Euler Project 11

Question

I'm busy with Project Euler, and i'm trying to solve problem 11, but the answer seems to be faulty. But I don't understand why, the code seems to be right.

vector<vector<int> > grid;
    ifstream stream("/home/uauser/workspace/Project_euler/grid.txt");
    string line;
    char *tok;
    if (stream.is_open())
    {
        while(stream.good())
        {
            getline(stream, line);
            tok = strtok((char *)line.c_str(), " ");
            vector<int> row;
            while (tok != NULL)
            {
                int field;
                stringstream ss;
                ss << tok;
                ss >> field;
                row.push_back(field);

                tok = strtok(NULL, " ");
            }
            grid.push_back(row);
        }
        stream.close();
    }

    int product = 0;
    for(unsigned int i = 0; i < grid.size(); i++)
    {
        for(unsigned int j = 0; j < grid.at(i).size(); j++)
        {
            if( i < 17)
            {
                product = max(product, grid.at(i).at(j) * grid.at(i + 1).at(j) * grid.at(i + 2).at(j) * grid.at(i + 3).at(j));
            }
            if( j < 17)
            {
                product = max(product, grid.at(i).at(j) * grid.at(i).at(j+1)* grid.at(i).at(j+2) * grid.at(i).at(j + 3));
            }
            if((j < 17) && (i < 17) && (j >= 3) && (i >= 3))
            {
                product = max(product, grid.at(i).at(j) * grid.at(i-1).at(j+1) * grid.at(i-1).at(j+2) * grid.at(i -3).at(j+3));
                product = max(product, grid.at(i).at(j) * grid.at(i+1).at(j-1) * grid.at(i+1).at(j-2) * grid.at(i +3).at(j-3));
                product = max(product, grid.at(i).at(j) * grid.at(i+1).at(j+1) * grid.at(i+1).at(j+2) * grid.at(i + 3).at(j + 3));
                product = max(product, grid.at(i).at(j) * grid.at(i-1).at(j-1) * grid.at(i-1).at(j-2) * grid.at(i -3).at(j-3));
            }
            cout<<product<<endl;

        }
    }
    cout<<"The Product is: "<<product<<endl;
}

sow the code read's the txt file without a problem, but when he need to find the max value is only get the wrong answer.

Because your I/O loop is testing `stream.good()` you may be [processing the last line twice](http://stackoverflow.com/questions/4324441/testing-stream-good-or-stream-eof-reads-last-line-twice). Use `while(getline(stream, line))` instead. — Blastfurnace, Jun 10 '12 at 22:13
Your string tokenizing has a serious problem too. Modifying the result of `.c_str()` is __undefined behavior__. You cast away the constness and pass it to `strtok()`, which modifies the string. If you search on SO you'll find many questions/answers about better C++ string tokenizing. — Blastfurnace, Jun 10 '12 at 23:30

score 5 · Accepted Answer · answered Jun 10 '12 at 22:03

5

The indices for the diagonals are wrong. For example:

... * grid.at(i-1).at(j+1) * grid.at(i-1).at(j+2) * grid.at(i -3).at(j+3))

The middle part should have i-2 instead of i-1. Similar issues with the other diagonals.

answered Jun 10 '12 at 22:03

Junuxx

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Thanks that was en stuppid mistake – Bjorn Jun 11 '12 at 10:02

Euler Project 11

1 Answers1