Using the while-statement to
process data files
General procedure to access a data file
• General procedure in computer programming to read data
from a data file
1. Open the data file
• You provide the name of the data file to an "open a
file" library function that will open the data file
(Different programming languages will provide a
different "open a file" library function)
• This "open a file" library function will return some
information (let's call it X) back to you that you can
use to access the opened file That information is used
in the computer to identified the opened file
General procedure to access a data file
2. With the information X, you can then use a "read
something from a file" method to read data from the
opened file
3. There are other helpful methods on an opened file.
Some method let you check if you have reached the end
of the file
Previously discussed: reading from the
keyboard
• Summary: reading a floating point number from the
keyboard
Previously discussed: reading from the
keyboard (cont.)
• The methods in the Scanner class can also be used to read
input data from a file
Opening a data file
• How to open a file in Java:
File myFile; // Define a "File" type variable to receive
// the opened file
myFile = new File("Path-name-of-the-file"); // Open the file
The variable myFile contains information about the opened file
The variable myFile will be used in read operations
Scanning an opened file
• Reading data from an opened file is a very complex task
• Fortunately:
• An opened file behaves exactly like a keyboard
(which is represented by the variable System.in in
Java)
Scanning an opened file (cont.)
• We can use the method in the Scanner class to help us read
and convert the data into the desired encoding
Scanning an opened file (cont.)
• Previously discussed:
• To construct a Scanner object using the keyboard
System.in:
Scanner in;
// Define a Scanner typed variable
in = new Scanner(System.in); // Construct a Scanner that read
// data from keyboard System.in
Scanning an opened file (cont.)
• Read from a data file:
• Construct a Scanner object using an opened file:
/* --------------------------------Open a data file
--------------------------------- */
File myFile; // Define a "File" type variable to receive
// the opened file
myFile = new File("Path-name-of-the-file"); // Open the file
/* -------------------------------------------------Construct a Scanner from the opened file "myFile"
-------------------------------------------------- */
Scanner in;
// Define a Scanner typed variable
in = new Scanner(myFile); // Construct a Scanner that read
// data from opened file"myFile"
Scanning an opened file (cont.)
• From this point onwards, you can use
• in.nextDouble() to read a floating point number from the
data file
• in.nextInt() to read an integer number from the data file
• in.next() to read a string (word) from the data file
Checking for available input data
• There are very useful methods available in the Scanner
class to test if the input file is empty (exhausted) or not.
Checking for available input data (cont.)
• Check function for input availability on Scanner typed
variable in:
• in.hasNextDouble()
• returns true if the Scanner object in
contains at least one double typed
value in the input.
• returns false otherwise (no more
double typed value in the input.)
Checking for available input data (cont.)
• in.hasNextInt()
• returns true if the Scanner object in
contains at least one int typed value in the
input.
• returns false otherwise (no more int typed
value in the input.)
Checking for available input data (cont.)
• in.hasNext()
• returns true if the Scanner object in
contains at least one String typed value in
the input.
• returns false otherwise (no more String
typed value in the input.)
Checking for available input data (cont.)
• OK, we have covered everything we need to process data
files in Java !
Programming example 1: print the content
of a data file
• Let's start simple:
• Write a Java program that read the data from the
file inp1 and...
• print each word read to the terminal.
Programming example 1: print the content
of a data file (cont.)
• Rough algorithm:
Open the file "inp1"
Construct a Scanner object using the opened file
as long as ( there is data in the Scanner object )
{
read a word from the Scanner object;
print the word;
}
Programming example 1: print the content
of a data file (cont.)
• Algorithm in Structure Diagram form:
(I have used the initialization form to define the variables
myFile and in for brevity.)
Programming example 1: print the content
of a data file (cont.)
• Java program:
import java.io.*;
import java.util.Scanner;
public class File01
{
public static void main(String[] args) throws IOException
{
File myFile = new File("inp1"); // Open file "inp1"
Scanner in = new Scanner(myFile); // Make Scanner obj with opened file
String x;
// Variable to receive a string
while ( in.hasNext() )
{
x = in.next();
// Read a string (word)
System.out.println(x); // Print string read
}
System.out.println("Done");
}
}
Programming example 1: print the content
of a data file (cont.)
• Additional programming code needed:
• The File class is contained inside the java.io package
We need to use the import statement:
import java.io.File;
Programming example 1: print the content
of a data file (cont.)
• However, the methods in the File class need other methods
in the java.io package
That's why we import all class in the java.io package with
the import statement
import java.io.*;
Programming example 1: print the content
of a data file (cont.)
• File read errors in the java.io package are reported by
exceptions
• Exceptions is a very advanced programming
concept that is not discussed in this course
• A method that uses methods in the java.io package must
contain the declaration
.... throws IOExceptions
Programming example 1: print the content
of a data file (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/07/Progs/
File01.java
– Input data file inp1:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/07/Progs/i
np1
• How to run the program:
• Right click on both links and save in a scratch
directory
• To compile: javac File01.java
• To run:
java File01
Programming example 1: print the content
of a data file (cont.)
