How to use getline to read from a file into an array

Question

I could use your help on this problem. I have a file that contains this:

1x+1y+1z=5
2x+3y+5z=8
4x+0y+5z=2

I must store that into a string. Once stored the output should be this:

1x+1y+1z=5
a=1 b=1 c=1 d=5
2x+3y+5z=8
a=2 b=3 c=5 d=8
4x+0y+5z=2
a=4 b=0 c=5 d=2

This is the code I have, however it is not outputting anything. Can anybody help me? It gives me an error on line 19 but I don't know how to fix it. The error states "no matching function for call."

|19|error: no matching function for call to 'std::basic_ifstream::getline(std::string [10], int)'| |22|error: no matching function for call to 'std::basic_istringstream::basic_istringstream(std::string [10])'|

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>

using namespace std;

int main()
{
    ifstream file("matrix.txt");

    if (!file)
    {
        cout << "Could not open input file\n";
        return 1;
    }


    for (string line[10]; file.getline(line, 10); )
    {
        cout << line << '\n';
        istringstream ss(line);

        double a, b, c, d;
        char x, y, z, eq;

        if (ss >> a >> x >> b >> y >> c >> z >> eq >> d)
        {
            if (x == 'x' && y == 'y' && z == 'z' && eq == '=')
            {
                cout << "a = " << a
                     << "  b = " << b
                     << "  c = " << c
                     << "  d = " << d << '\n';
            }
            else
            {
                cout << "parse error 2\n";
            }
        }
        else
        {
            cout << "parse error 1\n";
        }

    }

}

Whenever asking "Why doesn't this code work" please provide all errors you get when compiling/running — Tyler, Oct 03 '17 at 15:41
[`using namespace std;` is a bad practice](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice), never use it. — tambre, Oct 03 '17 at 15:43
`for (string line[10]; file.getline(line, 10); )` This line looks highly suspicious to me. — Borgleader, Oct 03 '17 at 15:43
Post the **exact** error you receive from the compiler character-for-character in a code block, not a re-phrased or a shortened one. — tambre, Oct 03 '17 at 15:44

Daniel Trugman · Answer 1 · 2017-10-03T16:01:09.877

1

There are a couple of mistakes.

Mistake no. 1

You cannot and should not declare string line[10]. You also should not use a for loop. Here is a classic implementation of reading string by string from a stream:

string line;
while (file >> line) {
  ... // do stuff
}

Mistake no. 2

Your numbers are ints, not doubles, so you should use:

int a, b, c, d;

Saving values into an array

First of all, let me say, that there is no good reason to use raw arrays. You should always prefer using standard ADTs, such as the std::vector.

In this scenario, you should not read the value right into the vector, since the value may be ill-formed.

Instead follow this sequence:

vector<string> lines;
string line;
while (file >> line) {
  ... // do stuff

  if (successful)
    lines.push_back(line);
}

edited Oct 03 '17 at 16:01

answered Oct 03 '17 at 15:47

Daniel Trugman

8,186
20
41

But I want to store the line into an array. – compscistudent Oct 03 '17 at 15:48
@compscistudent, updated the answer to add lines to a vector – Daniel Trugman Oct 03 '17 at 16:02
The thing is I still cannot use vectors, and I must store into an array to manipulate the numbers with pointers – compscistudent Oct 03 '17 at 16:04
@compscistudent, are you sure you want an array of `string`s? maybe you should use an array of `char`s? At least that will make sense in terms of manipulating numbers with pointers... – Daniel Trugman Oct 03 '17 at 16:10

John Fischer III · Answer 2 · 2021-05-26T03:53:30.047

Here is code to read a file and insert into an array.

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

const int MAX_LINES = 100;

int main() {
    std::string strArray[MAX_LINES];
    fstream newFile;
    newFile.open("matrix.txt", ios::in); //open a file to perform read operation
    if (newFile.is_open()) {   //checking whether the file is open
        string tp;
        for (int counter = 0; getline(newFile, tp); counter++) { // get new line
            strArray[counter] = tp; // read into string array
            cout << tp << "\n"; //print the string
        }
        newFile.close(); //close the file object.
    }
    return 0;
}

How to use getline to read from a file into an array

2 Answers2