1. Java LESSON 3 Loops

2. We now know: Sequence begin prompt user read from keyboard BEGIN
add 1
print result
end
BEGIN
PROMPT "Enter a number"
INPUT number from keyboard
increment number
PRINT number
END

3. We also know: Selection (aka: Conditonal)
begin
prompt user
read number from keyboard
BEGIN
PROMPT "Enter a number"
INPUT number from keyboard
IF number > 5
PRINT "more than 5"
ELSE
PRINT "5 or less"
END IF
END
true
input > 5 ?
false
print "more than 5"
print "5 or less"
end

4. read number from keyboard
New: Iteration
begin
prompt user
read number from keyboard
end
input > 5 ?
true
false
print "another one"
BEGIN DO
PROMPT "Enter a number"
INPUT number from keyboard
IF number > 5
PRINT "another one"
END IF
WHILE number > 5
END
More about converting iteration flow chart elements to pseudo code next week

5. Iteration Iteration means: repeat executing a block of program code
3 iteration types implemented in Java:
'while' loops
'do' - 'while' loops
'for' loops
as long as a condition is true
Boolean expression

6. Review of Boolean Expressions
6 Comparison Operators
> greater than
< smaller than
>= greater than or equal to
<= less than or equal
== equal to
!= not equal to
&& and
|| or
! not
3 Logical Operators
if ( !(a>b) || (c>d) && (x==y) ) {
// then do stuff here }

7. Structure of 'while' Loops
while ( boolean expresion is true ) {
// do stuff
// do some more stuff
// whatever
} // end of while loop

8. 'while' Loop Example Number is 5 Number is 4 Number is 3 Number is 2
int number= 5;
while ( number >= 0 ) {
System.out.println( "Number is " + number );
-- number;
} // end while number >= 0
System.out.println( "End" );
Number is 5
Number is 4
Number is 3
Number is 2
Number is 1
Number is 0
End
What gets printed?

9. Another 'while' Example int number = 1; while ( number <= 4 ) {
System.out.println( number );
....;
++ number;
} // end of while loop
System.out.println( "End" ); 1 2 3 4
End
What gets printed?

10. Same 'while' Example Again (well, almost, spot the difference)
int number = 1;
while ( number <= 4 ) {
++ number;
…. ;
System.out.println( number );
} // end of while loop
System.out.println( "End" ); 2 3 4 5
End
What gets printed?

11. Incrementing (subtle weirdness….)
int number = 0;
while ( ++number < 5 ) {
System.out.println( number );
....; }
++ number
1. increment
2. compare 1 2 3 4
"pre - operator"
int number = 0;
while ( number++ < 5 ) {
System.out.println( number );
....; }
number ++
1. compare
2. increment 1 2 3 4 5
"post - operator"

12. Stylish Loops Alignment & indentation Numbers in loops number <= 10
while ( … ) {
...; // code here }
Numbers in loops
If what you really want is to execute the loop 10 times, then use ’10’:
number <= 10
but don’t write:
number < 11

13. Task (5 minutes) Add together the sequence 1 + 1/2 + 1/4 + 1/8 + ...
while the terms you are adding together are larger than 1/100.
Use the following variables:
float total=0.0;
float counter = 1.0;

14. Solution float total = 0.0; float counter = 1.0;
while ( (1.0/counter) > (1.0/100.0) ) {
total = total + 1.0/counter;
counter = counter * 2.0; }
System.out.println( "Sum is " + total );

15. Task 2 (5 minutes) Read positive integer numbers from the keyboard
using UserInput.readInt()
Find out if the number just entered is larger than all
others before (use an 'if' statement)
Input terminates when a zero has been entered
Print the biggest number
Use the following variables
int input = 1, biggestSoFar = 0;

16. Solution int input=1; int biggestSoFar=0; while (input != 0) {
System.out.println("Enter a number : ");
input = UserInput.readInt();
if( input > biggestSoFar ) {
biggestSoFar = input; }
System.out.println("The biggest number
was " +biggestSoFar);

17. 'do' - 'while' Loops 6 AT LEAST ONCE ! int number = 1; do { ++number;
} while ( number <= 5 );
System.out.println(number); 6
'while'-'do' loops test at beginning of loop
'do' loops test at the end of the loop
the loop is always executed
AT LEAST ONCE !

18. Increment 'n Test int number = 1; do { 1 2
System.out.println( number );
} while ( ++number <= 3 );
System.out.println( number+ " End" ); 1 2 3
4 End

19. Notes
( ++number <= 3 ); is the form of combined "increment and test" that experienced JAVA programmers would prefer. For now, avoid it.
It is generally safer to test at the start of a loop; "while" loops are generally safer and more common than "do"-"while" loops.

