I too am attempting Project Euler #8. I know what the right answer is, but I'm getting something different. I'm using all ulong in a checked block, so this isn't related to overflow.

Here's the problem:

The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.

73167176531330624919225119674426574742355349194934 96983520312774506326239578318016984801869478851843 85861560789112949495459501737958331952853208805511 12540698747158523863050715693290963295227443043557 66896648950445244523161731856403098711121722383113 62229893423380308135336276614282806444486645238749 30358907296290491560440772390713810515859307960866 70172427121883998797908792274921901699720888093776 65727333001053367881220235421809751254540594752243 52584907711670556013604839586446706324415722155397 53697817977846174064955149290862569321978468622482 83972241375657056057490261407972968652414535100474 82166370484403199890008895243450658541227588666881 16427171479924442928230863465674813919123162824586 17866458359124566529476545682848912883142607690042 24219022671055626321111109370544217506941658960408 07198403850962455444362981230987879927244284909188 84580156166097919133875499200524063689912560717606 05886116467109405077541002256983155200055935729725 71636269561882670428252483600823257530420752963450

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

And here's my implementation (using LinqPad). I keep getting an answer of 10077696000 but I know the answer is actually 23514624000;

string digit = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355668966489504452445231617318564030987111217223831136222989342338030813533627661428280644448664523874930358907296290491560440772390713810515859307960866701724271218839987979087922749219016997208880937766572733300105336788122023542180975125454059475224352584907711670556013604839586446706324415722155397536978179778461740649551492908625693219784686224828397224137565705605749026140797296865241453510047482166370484403199890008895243450658541227588666881164271714799244429282308634656748139191231628245861786645835912456652947654568284891288314260769004224219022671055626321111109370544217506941658960408071984038509624554443629812309878799272442849091888458015616609791913387549920052406368991256071760605886116467109405077541002256983155200055935729725716362695618826704282524836008232575304207529634500";


int numDigits = 13;



ulong answer = 0;
int answerIndex = 0;
int currentIndex = 0;
checked
{
    while (currentIndex <= digit.Length - numDigits)
    {
        var substr = digit.Substring(currentIndex, numDigits);
        if (!substr.Contains('0'))
        {
            ulong product = substr.ToCharArray().Select(x => ulong.Parse(x.ToString())).Aggregate((x, y) => x * y);

            if (product > answer)
            {
                answerIndex = currentIndex;
                answer = product;
                var disp = string.Join(" x ", substr.ToCharArray());
                $"Start Index:\t{currentIndex.ToString().PadLeft(4)}\t\tDigits:\t{disp} = {product}".Dump();
            }
        }
        currentIndex  ;
    }
}

$"Answer is: {answer}".Dump();

CodePudding user response：

Your argument is incorrect, something went wrong when you copy pasted and you lost 7:

// current code
string digit = "...227443043556689664895044524452316173185640..." 
// should be
string digit = "...2274430435576689664895044524452316173185640..."
                              ^
                              |
                            missed 

Edit: To spot this I've solved the problem myself:..

      string digit = ...;

      int numDigits = 13;

      var result = Enumerable
        .Range(0, digit.Length - numDigits   1)
        .Select(start => digit
           .Skip(start)
           .Take(numDigits)
           .Aggregate(1L, (s, a) => s * (a - '0')))
        .Max();

...And I got the incorrect result (10077696000), then I copy pasted data from the original site and got the right outcome (23514624000). All I had to do is to compare inputs.