Strings and input files, Assign several variables values from txt file that contains strings

Question

I'm new to programming in C++ and I'm not sure how exactly the input and output stream work. I want to take a txt file, which contains strings and characters that are separated by a delimiter (': '), to assign class member variables.

So far I been able to have a txt file that contains a single double or integer per line and assigned them to variables. Currently what I have:

Current "Default.txt"

10

3

0.80

1.5

The code that I'm currently using for variable assignment:

#include <iostream>
#include <fstream>

RandomObject::RandomObject(){ //The Default Constructor

std::ifstream my_file;

my_file.open("Default.txt");

//Not sure if this the most effective way to do this...
    if (my_file.is_open())
    {
        while (~my_file.eof())
        {   my_file >> Var1;   //Var1 is now an int with value 10
            my_file >> Var2;   //Var2 is now an int with value 3
            my_file >> Var3;   //Var3 is now double with value 0.80
            my_file >> Var4;   //Var4 is now double with value 1.5
            break;
        }
    
    }
    my_file.close();
}

Now I want the txt file to be:

The new "Default.txt"

Number Per Dimension: 10

Number of Dimensions: 3

Number Density: 0.80

Initial Temperature: 1.5

I want the value after the colon to be assigned to a member variable:

int Var1 = 10;
int Var2 = 3;
double Var3 = 0.80;
double Var4 = 1.5;

and some output text indicating variable intialization of either:

"Variable 1 is set to 10"

or

"Number Per Dimension is 10".

How do I go about doing this? The txt file will have all the values in the same order, although it might be useful to have program read "Number Per Dimension" as the first string to automatically assign 10 to Var1 if thats possible.

I'm guessing the first step is somehow to split it into two strings with ': ' as the delimiter and store it in some temporary string array variable? I'm not sure what exactly is going on with the reading the line and storing the value in my code above.

I would like to understand how the file stream actually works. Sorry that the formatting is bad.

https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong — François Andrieux, Jun 08 '17 at 20:50
Use `while (!my_file.eof())` and not `~` which is a binary inversion, not a logical one. — tadman, Jun 08 '17 at 20:50

score 1 · Answer 1 · answered Jun 09 '17 at 20:28

I'm not going to write you some code since you have one above, so I'm going to explain to you how I would do it/and how those work and give you some useful links.

Use std::getline(file_name,line) to read a line. getline() has a default delimiter the \n. So it stores the entire line in the variable line.

Having stored in a string variable what getline() returned, use std::find to find the ":" and it's position.It will return a size_t position,use it with substr and/or erase functions to get these two parts in different variables.

String variable string var_text with your text will remain a it is, but the value since you want it as int or double needs modifications. You will have to compare first the string var_text with any string you want. Depending on the result, you can set the variable to either int or double etc. with stringstream

Now that you have your variable in the form you need, you can store it in the proper class member.

Repeat this for as long as getline() returns 1.

score 0 · Answer 2 · answered Jun 08 '17 at 21:02

Try something more like this instead:

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

RandomObject::RandomObject() //The Default Constructor
{
    std::ifstream my_file("Default.txt");
    std::string line;

    while (std::getline(my_file, line))
    {
        std::istringstream iss(line);
        std::string name;

        std::getline(iss, name, ':');

        if (name == "Number Per Dimension") {
            iss >> Var1;   //Var1 is now an int with value 10
            std::cout << name << " is " << Var1 << std::endl;
        }

        else if (name == "Number of Dimensions") {
            iss >> Var2;   //Var2 is now an int with value 3
            std::cout << name << " is " << Var2 << std::endl;
        }

        else if (name == "Number Density") {
            iss >> Var3;   //Var3 is now double with value 0.80
            std::cout << name << " is " << Var3 << std::endl;
        }

        else if (name == "Initial Temperature") {
            iss >> Var4;   //Var4 is now double with value 1.5
            std::cout << name << " is " << Var4 << std::endl;
        }
    }
}

Strings and input files, Assign several variables values from txt file that contains strings

Current "Default.txt"

The new "Default.txt"

2 Answers2