Thanks for sharing, nice having an ETA now

Cheers,
GG

The reason I put up a general guide of how it is solved is so that there is a hint for people with no idea of how to go about it! It’s like many problems in maths – the answer itself is (relatively) uninteresting, it’s the work to get there that’s the useful bit

Perhaps having both answer and working available would be most useful? If it was me, I’d be tempted to just put the answer in without doing the working which is why I omitted the answer. No harm done either way

Lastly, if you’ve no objections, I’ll package your code up into a text file and link that for download, or put it in the body of the post in a collapsible <div> ? The theme I’m using doesn’t seem to be great for code snippets unfortunately!

#include
using namespace std;

int grid [400] = {
8, 2, 22, 97, 38, 15, 0, 40, 0, 75, 4, 5, 7, 78, 52, 12, 50, 77, 91, 8,
49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48, 4, 56, 62, 0,
81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30, 3, 49, 13, 36, 65,
52, 70, 95, 23, 4, 60, 11, 42, 69, 24, 68, 56, 1, 32, 56, 71, 37, 2, 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, 3, 45, 2, 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, 2, 62, 12, 20, 95, 63, 94, 39, 63, 8, 40, 91, 66, 49, 94, 21,
24, 55, 58, 5, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72,
21, 36, 23, 9, 75, 0, 76, 44, 20, 45, 35, 14, 0, 61, 33, 97, 34, 31, 33, 95,
78, 17, 53, 28, 22, 75, 31, 67, 15, 94, 3, 80, 4, 62, 16, 14, 9, 53, 56, 92,
16, 39, 5, 42, 96, 35, 31, 47, 55, 58, 88, 24, 0, 17, 54, 24, 36, 29, 85, 57,
86, 56, 0, 48, 35, 71, 89, 7, 5, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58,
19, 80, 81, 68, 5, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77, 4, 89, 55, 40,
4, 52, 8, 83, 97, 35, 99, 16, 7, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66,
88, 36, 68, 87, 57, 62, 20, 72, 3, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69,
4, 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, 1, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57, 5, 54,
1, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52, 1, 89, 19, 67, 48};

int across (int x) {
int product;
product = grid[x]*grid[x+1]*grid[x+2]*grid[x+3];
return product;}

int down (int x) {
int product;
product = grid[x]*grid[x+20]*grid[x+40]*grid[x+60];
return product;}

int diagonalf (int x) {
int product;
product = grid[x]*grid[x+21]*grid[x+42]*grid[x+63];
return product;}

int diagonalb (int x) {
int product;
product = grid[x]*grid[x+19]*grid[x+38]*grid[x+57];
return product;}

int main () {
int pos=0, hia=0, storea, hid=0, stored, hidf=0, storedf, hidb=0, storedb;
for (; pos<400; pos++) {
int y;
y=pos%20;
if (y<19) {cout << grid[pos] << " ";}
if (y==19) {cout << grid[pos] << endl;}
}
cout << "The greatest product of 4 adjacent numbers is ";
pos=0;
for (; pos<400; pos++) {
int y, v;
y=pos%20;
if (yhia) {hia = v; storea = pos;}}
}
pos=0;
for (; poshid) {hid=v; stored=pos;}
}
pos=0;
for (; pos<340; pos++) {
int y, v;
y=pos%20;
if (yhidf) {hidf=v; storedf=pos;}}
}
pos=0;
for (; pos2) {v = diagonalb(pos);
if (v>hidb) {hidb=v; storedb=pos;}}
}
if (hia>hid && hia>hidf && hia>hidb) {cout << hia << "." << endl << "Starting at position "
<< storea << " and moving across (to the right)." << endl;
cout << "Values of " << grid[storea] << ", " << grid[storea+1] << ", " << grid[storea+2]
<< ", and " << grid[storea+3] <hia && hid>hidf && hid>hidb) {cout << hid << "." << endl << "Starting at position "
<< stored << " and moving down." << endl;
cout << "Values of " << grid[stored] << ", " << grid[stored+20] << ", " << grid[stored+40]
<< ", and " << grid[stored+60] <hia && hidf>hid && hidf>hidb) {cout << hidf << "." << endl << "Starting at position "
<< storedf << " and moving diagonally forward." << endl;
cout << "Values of " << grid[storedf] << ", " << grid[storedf+21] << ", " << grid[storedf+42]
<< ", and " << grid[storedf+63] <hia && hidb>hid && hidb>hidf) {cout << hidb << "." << endl << "Starting at row "
<< storedb/20+1 << ", column " << storedb%20+1 << " and moving diagonally backward." << endl;
cout << "Values of " << grid[storedb] << ", " << grid[storedb+19] << ", " << grid[storedb+38]
<< ", and " << grid[storedb+57] << endl;}
}

Answer (still unverified): 70,600,674
It was found starting at row 13, column 7 and moves diagonally down right to left from there. Values 89, 94, 97, and 87.