DSA 103 Object Oriented Programming :: Week 3

Object – Oriented Programming
Week 3 – While and for loop
Ferdin Joe John Joseph, PhD
Faculty of Information Technology
Thai-Nichi Institute of Technology

2
Loops – While, Do, For
• Repetition Statements
– While
– Do
– For
Faculty of Information Technology,

3
Repetition Statements
• Repetition statements allow us to execute a statement
or a block of statements multiple times
• Often they are referred to as loops
• Like conditional statements, they are controlled by
boolean expressions
• Java has three kinds of repetition statements:
while
do
for
• The programmer should choose the right kind of
loop statement for the situation

4
The while Statement
• A while statement has the following syntax:
• If the condition is true, the statement is executed
• Then the condition is evaluated again, and if it is
still true, the statement is executed again
• The statement is executed repeatedly until the
condition becomes false
while ( condition )
statement;

Logic of a while Loop
statement
true false
condition
evaluated
5Faculty of Information Technology,

6
The while Statement
• An example of a while statement:
• If the condition of a while loop is false
initially, the statement is never executed
• Therefore, the body of a while loop will
execute zero or more times
int count = 0;
while (count < 2)
{
System.out.println("Welcome to Java!");
count++;
}
Faculty of Information Technology, Thai-Nichi Institute of Technology

Trace while Loop
int count = 0;
while (count < 2)
{
count++;
}
Initialize count
animation

Trace while Loop, cont.
int count = 0;
while (count < 2)
{
count++;
}
(count < 2) is true
animation

int count = 0;
while (count < 2)
{
count++;
}
Print Welcome to Java
animation

int count = 0;
while (count < 2)
{
count++;
}
Increase count by 1
count is 1 now
animation

int count = 0;
while (count < 2)
{
count++;
}
(count < 2) is still true since count
is 1
animation

int count = 0;
while (count < 2)
{
count++;
}
Print Welcome to Java
animation

int count = 0;
while (count < 2)
{
count++;
}
Increase count by 1
count is 2 now
animation

int count = 0;
while (count < 2)
{
count++;
}
(count < 2) is false since count is 2
now
animation

Trace while Loop
int count = 0;
while (count < 2)
{
count++;
}
The loop exits. Execute the next
statement after the loop.
animation

16
Example (Average.java)

17
Infinite Loops
• Executing the statements in the body of a while
loop must eventually make the condition false
• If not, it is called an infinite loop, which will
execute until the user interrupts the program
• This is a common logical error
• You should always double check the logic of a
program to ensure that your loops will terminate

18
Infinite Loops
• An example of an infinite loop:
• This loop will continue executing until the
user externally interrupts the program.
int count = 1;
while (count <= 25)
{
System.out.println(count);
count = count - 1;
}

19
Nested Loops
• Similar to nested if statements, loops can
be nested as well
• That is, the body of a loop can contain
another loop
• For each iteration of the outer loop, the
inner loop iterates completely

20
Nested Loops
• How many times will the string "Here" be printed?
count1 = 1;
while (count1 <= 10)
{
count2 = 1;
while (count2 <= 20)
{
System.out.println ("Here");
count2++;
}
count1++;
}
10 * 20 = 200

Repetition with for loops
• So far, repeating a statement is redundant:
System.out.println("Homer says:");
System.out.println("I am so smart");
System.out.println("S-M-R-T... I mean S-M-A-R-T");
• Java's for loop statement performs a
task many times.
System.out.println("Homer says:");
for (int i = 1; i <= 4; i++) { // repeat 4 times
}
System.out.println("S-M-R-T... I mean S-M-A-R-T");

for loop syntax
for (initialization; test; update) {
statement;
statement;
...
statement;
}
– Perform initialization once.
– Repeat the following:
• Check if the test is true. If not, stop.
• Execute the statements.
• Perform the update.
body
header

Initialization
for (int i = 1; i <= 6; i++) {
System.out.println("I am so
smart");
}
• Tells Java what variable to use in the loop
– Performed once as the loop begins
– The variable is called a loop counter
• can use any name, not just i
• can start at any value, not just 1

Test
for (int i = 1; i <= 6; i++) {
System.out.println("I am so
smart");
}
• Tests the loop counter variable against a limit
– Uses comparison operators:
< less than
<= less than or equal to
> greater than
>= greater than or equal to

Increment and decrement
shortcuts to increase or decrease a
variable's value by 1
Shorthand Equivalent longer
version
variable++; variable = variable+1;
variable--; variable = variable - 1;
int x = 2;
x++; // x = x + 1;
// x now stores 3
double gpa = 2.5;
gpa--; // gpa = gpa - 1;
// gpa now stores 1.5

Modify-and-assign operators
shortcuts to modify a variable's value
Shorthand Equivalent longer version
variable += value; variable = variable + value;
variable -= value; variable = variable - value;
variable *= value; variable = variable * value;
variable /= value; variable = variable / value;
variable %= value; variable = variable % value;
x += 3; // x = x + 3;
gpa -= 0.5; // gpa = gpa - 0.5;
number *= 2; // number = number * 2;

Repetition over a range
System.out.println("1 squared = " + 1 * 1);
– Intuition: "I want to print a line for each number from 1 to 6"
• The for loop does exactly that!
for (int i = 1; i <= 6; i++) {
System.out.println(i + " squared = " + (i * i));
}
– "For each integer i from 1 through 6, print ..."

