My code

… was written in C++ and can be compiled with G++, Clang++, Visual C++. You can download it, too. Or just jump to my GitHub repository.

       
#include <iostream>
       
#include <string>
       
 
       
int main()
       
{
       
  unsigned int tests;
       
  std::cin >> tests;
       
  while (tests--)
       
  {
       
    // length of string
       
    unsigned int length;
       
    // number of relevant consecutive digits
       
    unsigned int span;
       
    // read number as a string
       
    std::string number;
       
    std::cin >> length >> span >> number;
       
 
       
    // results can be much bigger than 32 bits ... but 64 bits are enough, though
       
    unsigned long long best = 0;
       
    // loop ends when there are less than "span" digits left
       
    for (int start = 0; start + span < length; start++)
       
    {
       
      unsigned long long current = 1;
       
      // convert digits from ASCII to numbers and multiply
       
      for (unsigned int i = 0; i < span; i++)
       
        current *= number[start + i] - '0';
       
 
       
      // better than before ?
       
      if (best < current)
       
        best = current;
       
    }
       
 
       
    std::cout << best << std::endl;
       
  }
       
  return 0;
       
}

In order to run my code, execute
echo "1 1000 13 7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450" | ./euler-0008

(input format usually follows Hackerrank's requirements)

This solution contains 4 empty lines, 7 comments and 2 preprocessor commands.