c++ - Project Euler #8, I don't understand where I'm going wrong -

i'm working on project euler problem number eight, in ive been supplied ridiculously large number:

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

and supposed "find thirteen adjacent digits in 1000-digit number have greatest product." eg product of first 4 adjacent digits 7 * 3 * 1 * 6. code following:

int main() {     string num = /* ridiculously large number omitted */;     int greatestproduct = 0;     int product;     (int i=0; < num.length() -12; i++)     {         product = ((int) num[i] - 48);         (int j=i+1; j<i+13; j++)         {             product = product * ((int) num[j] - 48);             if (greatestproduct <= product)             {                 greatestproduct = product;             }         }     }     cout << greatestproduct << endl; } 

i keep getting 2091059712 answer project euler informs me wrong , suspect large anyway. appreciated.

edit: changed unsigned long int , worked. everyone!

in fact solution small rather big. answer pointed out in comments, there integer overflow, , clue in fact solution close largest possible value unsigned int: 2147483647. need use different type store product.

note answer below still 'correct' in code wrong, it's not causing wrong value. try taking (working) code http://codereview.stackexchange.com if folks there tell improve in approach , coding style.

previous answer

you checking new greatest product inside inner loop instead of outside. means maximum includes strings of less or equal ton 13 digits, rather 13.

this make difference if finding string has fewer 13 digits have large product, 0 @ either end. shouldn't count largest, code does. (i haven't checked if happen.)

for (int i=0; < num.length() -12; i++) {     product = ((int) num[i] - 48);     (int j=i+1; j<i+13; j++)     {         product = product * ((int) num[j] - 48);     }     if (greatestproduct <= product)     {         greatestproduct = product;     } }