Mislav Janjić Mislav Janjić - 19 days ago 7
C Question

Project Euler #8, I can't figure out why my code gives absurdly high values

Recently, I started with Project Euler and so far it's been going well, but I went on to the 8th problem, and have been stuck on it ever since.

In the problem there is a 1000-digit number and we need to find the greatest product of the adjacent 13 numbers.
The number is here:

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

My code goes as follows:

int main() {
char str[] = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";
unsigned long long int product,maxPrev=0;
int i = 0, j = 0;
product = (str[i] - 48) *(str[i+1] -48) *(str[i+2] -48) *(str[i+3] -48) *(str[i+4] -48) *(str[i+5]-48) *(str[i+6]-48) *(str[i+7] -48) *(str[i+8] -48) *(str[i+9]-48) *(str[i+10]-48) *(str[i+11]-48) *(str[i+12]-48);
maxPrev = product;
for(i=0;i<=986;i++){
product= (str[i] - 48) *(str[i+1] -48) *(str[i+2] -48) *(str[i+3] -48) *(str[i+4] -48) *(str[i+5]-48) *(str[i+6]-48) *(str[i+7] -48) *(str[i+8] -48) *(str[i+9]-48) *(str[i+10]-48) *(str[i+11]-48) *(str[i+12]-48);
if(product>maxPrev){
maxPrev = product;
}
}
printf("\n%llu",maxPrev);
getchar();
return 0;


}

So far, I noticed that somewhere in the code, an overflow occurs and I can't figure out why.

mch mch
Answer

It is because (str[i] - 48) evaluates to an int, not an unsigned long long.

You have to cast the expression

product = (unsigned long long)(str[i] - '0') *(str[i+1] - '0') *(str[i+2] - '0') *(str[i+3] - '0') *(str[i+4] - '0') *(str[i+5] - '0') *(str[i+6] - '0') *(str[i+7] - '0') *(str[i+8] - '0') *(str[i+9] - '0') *(str[i+10] - '0') *(str[i+11] - '0') *(str[i+12] - '0');

Also you should use '0' instead of 48.

Instead of this line you can use another loop

product = 1;
for (int j = 0; j < 13; j++)
{
    if (!isdigit(str[i + j]) || str[i + j] == '0')
    {
        product = 0;
        break;
    }
    product *= str[i + j] - '0';
}

to calculate the product. In this case the cast is not necessary, because product has already the correct type.

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