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Loops
Chapter 4
“while” structures
• It repeats a set of statements while a condition is true.
while (condition)
{
execute these statements;
}
“while” structures
• It repeats a set of statements while a condition is true.
while ( condition ) 1
{
execute these statements;
}
2
3
The dynamics of “while”
1. Evaluate condition:
• if TRUE go to 2
• If FALSE go to 3
2. Execute statements, and then go to 1
3. Continue with next statement.
“while” structures
• It repeats a set of statements while a condition is true.
int speedLimit = 55;
int speed
= 0;
while ( speed <= speedLimit )
{
speed = speed + 1;
}
// since we don’t want a ticket…
speed = speed - 1;
• What is the value of “speed” after the
last statement?
A.
B.
C.
D.
E.
55
54
56
0
none of the
above
“while” structures
• It repeats a set of statements while a condition is true.
int speedLimit = 55;
int speed
= 0;
1
initialize variables
in conditional
while ( speed < speedLimit )
2
{
modify variables
speed = speed + 1;
in conditional
}
// since we don’t want a ticket…
speed = speed - 1;
•
“while” structures:
Exercises
Determine the output of the following methods:
public void foo1() {
int i=0;
while (i <= 20) {
System.out.println( i );
i = i + 4; // or i+= 4;
}
}
A.
B.
C.
D.
E.
0 4 8 12 16 20
4 8 12 16
0 4 8 12 16 20 24
0 4 8 12 16
none of the above
(on separate lines)
public void foo2() {
A. 20 10 5 2.5 1.25
int i = 20;
B. 10 5 2 1 0
while (i > 0) {
C. 20 5 2 1
D. 20 5 2 1 0
i = i / 2;
System.out.println( i ); E. none of the above
(on separate lines)
}
}
Computer Work
1. Write a program that reads an integer, and displays (using
System.out.println) all numbers from the number down to 0.
• For example, if 8 is entered, the program should display
numbers 7, 6, 5, 4, 3, 2, 1, 0, in this order and with each
number in one line.
2. Write a program that reads an integer, and displays (using
System.out.println) all even numbers between 0 and the
number received, and finally the number of numbers displayed.
• For example, if the parameter was 8, the method should
display numbers 2, 4 and 6, in this order and with each
number in one line, and return a value of 3 (which is how
many even numbers were between 0 and 8).
“for” structures
• It (also) repeats statements while a condition is true. Works
best when loop will be repeated a known number of times
1
2
4
loop
initial
modify
for ( statement ; condition; statement )
{
statements;
3
}
5
The dynamics of “for”
1. Initialize condition variables.
2. Evaluate loop condition:
• if TRUE go to 3
• If FALSE go to 5
3. Execute statements; then go to 4
4. Modify condition variables; then go to 2
5. Continue with next statements.
prime using for
Write a program that prints “yes” or “no” to indicate
whether an integer parameter named number is a prime
number. Use a for loop.
How to write Java:
Answer the questions: Exactly how do you do this? What is a
step by step description?
WRITE IN ENGLISH. NOT Java
or Draw pictures.
or both
Debug the English: Trace through with numbers. especially
with going through the loop 0 times, 1 time, >= 1 times.
Fix the ENGLISH.
1. Brainstorm. How do you figure out if a
number is prime? prime(11) prime(12).
Talk to each other.
2. Write an algorithm for that
3. Translate the algorithm to Java
Prime Number Program
import java.util.Scanner;
public class Prime
{
public static void main(String[ ] args)
{
Scanner kbd = new Scanner(System.in);
System.out.print("Enter a number: ");
int nbr = kbd.nextInt( );
if (nbr < 2) // only look at positive numbers
{
System.out.println("no");
System.exit(0); // means leave the program right now
}
for (int i=2;i<nbr;i++)
for loop
{
if (nbr % i == 0)
{
System.out.println("no");
System.exit(0);
Leaves loop and program
}
}
System.out.println("yes");
outside the loop
}
}
Embedded loops
• Write a program to draw a square box after
reading in its size
1. Write the first line (for loop
printing n stars)
2. println
3. for each internal line (n-2 of them)
a. print a *
b. print n-2 spaces
c. println a *
4. print the last line
import java.util.Scanner;
public class DrawStars
{
public static void main(String[ ] args)
{
Scanner kbd = new Scanner(System.in);
System.out.print("Enter a number: ");
int nbr = kbd.nextInt( );
if (nbr < 2) // only look at positive numbers
System.exit(0); // means leave the program right now
// do first line
for (int i=0;i<nbr;i++)
System.out.print("*");
// add enter
System.out.println( ); // nothing in parentheses
// continued on next page
// print n-2 internal lines
for (int i=0;i<nbr-2;i++) // i is never used. it only counts
{
// nbr of times through loop
// print first character on line
System.out.print("*");
// print number of spaces minus one * on each end
for (int j=0;j<nbr-2;j++) // embedded loop for one internal line
System.out.print(" "); // single space in between quotes
// print last * on line -- also put enter in there
System.out.println("*");
}
// and outer for loop that does all internal lines
// print last line. same as first line
for (int i=0;i<nbr;i++)
System.out.print("*");
System.out.println( ); // nothing in parentheses
} // end main
} // end class
Embedded Loop Practice
• Write a loop that prints a times table 0-2:
0 01 2
1. Write the first line
0 00 0
2. for each number on the left (0, 1, 2)
1 01 2
a. print the left number (the LHS of
2 02 4
the table)
2a
b. for each number on the right (0,
1, 2)
i. print the product
c. println.