Lecture on repetition statements (loops)

Chapter 4
Repetition Statements (loops)
1

2
Loops – While, Do, For
• Repetition Statements
– While
– Do
– For

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
5

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++;
}

Trace while Loop
int count = 0;
while (count < 2)
{
count++;
}
Initialize count
animation
7

Trace while Loop, cont.
int count = 0;
while (count < 2)
{
count++;
}
(count < 2) is true
animation
8

int count = 0;
while (count < 2)
{
count++;
}
Print Welcome to Java
animation
9

int count = 0;
while (count < 2)
{
count++;
}
Increase count by 1
count is 1 now
animation
10

int count = 0;
while (count < 2)
{
count++;
}
(count < 2) is still true since
count is 1
animation
11

int count = 0;
while (count < 2)
{
count++;
}
Print Welcome to Java
animation
12

int count = 0;
while (count < 2)
{
count++;
}
Increase count by 1
count is 2 now
animation
13

int count = 0;
while (count < 2)
{
count++;
}
(count < 2) is false since count is
2 now
animation
14

Trace while Loop
int count = 0;
while (count < 2)
{
count++;
}
The loop exits. Execute the next
statement after the loop.
animation
15

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

Lecture on repetition statements (loops)

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