CS 1620

File I/O

So far this semesterall input has been from keyboardall output has been to computer screen

these are just two examples of where to retrieve/send data

ifstream – Input File Stream reads input from the location of your choice ifstream is a type

declare variables of type ifstreamuse variable for file input

Steps to read from a file:

1. declare a variable infile of type ifstream

2. call infile.open, and send the filename as its parameter

3. check to see if the open command worked with infile.fail

4. use infile to read in data

5. call infile.close to close the stream

ifstream infile;

infile.open("prices.dat");

if (infile.fail()) {

cout << "File failed to open"

<< endl;

exit(1);

// read data from infile

infile.close();

ifstream variable declared like any other variable

ifstream infile;

can use any legal variable name that you like I will use infile for all of my examples

ifstream kev; // legal, but not a good choice

kev.open("kev.txt");

requires #include<fstream>

.open takes a null-terminated character array as

inputstring literal: infile.open("prices.dat");character array variable char filename[] = "prices.dat";

infile.open(filename);will a string variable work? string filename = "prices.dat";

infile.open(filename);

c_str a string variable does not hold a null-terminated character

array string data type provides a member function called c_str() returns a null-terminated character array representing the

string to use, just affix .c_str() to the variable name

string filename = "prices.dat";

infile.open(filename.c_str());

.fail returns true if the file has been opened, false

otherwise

if (infile.fail()) {

<< endl;

exit(1);

exit terminates the program, and returns its argument value to the operating system

must #include<cstdlib> to use exit

.closecloses the current file stream

good programming practice to close files when you're done with them

O/S only allows finite number of open files

infile.close();

Example 1: Open a file called "prices.dat". If the file opens, print the message "File successfully opened!". If not, print "File failed to open".

#include <iostream>#include <fstream>#include <cstdlib> using namespace std;

int main() {

ifstream infile;

if (infile.fail()) { cout << "File failed to open" << endl; exit(1); }

cout << "File successfully opened!" << endl;

infile.close();

return 0;

int main() {

ifstream infile;

infile.close();

return 0;

1. declare a variable infile of type ifsteam

int main() {

ifstream infile;

infile.close();

return 0;

int main() {

ifstream infile;

infile.close();

return 0;

int main() {

ifstream infile;

infile.close();

return 0;

int main() {

ifstream infile;

infile.close();

return 0;

int main() {

ifstream infile;

infile.close();

return 0;

Example 2: Read a filename from the user, and open that file for reading. If the file opens, print the message "File successfully opened!". If not, print "File failed to open".

// included libraries: iostream, fstream, cstlib, string

int main() {

string filename;

cout << "Please enter the name of the file: "; cin >> filename;

ifstream infile;

cout << "File successfully opened!" << endl; infile.close();

return 0;

Read a filename from the user, and open that file for reading. If the file opens, print the message "File successfully opened!". If not, print "File failed to open".

int main() {

string filename;

ifstream infile;

return 0;

int main() {

string filename;

ifstream infile;

return 0;

int main() {

string filename;

ifstream infile;

return 0;

int main() {

string filename;

ifstream infile;

return 0;

int main() {

string filename;

ifstream infile;

return 0;

int main() {

string filename;

ifstream infile;

return 0;

How to read from a file ifstream is a subtype of istream

this means that anything you can do with an istream, you can do with an ifstream

cin is an istreamhence, any operation supported by cin, is

also supported by infile the >> operatorgetline

Example: Read in three values from a file called "data.txt", and print out the average of those three values.

// included libraries: iostream, fstream, cstlib

int main() {

ifstream infile;

infile.open("test.txt");

double val1, val2, val3; infile >> val1 >> val2 >> val3;

cout << "The average of these values is: " << (val1 + val2 + val3)/3 << endl;

infile.close();

return 0;

Code for reading in values goes here!

int main() {

ifstream infile;

double val1, val2, val3; cin >> val1 >> val2 >> val3;

infile.close();

return 0;

If we were reading from the keyboard, code would look like this:

int main() {

ifstream infile;

double val1, val2, val3; cin >> val1 >> val2 >> val3;

infile.close();

return 0;

Replace cin with infile, and you'll read from the file, instead of the keyboard.

int main() {

ifstream infile;

infile.close();

return 0;

Replace cin with infile, and you'll read from the file, instead of the keyboard.

int main() {

ifstream infile;

infile.close();

return 0;

File Input: Unknown file sizeExample: read in all of the numbers from a

file, and print their average.

int main() {

ifstream infile;

infile.close();

return 0;

This accommodates exactly three values.• if "test.txt" contains more than 3 values, they are ignored• if "test.txt" contains less than 3 values, program doesn't fill val3.

