDigits);

    return 0;
}