20. Task using 'do'-'while'
Read positive integer numbers from the keyboard
using UserInput.readInt()
Find out if the number just entered is larger than all
others before (use an 'if' statement)
Input terminates when a zero has been entered
Print the biggest number
Use the following variables
int input, biggestSoFar = 0;

21. Solution int input; int biggestSoFar=0; do {
Note the difference to the 'while' loop where we wrote:
int input=1;
int input;
int biggestSoFar=0;
do {
input = UserInput.readInt();
if( input > biggestSoFar ) {
biggestSoFar = input; }
} while (input != 0)
System.out.println("The biggest number
was " +biggestSoFar);

22. Advanced Stuff (1 of 3) On rare occasion you want an "emergency
exit" out of the (innermost) loop
Use the 'break' statement
while ( i > 0 ) {
....;
if ( j == ) {
break; // leave the loop
} ....;
} // end of the loop body
System.out.println( "continues here ...");

23. Advanced Stuff (2 of 3)
Also on rare occasions inside a loop you want to skip executing the remainder of the loop. Use the 'continue' statement.
A "while" or "do"-"while" loop moves directly to the next test at the head or foot of the loop, respectively.
while ( i > 0 ) {
.....;
if ( j == ) {
continue; }
i=i -1; // on ‘continue’ this line gets skipped

24. Advanced Stuff (3 of 3) On very rare occasion you want an
"emergency exit" out of the entire program
sort of "crashing out"
that is bad, bad style, mind.
Use the ‘System.exit(0)' statement
if ( j == ) {
System.exit(0); // leave program
} ....;

25. Check your knowledge now:
What is wrong here ?
int a;
int sum = 0;
while ( a< 10 ); {
sum = sum + a;
a=a-1;
if ( a = 5 ) {
a = 9; }
System.out.println( a ); // what's the output ?
a=0;
'a' not initialised: 'int a=0;'
';' ---> endless loop !
correct
a=a+1;
endless loop: should be 'a=a+1;'
if ( a==5) {
Should be 'a==5'
endless loop? --> no, OK

26. Intermediate Summary Today we have ...
revised 'boolean expressions' statements
introduced 'while' loops
introduced 'do' - 'while' loops
(looked at 'break', 'continue' and 'exit(0)')

27. The Problem with 'do'-Loops
begin
BEGIN DO
PROMPT "Enter a number"
INPUT number from keyboard
IF number > 5
PRINT "another one"
END IF
WHILE number > 5
END
prompt user
read number from keyboard
do {
System.out.println("Enter number ");
number = UserInput.readInt();
print "another one"
if ( number > 5 ) {
System.out.println( "another one" );
input > 5 ?
yes
} // end of if
} while ( number > 5 ); no
'if' ?
No sign of that in the Flow Chart!
end

28. Solution: Use 'while' - Loops and "Read Ahead"
begin
Solution: Use 'while' - Loops and "Read Ahead"
Prompt user and read number from keyboard
BEGIN
PROMPT "Enter a number"
INPUT number from keyboard
WHILE number > 5
PRINT "another one"
END WHILE
END
System.out.println("Enter number ");
number = UserInput.readInt();
input > 5 ? no
yes
print "another one"
while ( number > 5 ) {
System.out.println( "another one" );
System.out.println("Enter number ");
number = UserInput.readInt();
Prompt user and read number from keyboard
} // end of while
end

29. New Now: 'for' - Loops Java has three types of loop structures:
'while' - loops
'do' - 'while' - loops (avoid)
'for' - loops
Usually preferred when:
amount of iterations is not known a priori
for a known amount of iterations

30. Structure of the 'for' - Loop
Boolean expression, evaluated before loop runs, defining when to stop
Starting value of "loop variable" which has to be of integer type:
(char, byte, int, long)
Action performed on "loop variable" after each loop run
Single set { } of curly braces
for ( x=1; x<3; x ++) {
System.out.println("loop no. " +x); }
Keyword 'for' (all lowercase)
Single set ( ) of round braces, three items, ;
Body of the loop between { }

31. How the 'for' - Loop Works for ( x=1; x<3; x++ ) {
Starting value of "loop variable", initialisation only once, before first 'round' of loop only
Boolean expression, defining how often to loop. Executed before each 'round' of the loop
Action performed on "loop variable" after each 'round' of the loop
for ( x=1; x<3; x++ ) {
System.out.println("loop no. " +x); }

32. 'for' vs. ‘while’ Loops x=1; while ( x<3 ) {
System.out.println("loop no. " +x);
x++ ; }
for ( x=1; x<3; x++ ) {
System.out.println("loop no. " +x); }

