Aray in Programming

So far this semester
all input has been from keyboard
all 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 ifstream
 use 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
input
 string literal:
 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
<< endl;
exit(1);
}
 exit terminates the program, and returns its argument value to
the operating system
 must #include<cstdlib> to use exit

.close
closes 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;
cout << "File failed to open" << endl;
exit(1);
}
cout << "File successfully opened!" << endl;
infile.close();
return 0;
}
 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>
int main() {
ifstream infile;
exit(1);
}
infile.close();
return 0;
}
type ifsteam
the filename as its
parameter
command worked with
infile.fail
stream

#include <iostream>
#include <fstream>
#include <cstdlib>
int main() {
ifstream infile;
exit(1);
}
infile.close();
return 0;
}
1. declare a variable infile
of type ifsteam
the filename as its
parameter
command worked with
infile.fail
stream

#include <iostream>
#include <fstream>
#include <cstdlib>
int main() {
ifstream infile;
exit(1);
}
infile.close();
return 0;
}
type ifsteam
2. call infile.open, and
send the filename as its
parameter
command worked with
infile.fail
stream

#include <iostream>
#include <fstream>
#include <cstdlib>
int main() {
ifstream infile;
exit(1);
}
infile.close();
return 0;
}
type ifsteam
the filename as its
parameter
command worked with
infile.fail
5. call infile.close to close
the stream

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;
exit(1);
}
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;
cout << "Please enter the name of the file: ";
cin >> filename;
ifstream infile;
exit(1);
}
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".

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 istream
hence, any operation supported by cin, is
also supported by infile
 the >> operator
 getline

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("data.txt");
exit(1);
}
double val1, val2, val3;
infile >> val1 >> val2 >> val3;
cout << "The average of these values is: "
<< (val1 + val2 + val3)/3 << endl;
infile.close();
return 0;
 Example: Read in three values from a file called "data.txt", and print out
the average of those three values.
Code for reading in
values goes here!

int main() {
ifstream infile;
exit(1);
}
cin >> val1 >> val2 >> val3;
<< (val1 + val2 + val3)/3 << endl;
infile.close();
return 0;
If we were reading
from the keyboard,
code would look like
this:

int main() {
ifstream infile;
exit(1);
}
cin >> val1 >> val2 >> val3;
<< (val1 + val2 + val3)/3 << endl;
infile.close();
return 0;
Replace cin with infile,
and you'll read from
the file, instead of the
keyboard.

int main() {
ifstream infile;
exit(1);
}
<< (val1 + val2 + val3)/3 << endl;
infile.close();
return 0;
Replace cin with infile,
and you'll read from
the file, instead of the
keyboard.

int main() {
ifstream infile;
exit(1);
}
<< (val1 + val2 + val3)/3 << endl;
infile.close();
return 0;

File Input: Unknown file size
Example: read in all of the numbers from a
file, and print their average.

int main() {
ifstream infile;
infile.open("test.txt");
exit(1);
}
<< (val1 + val2 + val3)/3 << endl;
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 total
 count the number of values read
divide total by the number of values read

int main() {
ifstream infile;
exit(1);
}
double total = 0;
int count = 0;
infile >> val;
while ( ) {
total += val;
count++;
infile >> val;
}
if (count == 0) {
cout << "No values to average!" << endl;
} else {
<< total / count << endl;
}
infile.close();
What goes here?

.eof()
.eof() returns true when the file has been
read past its end
typical format for .eof() loops
Step 1: read a value from filestream
Step 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;
exit(1);
}
double total = 0;
int count = 0;
infile >> val;
while ( ) {
total += val;
count++;
infile >> val;
}
if (count == 0) {
} else {
}
infile.close();
Step 1: read a value from
filestream
Step 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;
exit(1);
}
double val, total = 0;
int count = 0;
infile >> val;
while ( ) {
total += val;
count++;
infile >> val;
}
if (count == 0) {
} else {
}
infile.close();
filestream
is not true)
with value

int main() {
ifstream infile;
exit(1);
}
int count = 0;
infile >> val;
while ( !infile.eof() ) {
total += val;
count++;
infile >> val;
}
if (count == 0) {
} else {
}
infile.close();
filestream
Step 2: while
( filestream.eof() is not true)
with value

int main() {
ifstream infile;
exit(1);
}
int count = 0;
infile >> val;
total += val;
count++;
infile >> val;
}
if (count == 0) {
} else {
}
infile.close();
filestream
is not true)
Step 2.1 Do something with
value

 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
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");
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;
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;
}
Replace cout with
outfile

int main() {
ofstream outfile;
exit(1);
}
outfile << "Kevin Grant" << endl;
outfile << "1234 Main Street" << endl;
outfile << "Disneyland, California, 90210" << endl;
outfile << "(123) 456-7890" << endl;
outfile.close();
return 0;
}

Kevin Grant
1234 Main Street
Disneyland, 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 reading
2) Open a file called output.txt for writing
3) Write each line in input.txt to output.txt
4) 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() {
ifstream infile;
infile.open("input.txt");
cout << "input.txt failed to open." << endl;
exit(1);
}
2) Open a file called output.txt for writing
4) Close files
return 0;

int main() {
ifstream infile;
exit(1);
}
ofstream outfile;
outfile.open("output.txt");
cout << "output.txt failed to open." << endl;
exit(1);
}
4) Close files
return 0;
}

int main() {
ifstream infile;
exit(1);
}
ofstream outfile;
exit(1);
}
3) For each line in input.txt
3.1) Write line to output.txt
4) Close files
return 0;
}

int main() {
ifstream infile;
exit(1);
}
ofstream outfile;
exit(1);
}
3) For each line in input.txt
4) Close files
return 0;
}
filestream
is not true)
with value

int main() {
ifstream infile;
exit(1);
}
ofstream outfile;
exit(1);
}
string buffer;
getline(infile, buffer);
}
4) Close files
return 0;
}

int main() {
ifstream infile;
exit(1);
}
ofstream outfile;
exit(1);
}
string buffer;
outfile << buffer << endl;
}
4) Close files
return 0;
}

int main() {
ifstream infile;
exit(1);
}
ofstream outfile;
exit(1);
}
string buffer;
outfile << buffer << endl;
}
infile.close();
outfile.close();
return 0;
}

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