Qfyilyi

I have to find the largest product of 13 adjacent numbers of a 1000-digit number below. My code for the problem is as follows:

#include <stdio.h>



int main()

{

    char arr[1000] =

        "731671765313306249192251196744265747423553491949349698352031277450"

        "632623957831801698480186947885184385861560789112949495459501737958"

        "331952853208805511125406987471585238630507156932909632952274430435"

        "576689664895044524452316173185640309871112172238311362229893423380"

        "308135336276614282806444486645238749303589072962904915604407723907"

        "138105158593079608667017242712188399879790879227492190169972088809"

        "377665727333001053367881220235421809751254540594752243525849077116"

        "705560136048395864467063244157221553975369781797784617406495514929"

        "086256932197846862248283972241375657056057490261407972968652414535"

        "100474821663704844031998900088952434506585412275886668811642717147"

        "992444292823086346567481391912316282458617866458359124566529476545"

        "682848912883142607690042242190226710556263211111093705442175069416"

        "589604080719840385096245544436298123098787992724428490918884580156"

        "166097919133875499200524063689912560717606058861164671094050775410"

        "022569831552000559357297257163626956188267042825248360082325753042"

        "0752963450";



    int i, j;

    long int max;

    max = 0;

    long int s = 1;

    for (i = 0; i < 988; i++) {

        int a = 0;

        for (j = 1; j <= 13; j++) {

            printf("%c", arr[i + a]);

            s = s * arr[i + a];

            a++;

        }

        printf("%c%d", '=', s);

        printf("n");

        if (s > max) {

            max = s;

        }

    }

    printf("nMaximum product is %d", max);

    getchar();

}

Some outputs are zero even if none of the input is zero. The second output happens to be negative. The answers don't even match. Any help is appreciated.

Many set of 13 digits in your char array arr contains zeroes and that is why the multiplication of these sets will result in 0.

There are a couple of issues with your code:

• You are using %d instead of %ld to print long int. Using the wrong conversion specifier will result in undefined behaviour.

If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.

• You are not converting the ASCII value of the digit into its actual value before multiplication. (ASCII value of '0' is 48). This results in integer overflow and is the cause for negative values to be printed.

So the statement:

s = s * arr[i + a];

should be changed to:

s = s * (arr[i + a] - '0');

• You are also not resetting the product s to 1 at the beginning of the inner for loop and because of this, you end up multiplying values from the results of different sets of 13.

After making these changes, you can see the live demo here.

There are a few issues to tackle in this code:

• Clean up spacing and variable names (an edit by another user helped resolve this issue). Remove redundant variables like a, which j could easily represent by iterating from 0 to 12 rather than 1 to 13. This seems cosmetic but will make it easier for you to understand your program state, so it's actually critical.




• Numerical overflow: As with all PE problems, you'll be dealing with extremely large numbers which may overflow the capacity of the long int datatype (2³¹ - 1). Use unsigned long long to store your max and s (which I'd call product) variables. Print the result with %llu.




• Convert chars to ints: arr[i+j] - '0'; so that you're multiplying the actual numbers the chars represent rather than their ASCII values (which are 48 higher).




• s (really product) is not reset on each iteration of the inner loop, so you're taking the product of the entire 1000-sized input (or trying to, until your ints start to overflow).