Passing an array from txt file to a function

Question

I have a simple function of the form:

double f(double x)
{
...
}

For that function, I want to use data from a txt file, which I pass to an array:

ifstream inFile;

    inFile.open("data.txt");


    //Counting lines
    string s;
    int nlines=0;

    while(!inFile.eof()){
        getline(inFile, s);
        nlines++;
    }

    inFile.seekg(0, ios::beg);

    while(!inFile.eof()){
        inFile >> a[entry_data];
        inFile >> b[entry_data++]; 
    }

    inFile.close();

If I put this code inside the function f, each time the function is called, it will have to open the file, create the array... and so on. I want to avoid this by defining the array just the first time the function is called, or before. How can I do this?

I know that if I define the array in the main scope and pass it to the function as an argument I can solve this, but I want to keep the main scope as clean as possible. Thanks!

Wrap your function in a class, and use a singleton there to get the data you want. — Matthieu Brucher, Nov 21 '18 at 18:05
OT but a bug: `while(!inFile.eof()){` https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong — drescherjm, Nov 21 '18 at 18:07
My guess is the OP can't use std::vector. That is not at all clear since the line count is not used and we don't know where `a` and `b` are declared or allocated. — drescherjm, Nov 21 '18 at 18:16
@drescherjm cannot use vector but can use seekg? Found an idiot professor. — n. 'pronouns' m., Nov 21 '18 at 18:17

Thomas Matthews · Answer 1 · 2018-11-21T18:13:19.100

0

Your f function takes 1 (one) value, not an array.

You want to use std::vector<double> instead of array, if you want to pass the data.

Your loop should be:

double value1, value2;
std::vector<double> a;
std::vector<double> b;
while (inFile >> value1 >> value2)
{
   a.push_back(value1);
   b.push_back(value2);
}

You can use your f function:

const size_t size = a.size();
for (size_t index = 0; index < size; ++size)
{
  double result = f(a[index]);
  //...
}

Edit 1: Function to Load
You could create an input function and call it once in main:

void input_data(std::istream& input, std::vector<double>& a, std::vector<double>& b)
{
  double value1, value2;
  while (input >> value1 >> value2)
  {
     a.push_back(value1);
     b.push_back(value2);
  }
}

edited Nov 21 '18 at 18:13

answered Nov 21 '18 at 18:10

Thomas Matthews

52,985
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I don't think that's what OP wants. `a` and `b` are read inside `f`, not outside. – Matthieu Brucher Nov 21 '18 at 18:13
Basically the function uses a bunch of parameters that are in a file, and OP doesn't want to load them each time the function is called. – Matthieu Brucher Nov 21 '18 at 18:15
1

This gives the OP some tools to work with. I don't like giving 100% solutions, I like to make the posters think. – Thomas Matthews Nov 21 '18 at 18:16
1

Also, there is a design smell that the OP should be using a `class` that models the record, instead of using parallel containers. – Thomas Matthews Nov 21 '18 at 18:19

Matthieu Brucher · Accepted Answer · 2018-12-03T22:04:49.907

0

Let's create an instance that will read your data.

class DataHolder
{
    DataHolder()
    {
    ifstream inFile;

    inFile.open("data.txt");

    double v1, v2;

    while(inFile >> v1 >> v2){
        a.push_back(v1); 
        b.push_back(v2);
    }
    }
public:
    static DataHolder& getInstance()
    {
        static DataHolder d;
        return d;
    }
    std::vector<double> a, b;
};

Then in your function, use the data holder:

double f(double x)
{
    auto& d = DataHolder::getInstance();
    // use the holders data
}

edited Dec 03 '18 at 22:04

answered Nov 21 '18 at 18:11

Matthieu Brucher

19,950
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This is what I need. Thank you! – Psyphy Nov 21 '18 at 21:15
I have been testing your idea, but I can't get it to work. I have tried different variations based on the same idea, but using literally what you have posted I get `error: 'DataHolder' does not refer to a value auto& d = DataHolder.getInstance();` – Psyphy Dec 03 '18 at 21:35
Oops, sorry, it's :: – Matthieu Brucher Dec 03 '18 at 22:04

Passing an array from txt file to a function

2 Answers2