the while-statement. syntax and meaning of the while-statement the loop-continuation-condition is a...
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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 while-statement
■ 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 while-statement 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 loop-statement 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"); } }