Week04

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Repetition Statements
• Repetition statements allow us to execute a
statement 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:
 the while loop
 the do loop
 the for loop
• The programmer should choose the right kind of
loop for the situation

The while Statement
• A while statement has the following syntax:
while ( condition )
statement;
• 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 until the
condition becomes false
• We use while loops when we cannot predict the
number of times the loop will execute, e.g.
•testing for bad user input
•processing input records from a database

Logic of a while Loop
statement
true
false
condition
evaluated

The while Statement
• An example of a while statement:
int count = 1;
while (count <= 5)
{
System.out.println (count);
count++;
}
• 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

How Often is the Body Run?
• 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
• Once inside the body of the while loop, if nothing
is done to modify the loop condition, we get an
infinite loop

Infinite Loops
• The body of a while loop eventually must 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
normally

Infinite Loops
• An example of an infinite loop:
int count = 1;
while (count <= 25)
{
count = count - 1;
}
• This loop will continue executing until interrupted
(Control-C) or until an underflow error occurs

Running Sum, Count, Max, Min
• In the Average program from Week 3's lab, we saw
how to keep a running count of the numbers
entered by the user
• We also kept a running sum of the numbers
entered. This allowed us to calculate the average
of the numbers entered
• The same idea can be used to calculate the
maximum and minimum of the numbers entered;
see Average2 from Week 4's lab
• We must start the running maximum small so that
the first number entered overwrites it
• The same thing goes for the running minimum
• Can you think of numbers which will break it?

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
• See PalindromeTester.java (page 235)

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

The do Statement
• A do statement has the following syntax:
do
{
statement;
}
while ( condition )
• The statement is executed once initially, and then
the condition is evaluated
• The statement is executed repeatedly until the

Logic of a do Loop
true
condition
evaluated
statement
false

The do Statement
• An example of a do loop:
• The body of a do loop executes at least once
• See ReverseNumber.java (page 244)
int count = 0;
do
{
count++;
} while (count < 5);

Comparing while and do
statement
true
falsecondition
evaluated
The while Loop
true
condition
evaluated
statement
false
The do Loop

When to Use Do or While?
• Use a do loop when the loop body must be
executed at least once, e.g
• Get the user's input before we can tell that it is
invalid
• Use either of these two loop forms when the
number of times the loop must execute is
unknown
• When the number of iterations through the loop is
known in advance, you should use a for loop

Looping N times, or over a range
• In many situations, you know in advance how many times
that you want to loop, e.g.
•Do something for every day in this month
• Alternatively, you need to loop over a range of values, e.g.
•Do some calculations for each possible age in the range 0
to 100 years old
• In these situations, we use the for loop.
• The for loop:
•has a loop counter, holding one value in the range each
time through the loop, and
•increments the loop counter for you

The for Statement
• A for statement has the following syntax:
for ( initialization ; condition ; increment )
statement;
The initialization
is executed once
before the loop begins
The statement is
executed until the
The increment portion is
executed at the end of each
iteration

Logic of a for loop
statement
true
condition
evaluated
false
increment
initialization

The for Statement
• A for loop is functionally equivalent to the
following while loop structure:
initialization;
while ( condition )
{
statement;
increment;
}

The for Statement
• An example of a for loop:
for (int count=1; count <= 5; count++)
• In this example, count goes from 1 to 5 in steps of 1,
and the count value is printed out
• The initialization section can be used to declare a
variable
• Like a while loop, the condition of a for loop is tested
prior to executing the loop body
• Therefore, the body of a for loop will execute zero or
more times

The for Statement
• The increment section can perform any calculation
• What does the above loop print out?
• A for loop is well suited for executing statements
a specific number of times that can be calculated
or determined in advance
• See Multiples.java (page 248)
• See Stars.java (page 250)
for (int num=100; num > 0; num -= 5)
System.out.println (num);

Nested For Loops
• Just like other control structures in Java, for loops
can be nested inside other loops or if statements
• What does this code do?
for (rows=1; rows<=10; rows++)
{
for (cols=1; cols<=20; cols++)
System.out.print(“*”);
System.out.println;
}
• Try it in the Java debugger and see

Nested For Loops
• It's important here to remember to treat the outside
loop counter as constant
• Watch what happens when we use the outside
loop counter as a constant in the inner loop:
• What does this code do?
{
for (cols=1; cols<=rows; cols++)
System.out.println;
}
• Try it in the Java debugger and see

Nested For Loops
{
for (cols=1; cols<=rows; cols++)
System.out.println;
}
• Note that the inner loop uses the value of the rows
variable to determine the number of stars to print

Class Exercise
• Write a program to print out a multiplication table,
with columns 1 2 3 4 5 6 7 8 9 10, and rows 1 .. 10, e.g.
x 1 2 3 4 5 6 7 8
1 1 2 3 4 5 6 7 8
2 2 4 6 8 10 12 14 16
3 3 6 9 12 15 18 21 24
• Hint: we need 2 variables to multiply together. What
can we name them?
• Hint: do we need loops? How many loops?
• Hint: when do we print a newline?
• Suggestion: don't print the blue numbers to start with

Class Exercise
• Modify the program to print out the row and
column labels. Include horizontal and vertical bars
if possible.
• Modify the program to ask the user for the upper
bound for the table size, instead of using 10.
• Modify the program to ensure that the user's input
is between 5 and 15.

Program Development Tips
• Think about the problem BEFORE you start writing
code!!
• What variables will be needed, how will they
change over time?
• What control structures do you think you'll need:
IF, WHILE, DO, FOR, and where?
• Develop your program one bit at a time. Test what
you have written before adding new stuff.
• Use the debugger to show you what the code is
actually doing. Remember: a computer DWIS not
DWIW.

Program Development Tips
• Use a top-down approach, e.g.
 Get the input from the user, make sure it's between 5 and
10
 Print out the heading (need a loop here, probably a FOR
loop as we know the table size)
 We need a loop for each line, again a FOR loop
 FOR each line
• We need another variable to be the column variable, and
we're going to need an inner loop now
• Do the multiplication inside the inner loop and print out the
result
• At the end of the line, i.e. when the inner loop has finished,
print out the newline
• Alternatively, if you know the code to write to do a
specific thing, then do it (bottom-up).
• You will usually do a bit top-down and a bit
bottom-up.

Break and Continue
• Inside a loop body, the break command instantly
exits the loop and starts the code following the
loop.
• Inside a loop body, the continue command
instantly takes you to the top of the loop. Any
condition at the top of the loop is evaluated.
• The textbook suggests you should avoid these two
commands. I disagree.
• Example: ask user for number between 1 and 100.
If they enter something else, tell them that they are
stupid and ask again.

The Break Command
int value;
while (true)
{
System.out.print(“Enter num 1-100: “);
value=scan.nextInt();
if ((value>=1) && (value<=100)
break;
System.out.println(“Idiot! Try again”);
}
• Note the loop looks like an infinite loop. But the
loop breaks when the user gets it right.
• Try doing the same with a normal WHILE or DO
loop

Week04

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