Loading a matrix from file

In summary, the author is trying to upload a matrix file to a forum, but is having trouble understanding why he is getting zeros in the output. He suspects that the problem is with the input file format and needs help from the forum community to figure out how to parse it correctly.
  • #1

BRN

108
10
Hello everybody,
I created this tamplet to upload a file matrix:

Load Matrix:
#include <sstream>
#include "fstream"
#include <vector>
#include <iostream>
#include <string>

template<class T >std::istream& readMatrix(std::vector<std::vector<T>>& outputMatrix, std::istream& inStream)
{
    if (inStream) {
        std::string line;

        while (std::getline(inStream, line)) {
            outputMatrix.emplace_back();

            // Break down the row into column values
            std::stringstream split(line);
            int value;

            while (split >> value)
                outputMatrix.back().push_back(value);
        }
    }
    return inStream;
}

Unfortunately it does not provide me with the correct output. I get this:
Output:
input:
1234
5678 
9123
4567

output:
1234 0 0 0 
5678 0 0 0 
9123 0 0 0 
4567 0 0 0

I can't understand why I get those zeros. Any idea?
 
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  • #2
Did you mean to write split > value or split >> value?

I don't understand where the split values are assigned.

It seems you've called a split(line) function which I assume returns an array of values.
 
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  • #3
Your matrix file format is ambiguous unless you can absolutely guarantee that every entry will only ever be a single digit number. If one line is 12345 should that be 12,3,4,5 or 1,23,4,5 or 1,2,34,t or 1,2,3,45? You should put in delimiters such as spaces or commas.

I think what's happening is that you have told your stream to read values from each line in your input until it runs out of input. It reads the first value 1234, which is a valid four digit number, and finds nothing else on the line so goes on to the next line. You need to look at how std::stringstream's operator>> works with delimiters if you want to potentially use more than one digit, or consider using get() instead if you genuinely only want single digit numbers and no delimiters.
 
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  • #4
The code looks fine to me. And when I run it, it works as expected: I created a file data.dat with the entries
1234
5678
9012
3456

and ran it with your code, extended by a main
Code:
int main() {
    using namespace std;
    ifstream file("data.dat");
    vector<vector<int>> matrix;
    readMatrix(matrix, file);
    for(const auto& row: matrix) {
        for (const auto& entry: row) {
            cout<<entry<<" ";
        }
        cout<<endl;
    }
}
When I run the program, I get exactly the expected 1x4 matrix that is in data.dat - no zeroes added. I suspect the problem is in your input stream, not in the code you use to parse it.
 
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  • #5
Timo said:
The code looks fine to me.
The splitting looks fine to me too. And a similar example runs as it should.
 
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  • #6
On 2nd though, here is another suspicion what goes wrong: Check the code where you output your matrix: Maybe your code assumes a 4x4 matrix and fills the missing entries with 0 or access to uninitialized memory locations like matrix[0][1]?
 
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  • #7
Ibix said:
Your matrix file format is ambiguous unless you can absolutely guarantee that every entry will only ever be a single digit number. If one line is 12345 should that be 12,3,4,5 or 1,23,4,5 or 1,2,34,t or 1,2,3,45? You should put in delimiters such as spaces or commas.
Exactly, this was the problem. I inserted a space between each matrix element in the file and is now loaded correctly.

Timo said:
Maybe your code assumes a 4x4 matrix and fills the missing entries with 0 or access to uninitialized memory locations like matrix[0][1]?
Yes, this happened. My code interpreted my file as a single column of 4 values and consequently added three other columns of zeros.

Thank you all for your answers!
 
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What is the purpose of loading a matrix from a file?

Loading a matrix from a file allows scientists to access and manipulate data that has been previously stored in a file. This can save time and effort by eliminating the need to manually input the data into a program or spreadsheet.

What file formats can be used to store matrices?

Matrices can be stored in a variety of file formats, such as CSV, TXT, and XLSX. The choice of file format will depend on the specific needs and preferences of the scientist.

What are the steps involved in loading a matrix from a file?

The general steps for loading a matrix from a file are: 1) opening the file, 2) reading the data from the file, 3) converting the data into a matrix format, and 4) storing the matrix in a variable for further analysis or manipulation.

What should be considered when loading a large matrix from a file?

When working with large matrices, it is important to consider the amount of memory and processing power required to load and manipulate the data. It may be necessary to use specialized software or techniques to handle large datasets efficiently.

Are there any potential errors or issues that can occur when loading a matrix from a file?

Yes, there are a few common errors that can occur when loading a matrix from a file, such as incorrect file format, missing data, or incompatible data types. It is important to carefully check the file and the code used for loading the matrix to prevent these errors.

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