The while-statement
Syntax and meaning of the while-statement
•
The LOOP-CONTINUATION-CONDITION is a Boolean expression
(exactly the same as in the condition clause of an if-statement)
Flow chart of a while-statement (1)
Flow chart of a while-statement (2)
• When the loop-cont-condition is true, the execution takes
the downward branch:
■ It executes the statements which for the body of the whilestatement
■ Then it goes back and retests the loop-cont-condition
• When the loop-cont-condition is false, the execution takes
the side branch
■ In this case, the execution proceeds (continues) with the
statement following the while-statementI.e.,: the whilestatement is completed
Structure diagram representing a while-statement
Example while-statement: print the numbers 1 through 10
public class While01
{
public static void main(String[] args)
{
int a;
a = 1;
while ( a <= 10 )
// While-statement
{
System.out.println(a); // Print a
a++;
// Increment a
}
System.out.println("Done");
System.out.println("Exit: a = " + a);
}
}
Computer programming (1):
Find all divisor of the number 12
I think you would have done this:
Check if 12 is divisible by 1
Check if 12 is divisible by 2
...
Check if 12 is divisible by 12
Note:
We do not need to check numbers > 12 because only number ≤ 12 can be
divisors !
When the remainder of the division is equal to 0, we print out the divisor
Computer programming (2):
Rough algorithm (pseudo code) to find all divisors:
input: n = some integer number
for (x = 1, 2, 3, ..., n) do
{
if ( n is divisible by x ) then
print x;
(because x is a divisor !)
}
We have basically written down what we would do to find all divisors in a
pseudo programming language !!!
Computer programming (3):
int x = 1
while ( x <= n )
// Run x = 1, 2, ..., n
{
if ( n % x == 0 )
{
System.out.println(x); // Print x (because it's a divisor)
}
x++;
// Make sure we more to the next number !!
}
Programming example 2:
find all common divisors of 2 numbers (1)
• Problem description:
■ Write a Java program that reads in 2 numbers x and y...
■ and prints all numbers that are divisors of both x and y
• A concrete example:
■ Input: x = 24 and y = 16
■ Output: 1, 2, 4, 8
Programming example 2:
find all common divisors of 2 numbers (2)
input x, y;
min = min(x, y); // this is the range of the brute force search
for (a = 1, 2, ...., min) do
{
if (x and y are divisible by a)
{
print a;
}
}
Programming example 2:
find all common divisors of 2 numbers (3)
while ( a <= min )
// Run a = 1, 2, ..., min(x,y)
{
if ( x % a == 0 && y % a == 0 )
{ // a is a divisor of x and y
System.out.println(a);
}
a++;
}
The break statement
When the break statement is executed inside a loop-statement, the loopstatement is terminated immediately
• The execution of the program will continue with the statement following the
loop-statement
The continue statement
Using the while-statement to process data files
What is a file:
■ File = an electronical document stored inside a computer system that
contains information (data)
■ A file can be created by humans using a computer program called an
editor (e.g., gedit)
■ A file can also be created when a computer program needs to store its
output data.
General procedure to access a data file
•
•
•
•
General procedure in computer programming to read data
from a data file
Open the data file
With the information X, you can then use a "read something
from a file" method to read data from the opened file
There are other helpful methods on an opened file.Some
method let you check if you have reached the end of the file
Opening a data file
•
How to open a file in Java:
File myFile;
// Define a "File" type variable
myFile = new File("Path-name-of-the-file");
The variable myFile contains information about the opened file
The variable myFile will be used in read operations
Scanning an opened file (1)
Construct a Scanner object using an opened file:
File myFile; // Define a "File" type variable
myFile = new File("Path-name-of-the-file"); // Open the file
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 (2)
•
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.
• Check function for input availability on Scanner typed variable in:
■ in.hasNextDouble()
■ in.hasNextInt()
■ in.hasNext()
Programming example : print the content of a data file (1)
•
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 : print the content of a data file (2)
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);
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");
}
}