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Background:

I'm attempting to read through a 2d array and find values that coincide with row numbers and column numbers.

Question:

How do I read values off a file and acquire, for example, 1 and 4 vs. 14?

Here's what I have so far...

All constructive criticism is welcome. 

int arrayOfNum[5][5] = {
    {34,21,32,41,25},
    {14,42,43,14,31},
    {54,45,52,42,23},
    {33,15,51,31,35},
    {21,52,33,13,23}};

ofstream arrayFile;
arrayFile.open("arrays.txt");

if (arrayFile.is_open()) {
    cout << "File opened successfully..." << endl;
}

for (int i = 0; i <= 4; i++) {

    arrayFile << endl;

    for (int j = 0; j <= 4; j++) {

        arrayFile << arrayOfNum[j][i] << ' ';
    }
}
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6
  • 1
    From an std::ifstream, you may use operator >>... Commented Aug 2, 2014 at 16:53
  • Reading from an ofstream ?? Are you sure ? Commented Aug 2, 2014 at 16:54
  • You have the output operator << for writing, what do you think you would use for operator to read from an input file stream? Hint: If you have used std::cin anytime you already know how to do it. Commented Aug 2, 2014 at 16:55
  • I'm not sure what the question is. Is about how to read data from a file, or is it about how to make a proper data layout in order to retrieve e.g. the dimensionality of the matrix? Commented Aug 2, 2014 at 17:39
  • I'm trying to output a 2d array to a file and then reading the values off and assigning them to a variable. The main objective is to find a value on the array such as '25', check if '25' is in row 2 and column 5...if not then the program goes to row 2 column 5 and checks that number until it finds a number where the row and column coincide with a value in the array. I need to read '25' as 2 and 5 in the file. Commented Aug 2, 2014 at 18:21

1 Answer 1

Reset to default
1

You are writing rows in columns as you wrote "arrayOfNum[j][i]", but I'm writing the code for correct direction. You can change the condition in following code but it's working perfect if you write "arrayOfNum[i][j]" while outputting in 'array.txt'

ifstream arrayFile;
arrayFile.open("arrays.txt");
for (int i = 0; i <= 4; i++)
{
    for (int j = 0; j <= 4; j++)
    {
        arrayFile >> arrayOfNum[i][j];
        cout<<arrayOfNum[i][j]<<" ";
        if(i+1 == arrayOfNum[i][j] / 10 && j+1 == arrayOfNum[i][j] % 10)
        {
            cout<<"\n Matched: "<<arrayOfNum[i][j]<<endl;
        }
    }
    cout<<endl;
}

And yes it will work for only 2 digit numbers, if you want for N digit numbers then I can do it for you too! :)

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4 Comments

This is a good answer but should explain the operators used. division gives you the first digit. % is the modulus operator and it will give you the second digit.
You used ifstream class for arrayFile so does that mean it's only an input file? Would I be able to still acquire values from the file and assign variables to them or should I switch the class to fstream? And one last thing, can I get a general formula for what you did in the if statement? Did you just divide by 10 and use the modulus because it was 2 digits?
1- Yes 'ifstream' class can only input data from file. 2- Obviously, if you're inputting using 'ifstream' you can acquire values from the file one by one using 'arrayFile>>variableName;' which means you're assigning value taken from file to 'variableName' variable. I don't understand why did you asked this question, I think you want to ask "Can I still write to file?". You can ask me again if you meant this. 3- And sure you can make a general formula like this in if statement: (log10(arrayOfNum[i][j])*10) You'll need #include <math.h> where log10 will give you (numberOfDigits-1)
But you should make sure what will be the meaning of each digit if your numbers are more than 99 i.e 100, 101, 102...

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