• Example run:
• Input file inp1:
Hello Class
This is your first input data file !
Programming example 1: print the content
of a data file (cont.)
• Output of the program:
Hello
Class
This
is
your
first
input
data
file
!
Done
Programming example 2: sum the (floating
point) data in a data file
• Programming problem:
• Given a data file inp2 that contains a (unspecified
number) floating point numbers
• Write a Java program that read the data from the file
inp2 and...
• print the sum of the numbers to the terminal.
Programming example 2: sum the (floating
point) data in a data file (cont.)
• Rough algorithm:
Open the file "inp2"
Construct a Scanner object using the opened file
sum = 0;
// Initial value ("clear the slate")
as long as ( there is a double datum in the Scanner object )
{
Read a floating point number from the Scanner object;
Add the number to the sum;
}
Print sum;
Programming example 2: sum the (floating
point) data in a data file (cont.)
• Algorithm in Structure Diagram form:
Programming example 2: sum the (floating
point) data in a data file (cont.)
• Java program:
import java.io.*;
import java.util.Scanner;
public class File01
{
public static void main(String[] args) throws IOException
{
File myFile = new File("inp2"); // Open file "inp2"
Scanner in = new Scanner(myFile); // Make Scanner obj with
opened file
double x;
// Variable to receive a floating point number
double sum; // Running sum
Programming example 2: sum the (floating
point) data in a data file (cont.)
sum = 0.0;
// Initialize ("clear the slate")
while ( in.hasNextDouble() )
{
x = in.nextDouble(); // Read a floating point
number
sum = sum + x;
// Add to the running sum
}
System.out.println("Sum = " + sum);
}
}
Programming example 1: print the content
of a data file (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/07/Progs/
File02.java
– Input data file inp1:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/07/Progs/i
np2
• How to run the program:
• Right click on both links and save in a scratch
directory
• To compile: javac File02.java
• To run:
java File02
Programming example 1: print the content
of a data file (cont.)
• Example run:
• Input file inp2:
2.1
4.2
5.2
Output of the program:
Sum = 17.6
6.1
Programming example 3: compute the
average of (floating point) data in a data file
• Programming problem:
• Given a data file inp3 that contains a (unspecified
number) floating point numbers
• Write a Java program that read the data from the file
inp3 and...
• print the average of the numbers to the terminal.
Programming example 3: compute the average
of (floating point) data in a data file (cont.)
• Computing the average:
• The average of a series of numbers can be computed by
maintaining the following information:
• the running sum
• the number of items that was added into
the running sum
Programming example 3: compute the average
of (floating point) data in a data file (cont.)
• Example:
• Input: 2.1, 3.2, 2.2, 1.3
• Running sum = 10.8
• Number of items = 4
• Average = 10.8/4 = 2.7
Programming example 3: compute the average
of (floating point) data in a data file (cont.)
• Rough algorithm:
Open the file "inp3"
Construct a Scanner object using the opened file
sum = 0;
N=0
// Initial value ("clear the slate")
// # items added into sum
as long as ( there is a double datum in the Scanner object )
{
Read a floating point number from the Scanner object;
Add the number to the sum;
N++;
}
Print sum/N;
// print average
Programming example 3: compute the average
of (floating point) data in a data file (cont.)
• Algorithm in Structure Diagram form:
Programming example 3: compute the average
of (floating point) data in a data file (cont.)
• Java program:
import java.io.*;
import java.util.Scanner;
public class File01
{
public static void main(String[] args) throws IOException
{
File myFile = new File("inp2"); // Open file "inp2"
Scanner in = new Scanner(myFile); // Make Scanner obj with opened file
double x;
// Variable to receive a floating point number
double sum; // Running sum
int N;
// # items added **** New code
Programming example 3: compute the average
of (floating point) data in a data file (cont.)
sum = 0.0;
N = 0;
// Initialize ("clear the slate")
// No items added **** New code
while ( in.hasNextDouble() )
{
x = in.nextDouble(); // Read a floating point number
sum = sum + x;
// Add to the running sum
N++;
// One more item added **** New code
}
System.out.println("Average = " + sum/N);
}
}
Programming example 3: compute the average
of (floating point) data in a data file (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/07/Progs/
File03.java
– Input data file inp1:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/07/Progs/i
np2
• How to run the program:
• Right click on both links and save in a scratch
directory
• To compile: javac File03.java
• To run:
java File03
Programming example 3: compute the average
of (floating point) data in a data file (cont.)
• Example run:
• Input file inp3:
2.1 4.2 5.2
6.1
Output of the program:
Average = 4.4