Week04

Week 4

Loops

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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

falseconditionevaluated


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){ System.out.println (count); 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 condition becomes false


Logic of a do Loop

trueconditionevaluated

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++; System.out.println (count);} while (count < 5);


Comparing while and do

statement

true

falseconditionevaluated

The while Loop

true

conditionevaluated

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 initializationis executed once

before the loop begins

The statement isexecuted until the

condition becomes false

The increment portion is executed at the end of each

iteration


Logic of a for loop

statement

true

conditionevaluated

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++) System.out.println (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 (rows=1; rows<=10; rows++){ for (cols=1; cols<=rows; cols++) System.out.print(“*”); System.out.println;}

• Try it in the Java debugger and see


Nested For Loops

for (rows=1; rows<=10; rows++){ for (cols=1; cols<=rows; cols++) System.out.print(“*”); 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 Commandint 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

Technology

Transcript of Week04