Reading input from fstream

Question

So it is a simple problem. Simply read input from a specific file. Use the input as a Celsius temperature and convert this temperature into Fahrenheit then print the results to the user.

The problem seems to occur when I try to save input from the file. Located in the while block. I get an error I know may be caused by trying to use an int value in getline. I am fairly new to c++ and not sure how to do this. I have tried a myriad of ways but none seem to work. Any help would be greatly appreciated!

I did #include <fstream>.

The file contains these three values '0 50 100'.

This is the section of the code I have been working with:

//values for the files three input values
int i = 0, inVal1 = 0 , inVal2 = 0, inVal3 = 0,
    farenheit1 = 0, farenheit2 =0,farenheit3 = 0; 

ifstream inFile; //Input file variable


inFile.open("celsius_input.dat"); //open the file

if(!inFile){
    cout << "Unable to open file";
    exit(1); //terminate with error
}//end if
while (inFile)
{
    cin.ignore();
    getline(inFile, inVal1);
    getline(inFile, inVal2);
    getline(inFile, inVal3); // read the files values

    inFile.close();//Close file


} //end while       



farenheit1 = (9.0/5.0) * inVal1 + 32.0; //formula 
farenheit2 = (9.0/5.0) * inVal2 + 32.0; //formula
farenheit3 = (9.0/5.0) * inVal3 + 32.0; //formula


cout << "The first Inputed Value, " << inVal1
    << " degrees, Converted Into Farenheit Is "
    << farenheit1 << " Degrees!" << endl; //output of results
cout << "     " << endl;

cout << "The Second Inputed Value, " << inVal2
    << " degrees, Converted Into Farenheit Is "
    << farenheit2 << " Degrees!" << endl; //output of results
cout << "     " << endl;

cout << "Teh Third Inputed Value, " << inVal3
    << " degrees, Converted Into Farenheit  Is "
    << farenheit3 << " Degrees!" << endl; //output of results
cout << "     " << endl;

the first getline in the while loop. it says no matching function. — user2445852, Jun 02 '13 at 20:44
`getline` takes in a `string` as its second parameter, not an int. — Mohammed Hossain, Jun 02 '13 at 20:52

sehe · Answer 1 · 2013-06-02T20:54:32.140

I'd suggest the simplest that would work is:

#include <fstream>
#include <iostream>

int main()
{
    std::ifstream inFile("celsius_input.dat"); //open the file

    double celsius;
    while (inFile >> celsius)
    {
        double fahrenheit = (9.0/5.0) * celsius + 32.0;
        std::cout << "The input value " << celsius << " degrees, converted into fahrenheit is " << fahrenheit << " degrees" << std::endl;
    }
}

If you really must read a line first, do it like:

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

int main()
{
    std::ifstream inFile("celsius_input.dat"); //open the file

    std::string input;
    while (std::getline(inFile, input))
    {
        double celsius = std::strtod(input.c_str(), nullptr);
        double fahrenheit = (9.0/5.0) * celsius + 32.0;
        std::cout << "The input value " << celsius << " degrees, converted into fahrenheit is " << fahrenheit << " degrees" << std::endl;
    }
}

score 0 · Accepted Answer · edited May 23 '17 at 10:25

The std::getline function you're using saves an input to a string (see: http://www.cplusplus.com/reference/string/string/getline/). If the argument you pass to the function was a string, it would get the whole line from your file, which is "0 50 100" and put it in your string.

You could trying to save it to a string and then splitting it to three parts and converting to ints using atoi or std::stoi in C++11 (check Convert string to int C++) - this way it would be easier to handle possible errors.

But there's an easier way to do it - assuming your numbers are split by spaces and pretty much everything is correct with them, the ">>" operator breaks on spaces. Try:

inFile >> inVal1;
inFile >> inVal2;
inFile >> inVal3;

Also, using cin.ignore() is not necessary when using the inFile buffer. Every stream has a different buffer associated with it (and cin != inFile) so you don't need to clear the cin buffer to read from your file.

Thank you everyone for your help I really appreciate it. I just tried your example and it worked perfectly. I did not know I could get input from just using my file variable but now looking at it it makes sense! — user2445852, Jun 02 '13 at 21:11

Reading input from fstream

2 Answers2