C++ || Snippet – How To Read & Write Data From A File

February 12, 2012 admin No comments

This page will consist of a demonstration of a simple quadratic formula program, which highlights the use of the input/output mechanisms of manipulating a text file. This program will read in data from a file (numbers), manipulate that data, and output new data into a different text file.

REQUIRED KNOWLEDGE FOR THIS SNIPPET

Fstream Ifstream Ofstream Working With Files

NOTE: The data file that is used in this example can be downloaded here.

Also, in order to read in the data .txt file, you need to save the .txt file in the same directory (or folder) as your .cpp file is saved in. If you are using Visual C++, this directory will be located in

Documents > Visual Studio > Projects > [Your project name] > [Your project name]





		
		
			Input/Output Manipulation
			
C++
			
			// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Feb 12, 2012
//    Taken From: http://programmingnotes.org/
//    File:  FileInput.cpp
//    Description: Demonstrates how to read data from a file
// ============================================================================
#include <iostream>
#include <fstream>
#include <cstdlib> // used for the 'exit' command
#include <cmath>
using namespace std;

int main ()
{ 
	// declare variables
	ifstream infile;
	ofstream outfile;
	double a=0,b=0,c=0;
	double root1=0, root2=0;

	// this opens the input file
	// YOUR INPUT FILE NAME GOES BELOW
	infile.open("INPUT_Quadratic_programmingnotes_freeweq_com.txt");

	// check to see if the file even exists, & if not then EXIT
	if(infile.fail()) 
	{
		cout<<"\nError, the input file could not be found!\n";
		exit(1); // exits the program
	}

	// this opens the output file
	// if the file doesnt already exist, it will be created
	outfile.open("OUTPUT_Quadratic_programmingnotes_freeweq_com.txt");

	// this loop reads in data until there is no more 
	// data contained in the file
	while(infile.good())
	{
		// this assigns the incoming data to the
		// variables 'a', 'b' and 'c'
		// NOTE: it is just like a cin >> statement
		infile >> a >> b >> c;

		// NOTE: if you want to read in data into an array
		// your declaration would be like this
		// ------------------------------------ 
		// infile >> a[counter] >> b[counter] >> c[counter];
		// ++counter;
	}

	// this does the quadratic formula calculations
	root1 = ((-b) + sqrt(pow(b,2) - (4*a*c)))/(2*a);
	root2 = ((-b) - sqrt(pow(b,2) - (4*a*c)))/(2*a);

	// this displays the numbers to screen via cout
	cout <<"For the numbers\na = "<<a<<"\nb = "<<b<<"\nc = "<<c<<endl;
	cout <<"\nroot 1 = "<<root1<<"\nroot 2 = "<<root2<<endl;
	
	// this saves the data to the output file
	// NOTE: its almost exactly the same as the cout statement
	outfile <<"For the numbers\na = "<<a<<"\nb = "<<b<<"\nc = "<<c<<endl;
	outfile <<"\nroot 1 = "<<root1<<"\nroot 2 = "<<root2<<endl;

	// close the input/output files once you are done using them
	infile.close();
	outfile.close();

	// stops the program from automatically closing
	cin.get();

	return 0;
}// http://programmingnotes.org/
			
				
					
				
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						// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Feb 12, 2012
//    Taken From: http://programmingnotes.org/
//    File:  FileInput.cpp
//    Description: Demonstrates how to read data from a file
// ============================================================================
#include <iostream>
#include <fstream>
#include <cstdlib> // used for the 'exit' command
#include <cmath>
using namespace std;
 
int main ()
{ 
	// declare variables
	ifstream infile;
	ofstream outfile;
	double a=0,b=0,c=0;
	double root1=0, root2=0;
 
	// this opens the input file
	// YOUR INPUT FILE NAME GOES BELOW
	infile.open("INPUT_Quadratic_programmingnotes_freeweq_com.txt");
 
	// check to see if the file even exists, & if not then EXIT
	if(infile.fail()) 
	{
		cout<<"\nError, the input file could not be found!\n";
		exit(1); // exits the program
	}
 
	// this opens the output file
	// if the file doesnt already exist, it will be created
	outfile.open("OUTPUT_Quadratic_programmingnotes_freeweq_com.txt");
 
	// this loop reads in data until there is no more 
	// data contained in the file
	while(infile.good())
	{
		// this assigns the incoming data to the
		// variables 'a', 'b' and 'c'
		// NOTE: it is just like a cin >> statement
		infile >> a >> b >> c;
 
		// NOTE: if you want to read in data into an array
		// your declaration would be like this
		// ------------------------------------ 
		// infile >> a[counter] >> b[counter] >> c[counter];
		// ++counter;
	}
 
	// this does the quadratic formula calculations
	root1 = ((-b) + sqrt(pow(b,2) - (4*a*c)))/(2*a);
	root2 = ((-b) - sqrt(pow(b,2) - (4*a*c)))/(2*a);
 
	// this displays the numbers to screen via cout
	cout <<"For the numbers\na = "<<a<<"\nb = "<<b<<"\nc = "<<c<<endl;
	cout <<"\nroot 1 = "<<root1<<"\nroot 2 = "<<root2<<endl;
	
	// this saves the data to the output file
	// NOTE: its almost exactly the same as the cout statement
	outfile <<"For the numbers\na = "<<a<<"\nb = "<<b<<"\nc = "<<c<<endl;
	outfile <<"\nroot 1 = "<<root1<<"\nroot 2 = "<<root2<<endl;
 
	// close the input/output files once you are done using them
	infile.close();
	outfile.close();
 
	// stops the program from automatically closing
	cin.get();
 
	return 0;
}// http://programmingnotes.org/

QUICK NOTES:
The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

Once compiled, you should get this as your output

For the numbers a = 2 b = 4 c = -16
root 1 = 2 root 2 = -4

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Posted in: Basic, C++, C++, C++, Getting Started, Snippets Tags: c++, File, Output, Text, text file, While Loop

C++ || Snippet – How To Read & Write Data From A File

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