33. The Output is Therefore….
1. Initialisation of 'loop variable' to 1 "x=1;"
2. Evaluate boolean expression "1<3 ?" -->true
3. Print "loop no. 1"
4. Execute "++x". ‘x’ is now 2
5. Boolean expression "2<3 ?" -->true
6. Print "loop no.2"
7. Execute "++x". ‘x’ is now 3
8. Boolean expression "3<3 ?" -->false
9. Loop ends and program continues
for ( x=1; x<3; x++ ) {
System.out.println("loop no. " +x); }

34. Example 1: Counting Backwards
int i;
System.out.println("Start");
for ( i=4; i>=1; --i ) {
System.out.println("counter=" +i); }
System.out.println("End");
Start
counter=4
counter=3
counter=2
counter=1
End

35. Example 2: Counting in Steps of 3
int i;
System.out.println("Start");
for ( i=0; i<10; i=i+3 ) {
System.out.println("counter=" +i); }
System.out.println("End");
Start
counter=0
counter=3
counter=6
counter=9
End

36. Example 3: Exponential Growth
int i;
System.out.println("Start");
for ( i=2; i<100000; i=i*i ) {
System.out.println("i is " +i); }
System.out.println("End");
Start
i is 2
i is 4
i is 16
i is 256
i is 65536
End

37. Example 4: Stats Complete it
int i, min=1000, max=0, sum=0, in;
System.out.println("Enter 10 numbers between 0 and 1000");
for ( i=0; i<10; i ++) { }
System.out.println("Largest number was" +max);
System.out.println("Smallest number was" +min);
System.out.println("Average was" +(sum/10));
System.out.println("Enter number " +(i+1));
in = UserInput.readInt();
if ( in>max ) max=in; // no curlies?
if ( in<min ) min=in; // none here either?
sum=sum + in;

38. Task Now:
Write a 'for' - loop that adds all numbers between (and including) 1 and 10. Print the sum for each 'round' of the loop.
Sum=1
Sum=3
Sum=6
Sum=10
Sum=15
Sum=21
Sum=28
Sum=36
Sum=45
Sum=55
Final=55
int i, sum=0;
for ( i=1; i<=10; i ++) {
sum=sum + i;
System.out.println("Sum=" +sum); }
System.out.println("Final=" +sum);

39. Nesting
Value of shares which increase by 10% every month after ten years. Starting value: £10.
int year, month;
float sValue=10.0;
for ( year=1; year<=10; year++ ) { }
System.out.println("After 10 years: " +sValue);
£ 927,091.50
for ( month=1; month<=12; ++month ) {
sValue=sValue+ sValue *0.1; }

40. 'break' - The Great Escape
On occasion you want an "emergency
exit" out of the (innermost) loop
int month;
float sValue=10.0;
for ( month=1; month<=1000; month++ ) {
sValue=sValue+ sValue *0.1;
if (sValue > ) break; }
System.out.println("1 million after "+month+“months");
1 million after 121 months
Is this a good idea?

41. Use 'while' - Loops When the Amount of Iterations is Unknown
int month=0;
float sValue=10.0;
while ( sValue < ) {
sValue=sValue+ sValue *0.1;
++month; }
System.out.println("1 million after"+month+"months");

42. Initialising the Loop Variable
int i;
System.out.println("Start");
for ( i=4; i>=1; i --) {
System.out.println("counter=" +i); }
System.out.println("i is " +i);
Alternative 1:
the 'normal' way
'i' is known inside and outside the loop
System.out.println("Start");
for ( int i=4; i>=1; i-- ) {
System.out.println("counter=" +i); }
System.out.println("i is" +i);
Alternative 2:
initialisation inside the 'for' statement
'i' is known only inside the loop

43. Check your knowledge now:
What is wrong here ?
float a;
System.out.println("This loop will run 5 times");
for ( int k=1 k<5 ++k ); {
k=k-1;
if ( k != 0 ) {
a = 15 / k;
System.out.println( a ); // what's the output ? }
System.out.println( k ); // what's the output ?
'for' - loop runs 4 times only
both ';' inside ( ) missing, but misplaced outside ( )
for ( int k=0; k<5; k ++) {
causes endless loop, delete.
correct - jumps division by 0
mixing float and int in calc.
a = 15.0f / (float) k;

44. Summary 1 Today we have ... revised 'boolean expressions' statements
introduced 'while' loops
introduced 'do' - 'while' loops
(looked at 'break', 'continue' and 'exit(0)')

45. Summary 2 Today we also have ... introduced 'for' loops
structure: for, ( ), three statements ';' separated, { }
counting forwards / backwards / in steps
loops inside loops: nesting
introduced 'in-loop' loop initialisation: for( int i=...
revised 'break'

46. END OF LESSON 3