Project Euler Problem #11

Source: http://projecteuler.net/index.php?section=problems&id=11

Quick overview: Take a 20x20 grid of numbers and compute the largest product of 4 pairs of numbers in either horizontal, vertical, or diagonal.

My current approach is 开发者_Go百科to divide the 20x20 grid up into single rows and single columns and go from there with a much more manageable grid. The code I'm using to divide the rows into rows is

void fillRows
    ( string::const_iterator& fieldIter, 
      list<int>& rowElements, 
      vector<list<int>>& rows )
{
    // fieldIter is already initialized.
    int count(0);
    for( ; fieldIter < field.end(); ++fieldIter )
    {
        if(isdigit(field[*fieldIter]))
        {           
            rowElements.push_back(toInt(field[*fieldIter]));
            ++count;
        }
        if(count == 40)
        {
            rows.push_back(rowElements);
            count = 0;
            rowElements.clear();
        }
    }
}

Short explanation: I have the field set as static const std::string field and I am filling a vector with lists of rows. Why a list? Because the queue doesn't have a clear function. Also practice using STL container lists and not ones I write myself.

However, this thing isn't working. Oftentimes I see it omitting a character( function toInt parses the const char as int ) and I end up with 18 rows, two rows short of the 20x20 grid. The length of the rows seem good.

Rows: 18

RowElements[0]: 40 (instead of pairs I saved each number individually. Will fix that later)

What am I doing wrong?

---

You know the grid is 20x20, so just paste it into a text file and read it like you would normally read a matrix:

for (int i = 0; i < 20; ++i)
    for (int j = 0; j < 20; ++j)
        inFile >> mat[i][j];

Then do the obvious: for each element [i][j], go 4 elements down, 4 to the right, 4 diagonally to the right and 4 diagonally to the left and find the product. If you can't get 4 elements because of boundaries, ignore those you can get.

There's no need to complicate this like you seem to be doing. Keep it simple, because this is a simple problem, and if you overthink it you will only make your life harder.

---

I have written this code. I hope it helps, though it is a long one....

#include <iostream>
#include <vector>
using namespace std;



void main() 
{

int num_container[20][20] = {
                            { 8,02,22,97,38,15,00,40,00,75,04,05,07,78,52,12,50,77,91, 8},
                            {49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48,04,56,62,00},
                            {81,49,31,73,55,79,14,29,93,71,40,67,53,88,30,03,49,13,36,65},
                            {52,70,95,23,04,60,11,42,69,24,68,56,01,32,56,71,37,02,36,91},
                            {22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80},
                            {24,47,32,60,99,03,45,02,44,75,33,53,78,36,84,20,35,17,12,50},
                            {32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70},
                            {67,26,20,68,02,62,12,20,95,63,94,39,63, 8,40,91,66,49,94,21},
                            {24,55,58,05,66,73,99,26,97,17,78,78,96,83,14,88,34,89,63,72},
                            {21,36,23, 9,75,00,76,44,20,45,35,14,00,61,33,97,34,31,33,95},
                            {78,17,53,28,22,75,31,67,15,94,03,80,04,62,16,14, 9,53,56,92},
                            {16,39,05,42,96,35,31,47,55,58,88,24,00,17,54,24,36,29,85,57},
                            {86,56,00,48,35,71,89,07,05,44,44,37,44,60,21,58,51,54,17,58},
                            {19,80,81,68,05,94,47,69,28,73,92,13,86,52,17,77,04,89,55,40},
                            {04,52, 8,83,97,35,99,16,07,97,57,32,16,26,26,79,33,27,98,66},
                            {88,36,68,87,57,62,20,72,03,46,33,67,46,55,12,32,63,93,53,69},
                            {04,42,16,73,38,25,39,11,24,94,72,18, 8,46,29,32,40,62,76,36},
                            {20,69,36,41,72,30,23,88,34,62,99,69,82,67,59,85,74,04,36,16},
                            {20,73,35,29,78,31,90,01,74,31,49,71,48,86,81,16,23,57,05,54},
                            {01,70,54,71,83,51,54,69,16,92,33,48,61,43,52,01,89,19,67,48},
                                                                                        };


int test = num_container[6][8] * num_container[7][9] * num_container[8][10] * num_container[9][11];
cout<<test<<endl;
system("pause");

int start = 0;
int end = 3;
long long mul_result = 1;

vector<long long>final_results;

/////////////////////UP/DOWN/////////////////////
for(int k=0; k<20; k++)
{
    for(int i=0; i<=16; i++)
    {
        for(int j=start; j<=end; j++)
        {
            mul_result = mul_result * num_container[k][j];
            if (j == end)
                final_results.push_back(mul_result);
        }
        mul_result = 1;
        start++;
        end++;
    }
    start = 0;
    end = 3;

    for(int i=0; i<=16; i++)
    {
        for(int j=start; j<=end; j++)
        {
            mul_result = mul_result * num_container[j][k];
            if (j == end)
                final_results.push_back(mul_result);
        }
        mul_result = 1;
        start++;
        end++;
    }
    start = 0;
    end = 3;

}
/////////////////////UP/DOWN Ends here//////////////////////



///////////////////Both Ways Diagonal Starts here//////////////////////
int current_row = 0;

for(int i=0; i<=16; i++)
{
    for(int j=0; j<=16; j++)
    {
        current_row = i;
        for(int k=start; k<=end; k++)
        {
            mul_result = mul_result * num_container[current_row][k];
            current_row++;
            if (k==end)
                final_results.push_back(mul_result);
        }
        mul_result = 1;
        start++;
        end++;          
    }
    start = 0;
    end = 3;

    for(int j=0; j<=16; j++)
    {
        current_row = i+3;
        for(int k=start; k<=end; k++)
        {
            mul_result = mul_result * num_container[current_row][k];
            current_row--;
            if (k==end)
                final_results.push_back(mul_result);
        }
        mul_result = 1;
        start++;
        end++;          
    }
    start = 0;
    end = 3;
}
/////////////////////Both Ways diagonal ends here///////////////////


////////////////////Compare Thning Starts here//////////////////////

long long the_big_one = 0;
for(int i=0; i<final_results.size(); i++)
{
    if (final_results[i] > the_big_one)
        the_big_one = final_results[i];
}





cout<<endl<<endl<<"The big one is: "<<the_big_one<<endl;

system("pause");
}

---

Why not use a std::vector instead of a list? Lists and queues are a bad choice for this question as you require random-access, for which you need a vector.

Why are you moving to the next row when count == 40? Shouldn't it be 20?

You are using iterators wrong. You don't use field[*fieldIter] to get the element at the iterator, you just use *fieldIter.

You should use fieldIter != field.end() instead of <. For a string, it's the same either way, but for other containers (such as a list), < won't work because the list nodes aren't ordered in memory.

Where is your toInt function?

Anyway, why make this so complicated?

int grid[20][20];

for (int i = 0; i < 20; ++i)
  for (int j = 0; j < 20; ++j)
    std::cin >> grid[i][j];