Solution: loop through all values in the file

add each value read to a variable totalcount the number of values read

divide total by the number of values read

int main() {

ifstream infile;

double total = 0; int count = 0;

infile >> val;

while ( ) { total += val; count++; infile >> val; } if (count == 0) { cout << "No values to average!" << endl; } else { cout << "The average of these values is: " << total / count << endl; }

infile.close();

return 0;

What goes here?

.eof() .eof() returns true when the file has been

read past its endtypical format for .eof() loops

Step 1: read a value from filestreamStep 2: while ( filestream.eof() is not true)

Step 2.1 Do something with value Step 2.2 read another value from filestream

int main() {

ifstream infile;

double total = 0; int count = 0;

infile >> val;

infile.close();

return 0;

int main() {

ifstream infile;

double val, total = 0; int count = 0;

infile >> val;

infile.close();

return 0;

int main() {

ifstream infile;

infile >> val;

while ( !infile.eof() ) { total += val; count++; infile >> val; } if (count == 0) { cout << "No values to average!" << endl; } else { cout << "The average of these values is: " << total / count << endl; }

infile.close();

return 0;

int main() {

ifstream infile;

infile >> val;

infile.close();

return 0;

Step 1: read a value from filestreamStep 2: while ( filestream.eof() is not true)Step 2.1 Do something with value

Step 2.2 read another value from filestream

int main() {

ifstream infile;

infile >> val;

infile.close();

return 0;

Step 1: read a value from filestreamStep 2: while ( filestream.eof() is not true)Step 2.1 Do something with value

Step 2.2 read another value from filestream

File Output so far, all of our information has gone to the screen we can redirect this to a file, in a similar manner as

redirecting input from a file use ofstream variable

ofstream is a type, of subtype ostream cout is an ostream variable anything you can do with cout, you can do with an ofstream

variable this includes <<, and all formatting flags

Steps to write to a file:

1. declare a variable outfile of type ofstream

2. call outfile.open, and send the filename as its parameter

3. check to see if the open command worked with outfile.fail

4. use outfile to output data

5. call outfile.close to close the stream

ofstream outfile;

outfile.open("prices.dat");

if (outfile.fail()) {

<< endl;

exit(1);

// write data to outfile

outfile.close();

Example: Write your name, address, and phone number to a file called "name.txt"

int main() {

ofstream outfile;

outfile.open("name.txt");

if (outfile.fail()) { cout << "File failed to open" << endl; exit(1); }

cout << "Kevin Grant" << endl; cout << "1234 Main Street" << endl; cout << "Disneyland, California, 90210" << endl; cout << "(123) 456-7890" << endl;

outfile.close();

return 0;

If we were writing to the screen, we would do this.

int main() {

ofstream outfile;

cout << "Kevin Grant" << endl; cout << "1234 Main Street" << endl; cout << "Disneyland, California, 90210" << endl; cout << "(123) 456-7890" << endl;

outfile.close();

return 0;

Replace cout with outfile

int main() {

ofstream outfile;

outfile << "Kevin Grant" << endl; outfile << "1234 Main Street" << endl; outfile << "Disneyland, California, 90210" << endl; outfile << "(123) 456-7890" << endl;

outfile.close();

return 0;

Kevin Grant1234 Main StreetDisneyland, California, 90210(123) 456-7890

name.txt

Example: Copy the contents of a file called "input.txt" to a file called "output.txt"

Steps:1) Open a file called input.txt for reading2) Open a file called output.txt for writing3) Write each line in input.txt to output.txt4) Close files

int main() {

1) Open a file called input.txt for reading

2) Open a file called output.txt for writing

3) Write each line in input.txt to output.txt

4) Close files

return 0;}

int main() {

1) Open a file called input.txt for reading

4) Close files

return 0;}

int main() {

ifstream infile; infile.open("input.txt"); if (infile.fail()) { cout << "input.txt failed to open." << endl; exit(1); }

4) Close files

return 0;}

int main() {

4) Close files

return 0;}

int main() {

ofstream outfile; outfile.open("output.txt"); if (outfile.fail()) { cout << "output.txt failed to open." << endl; exit(1); }

4) Close files return 0;}

int main() {

3) For each line in input.txt 3.1) Write line to output.txt

int main() {

3) For each line in input.txt 3.1) Write line to output.txt

int main() {

string buffer; getline(infile, buffer);

while ( !infile.eof() ) { 3.1) Write line to output.txt getline(infile, buffer); }

int main() {

string buffer; getline(infile, buffer);

while ( !infile.eof() ) { 3.1) Write line to output.txt getline(infile, buffer); }

int main() {

ofstream outfile; outfile.open("output.txt"); if (outfile.fail()) { cout << "output.txt failed to open." << endl; exit(1); } string buffer; getline(infile, buffer);

while ( !infile.eof() ) { outfile << buffer << endl; getline(infile, buffer); }

int main() {

infile.close(); outfile.close(); return 0;}

CS 1620

file opens

message file

file inputsteps

file ioso

datif infile

failuse infile

datacall infile

open command

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