Loop walkthrough
for (int i = 1; i <= 4; i++) {
System.out.println(i + " squared = " + (i * i));
}
System.out.println("Whoo!");
Output:
1 squared = 1
2 squared = 4
3 squared = 9
4 squared = 16
Whoo!
1
1
2
2
3
3
4
4
5
5

Multi-line loop body
System.out.println("+----+");
for (int i = 1; i <= 3; i++) {
System.out.println(" /");
System.out.println("/ ");
}
System.out.println("+----+");
– Output:
+----+
/
/
/
/
/
/
+----+

Expressions for counter
int highTemp = 5;
for (int i = -3; i <= highTemp / 2; i++) {
System.out.println(i * 1.8 + 32);
}
– Output:
26.6
28.4
30.2
32.0
33.8
35.6

System.out.print
• Prints without moving to a new line
– allows you to print partial messages on the same line
int highestTemp = 5;
for (int i = -3; i <= highestTemp / 2; i++) {
System.out.print((i * 1.8 + 32) + " ");
}
• Output:
26.6 28.4 30.2 32.0 33.8 35.6
• Concatenate " " to separate the numbers

Counting down
• The update can use -- to make the loop count down.
– The test must say > instead of <
System.out.print("T-minus ");
for (int i = 10; i >= 1; i--) {
System.out.print(i + ", ");
}
System.out.println("blastoff!");
System.out.println("The end.");
– Output:
T-minus 10, 9, 8, 7, 6, 5, 4, 3, 2, 1,
blastoff!
The end.

Nested loops
• nested loop: A loop placed inside another loop.
for (int i = 1; i <= 5; i++) {
for (int j = 1; j <= 10; j++) {
System.out.print("*");
}
System.out.println(); // to end the line
}
• Output:
**********
**********
**********
**********
**********
• The outer loop repeats 5 times; the inner one 10 times.
– "sets and reps" exercise analogy

Nested for loop exercise
• What is the output of the following nested
for loops?
for (int i = 1; i <= 5; i++) {
for (int j = 1; j <= i; j++) {
}
System.out.println();
}
• Output:
*
**
***
****
*****

• What is the output of the following nested
for loops?
for (int i = 1; i <= 5; i++) {
for (int j = 1; j <= i; j++) {
System.out.print(i);
}
}
• Output:
1
22
333
4444
55555

Common errors
• Both of the following sets of code produce
infinite loops:
for (int i = 1; i <= 5; i++) {
for (int j = 1; i <= 10; j++) {
}
}
for (int i = 1; i <= 5; i++) {
for (int j = 1; j <= 10; i++) {
}
}

Complex lines
• What nested for loops produce the
following output?
....1
...2
..3
.4
5
• We must build multiple complex lines of output using:
– an outer "vertical" loop for each of the lines
– inner "horizontal" loop(s) for the patterns within each line
outer loop (loops 5 times because there are 5 lines)
inner loop (repeated characters on each line)

Outer and inner loop
• First write the outer loop, from 1 to the number of lines.
for (int line = 1; line <= 5; line++) {
...
}
• Now look at the line contents. Each line has a pattern:
– some dots (0 dots on the last line), then a number
....1
...2
..3
.4
5
– Observation: the number of dots is related to the line number.

Mapping loops to numbers
for (int count = 1; count <= 5; count++)
{
System.out.print( ... );
}
– What statement in the body would cause the loop to
print:
4 7 10 13 16
for (int count = 1; count <= 5; count++)
{
System.out.print(3 * count + 1 + "
");
}

Loop tables
• What statement in the body would cause the loop to print:
2 7 12 17 22
• To see patterns, make a table of count and the numbers.
– Each time count goes up by 1, the number should go up by 5.
– But count * 5 is too great by 3, so we subtract 3.
count number to print 5 * count
1 2 5
2 7 10
3 12 15
4 17 20
5 22 25
5 * count - 3
2
7
12
17
22

Loop tables question
• What statement in the body would cause the loop to print:
17 13 9 5 1
• Let's create the loop table together.
– Each time count goes up 1, the number printed should ...
– But this multiple is off by a margin of ...
count number to print
1 17
2 13
3 9
4 5
5 1
-4 * count -4 * count + 21
-4 17
-8 13
-12 9
-16 5
-20 1
-4 * count
-4
-8
-12
-16
-20

• Make a table to represent any patterns on each line.
....1
...2
..3
.4
5
• To print a character multiple times, use a for loop.
for (int j = 1; j <= 4; j++) {
System.out.print("."); // 4 dots
}
line # of dots
1 4
2 3
3 2
4 1
5 0
-1 * line
-1
-2
-3
-4
-5
-1 * line + 5
4
3
2
1
0

Nested for loop solution
• Answer:
for (int j = 1; j <= (-1 * line + 5); j++) {
System.out.print(".");
}
System.out.println(line);
}
• Output:
....1
...2
..3
.4
5

• What is the output of the following nested for loops?
for (int j = 1; j <= (-1 * line + 5); j++) {
}
for (int k = 1; k <= line; k++) {
System.out.print(line);
}
}
• Answer:
....1
...22
..333
.4444
55555

• Modify the previous code to produce this output:
....1
...2.
..3..
.4...
5....
• Answer:
for (int j = 1; j <= (-1 * line + 5); j++) {
}
System.out.print(line);
for (int j = 1; j <= (line - 1); j++) {
}
}

DSA 103 Object Oriented Programming :: Week 3

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