1. Simple Control Structures
booleans, the if statement, and the while loop
2-May-19

2. What are control structures?
You can’t do very much if your program consists of just a list of commands to be done in order
The program cannot choose whether or not to perform a command
The program cannot perform the same command more than once
Such programs are extremely limited!
Control structures allow a program to base its behavior on the values of variables

3. For C and C++ programmers only
Statement types are almost identical to those in C and C++
Main difference: true/false conditions must be boolean, not numeric!
Some unusual uses of the comma in for statements are not permitted in Java
There are two new statement types (try and assert) which we won’t talk about today

4. boolean boolean is one of the eight primitive types
booleans are used to make yes/no decisions
All control structures use booleans
There are exactly two boolean values, true (“yes”) and false (“no”)
boolean, true, and false are all lowercase
booleans are named after George Boole, the founder of Boolean logic

5. Declaring boolean variables
boolean variables are declared like any other kind of variable:
boolean hungry;
boolean passingGrade;
boolean taskCompleted = false;
boolean values can be assigned to boolean variables:
taskCompleted = true;

6. Numeric comparisons
The following numeric comparisons each give a boolean result:
x < y // is x less than y?
x <= y // is x less than or equal to y?
x == y // is x equal to y? (do not use =)
x != y // is x unequal to y?
x >= y // is x greater than or equal to y?
x > y // is x greater than y?
Reminder: Don’t use == or != for floating-point numbers

7. The if statement The if statement has the form: Examples:
if (boolean-expression) statement
Examples:
if (passingGrade) System.out.println("Whew!");
if (x > largest) largest = x;
if (citBook.price < 40.00) citBook.purchase();
The if statement controls one other statement
Often this isn’t enough; we want to control a group of statements

8. Compound statements
We can use braces to group together several statements into one “compound” statement:
{ statement; statement; ...; statement; }
Braces can group any number of statements:
{ } // OK--this is an “empty” statement
{ x = 0; } // OK--braces don’t hurt
{ temp = x; x = y; y = temp; } //typical use
The compound statement is the only kind of statement that does not end with a semicolon

9. The if statement again
The if statement controls one other statement, but it can be a compound statement
Example:
if (cost < amountInPocket) { System.out.println("Spending $" + cost); amountInPocket = amountInPocket - cost; }
It’s good style to use braces even if the if statement controls only a single statement:
if (cost > amountInPocket) { System.out.println("You can't afford it!"); }
I personally make an exception to this style rule when the controlled statement fits easily on the same line with the if:
if (x < 0) x = -x; // use absolute value of x

10. Flowchart for the if statement
condition?
statement
true
false

11. The if-else statement
The if-else statement chooses which of two statements to execute
The if-else statement has the form:
if (condition) statement-to-execute-if-true ; else statement-to-execute-if-false ;
Either statement (or both) may be a compound statement
Notice the semicolon after each controlled statement

12. Example if-else statements
if (x >= 0) absX = x; else absX = -x;
if (itemCost <= bankBalance) { writeCheck(itemCost); bankBalance = bankBalance - itemCost; } else { callHome(); askForMoreMoney(2 * itemCost); }

13. Flowchart for the if-else statement
condition?
true
statement-1
statement-2
false

14. Aside: the “mod” operator
The modulo, or “mod,” operator returns the remainder of an integer division
The symbol for this operation is %
Examples:
57 % 10 gives 7
20 % 6 gives 2
Useful rule: x is divisible by y if x % y == 0
If the left operand is negative, the result is negative (or zero)
Examples: -20 % 3 = -2, 20 % -3 = 2, -20 % -3 = -2

15. Nesting if (or if-then) statements
A year is a leap year if it is divisible by 4 but not by 100, unless it is also divisible by 400
if (year % 4 == 0) { if (year % 100 == 0) { if (year % 400 == 0) leapYear = true; else leapYear = false; } else leapYear = true; } else leapYear = false;

16. Operations on booleans
Assume p and q are booleans
There are four basic operations on booleans:
Negation (“not”): !p is true if p is false (and false otherwise)
Conjunction (“and”): p && q is true if both p and q are true
Disjunction (“or”): p || q is true if either of p and q is true
Exclusive or (“xor”): p ^ q is true if just one of p and q is true

17. Simpler tests
A simpler leap-year test:
if (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) leapYear = true; else leapYear = false;
An even simpler leap-year test:
leapYear = year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);

18. The while loop
This is the form of the while loop: while (condition) statement ;
If the condition is true, the statement is executed, then the whole thing is done again
The statement is executed repeatedly until the condition becomes false
If the condition starts out false, the statement is never executed at all

19. Flowchart for the while loop
condition?
statement
true
false

20. Countdown example
seconds = 5; while (seconds > 0) { System.out.print(seconds + "..."); seconds = seconds - 1; } System.out.println("Blast off!");
Result: Blast off!

21. The End
“640K ought to be enough for anybody.”
--Bill Gates, 1981