Project Euler 8 in c++

Question

I'm trying to solve problem 8 from project euler but I'm getting way too big numbers as results and I don't know why.

The problem is "Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?"

My code :

#include <iostream>
#include <string>

int main()
{
    std::string str = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";

    long long a = 1;
    long long fin = 0;

    for (int c = 0; c < 988; c++)
    {
        for (int d = 0; d < 13; d++)
        {
            a = a * str.at(c + d);
        }

        if (a > fin)
        {
            fin = a;
            std::cout << fin << " at " << c << std::endl;
        }

        a = 1;
    }

    system("pause");
}

The output :

7948587103611909356 at 0
8818137127266647872 at 15
8977826317031653376 at 71
9191378290313403392 at 214
9205903071867879424 at 573
Press any key to continue...

I do love meaningful variable names. Do you? Along with constants plucked out of the air — Ed Heal, Dec 25 '16 at 04:44
Why do you have the line ` system("pause");`? http://stackoverflow.com/questions/4118073/how-to-stop-console-from-closing-on-exit — Ed Heal, Dec 25 '16 at 04:53
@EdHeal.: I guess he is using Dev cpp ...so to prevent auto exit from output window... — user2736738, Dec 25 '16 at 04:56
I'm surprised that you didn't get a "time out" error for this code submission. Note that any string with 0 in it would justify a much larger increment in the loop than "+1", since any number multiplied by 0 is 0. — PaulMcKenzie, Dec 25 '16 at 05:32

score 3 · Accepted Answer · edited Dec 25 '16 at 04:58

The problem is that the characters '0' through '9' are not the same as the integers 0 through 9; rather, '0' has the value 48, '1' has the value 49, and so on. (These are the ASCII values of those characters.)

So to convert from a digit character to the desired number — for example, to extract e.g. 3 from '3' — you need to subtract '0'. In other words, you need to change this:

            a = a * str.at(c + d);

to this:

            a = a * (str.at(c + d) - '0');

Project Euler 8 in c++

1 Answers1