Lecture 4
Loops
Looping Constructs
The while Loop
Years to Double Investment
public class DoubleInvestment
{
public static void main(String[] args)
{
final double RATE = 5;
final double INITIAL_BALANCE = 10000;
final double TARGET = 2 * INITIAL_BALANCE;
double balance = INITIAL_BALANCE;
int year = 0;
// Count the years required for the investment to double
while (balance < TARGET)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest;
}
System.out.println("The investment doubled after " + year + " years.");
}
}
Hand-Tracing Code Execution
This program computes the sum of the digits in an integer.
Example: The Number Guessing Game
Algorithm Design
The Problem: Write a program that guesses the user's secret number between 1
and 100. The program should be able to guess the number in seven tries or less.
The Algorithm:
1. Set the initial range between lo = 0 and hi = 101
2. The program makes a guess in the middle of the range
guess = (lo + hi)/2
3. If guess is too high, let hi = guess
4. If guess is too low, let lo = guess
5. If guess is correct claim victory and quit else return to Step 2.
Example: The Number Guessing Game
Implementation
This is the setup
Let the game begin...
Example: The Number Guessing Game
Analysis
Starting with 100 possible answers, each wrong guess eliminates 1/2 of the
possibilities.
Starting with 100, how many times would be have to divide by 2 before the
resulting value is <= 1?
100
=1
2𝑛
100 = 2𝑛
n = 𝑙𝑔2 100
n = 6.643856...
100 -> 50 -> 25 -> 12 -> 6 -> 3 -> 1
Example: Building a StopWatch
Whenever currentTimeMillis() is called it returns the current time in milliseconds.
The stopwatch gets the current time when the first OK is pressed (startTime) and it gets
the current time again when the second OK is pressed (finishTime).
The elapsed time is computed as the difference between these two times converted to
seconds.
Example: Building a StopWatch
Whenever currentTimeMillis() is called it returns the current time in milliseconds.
The stopwatch gets the current time when the first OK is pressed (startTime) and it gets
the current time again when the second OK is pressed (finishTime).
The elapsed time is computed as the difference between these two times converted to
seconds.
A Delay Timer
time delay in milliseconds
setting the time when loop is entered
stay in the loop for delMsec (Msec)
This method is very useful for controlling the speed of execution of a program. Control
remains in the do-while( ) loop for delMsec milliseconds.
Example: Reading Integers from a Textfile
necessary when reading
data from a file
data.txt
Blanks, carriage returns, and other white-space will be ignored by
Scanner inputs for numeric data types such as integer
for Loop
The for-loop is the oldest looping construct. It has been implemented in every procedural
high-level programming language since FORTRAN (1957). For-loops are commonly used
when you know the number of iterations to be performed before beginning the loop.
for Loop Examples
for Loop Test Program
The do while Loop
do while Test Program
Nested Loops
body of outer loop executes 10 times
body of inner loop executes 10 times
each time it is called by the outer loop
inner loop prints 10 stars on a line
outer loop executes inner loop to print
a line of stars and then starts a new line
more Nested Loop Examples
even more Nested Loop Examples
Multiplication Table Generator
Multiplication Table
1
2
3
4
5
6
7
8
9
----------------------------------------1 |
1
2
3
4
5
6
7
8
9
2 |
2
4
6
8 10 12 14 16 18
3 |
3
6
9 12 15 18 21 24 27
4 |
4
8 12 16 20 24 28 32 36
5 |
5 10 15 20 25 30 35 40 45
6 |
6 12 18 24 30 36 42 48 54
7 |
7 14 21 28 35 42 49 56 63
8 |
8 16 24 32 40 48 56 64 72
9 |
9 18 27 36 45 54 63 72 81
Greatest Common Divisor
Using break & continue
x = 369
x = 531
x = 89
x = 453
x = 801
x = 777
x = 28
x = 241
x = 597
x = 989
x = 657
x = 597
x = 335
Must have gotten a 42
break - break out of the current loop
continue - continue immediately to the next iteration of the loop
Controlling Loops with Sentinel Values