Simple Control Structures
booleans, the if statement, and the while loop
26-Jul-16
What are control structures?
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You can’t do very much if your program consists of just
a list of commands to be done in order
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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
For C and C++ programmers only
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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
boolean
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boolean is one of the eight primitive types
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There are exactly two boolean values, true (“yes”) and
false (“no”)
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booleans are used to make yes/no decisions
All control structures use booleans
boolean, true, and false are all lowercase
booleans are named after George Boole, the founder of
Boolean logic
Declaring boolean variables
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boolean variables are declared like any other kind of
variable:
boolean hungry;
boolean passingGrade;
boolean taskCompleted = false;
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boolean values can be assigned to boolean variables:
taskCompleted = true;
Numeric comparisons
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The following numeric comparisons each give a
boolean result:
x
x
x
x
x
x
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<y
<= y
== y
!= y
>= y
>y
//
//
//
//
//
//
is
is
is
is
is
is
x
x
x
x
x
x
less than y?
less than or equal to y?
equal to y? (do not use =)
unequal to y?
greater than or equal to y?
greater than y?
Reminder: Don’t use == or != for floating-point
numbers
The if statement
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The if statement has the form:
if (boolean-expression) statement
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Examples:
if (passingGrade) System.out.println("Whew!");
if (x > largest) largest = x;
if (citBook.price < 40.00) citBook.purchase();
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The if statement controls one other statement
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Often this isn’t enough; we want to control a group of
statements
Compound statements
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We can use braces to group together several
statements into one “compound” statement:
{ statement; statement; ...; statement; }
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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
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The compound statement is the only kind of
statement that does not end with a semicolon
The if statement again
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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
Flowchart for the if statement
condition?
false
true
statement
The if-else statement
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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 ;
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Either statement (or both) may be a compound
statement
Notice the semicolon after each controlled statement
Example if-else statements
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if (x >= 0) absX = x;
else absX = -x;
if (itemCost <= bankBalance) {
writeCheck(itemCost);
bankBalance = bankBalance - itemCost;
}
else {
callHome();
askForMoreMoney(2 * itemCost);
}
Flowchart for the if-else statement
true
statement-1
condition?
false
statement-2
Aside: the “mod” operator
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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
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Useful rule: x is divisible by y if x % y == 0
If the left operand is negative, the result is negative (or
zero)
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Examples: -20 % 3 = -2, 20 % -3 = 2, -20 % -3 = -2
Nesting if (or if-then) statements
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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;
Operations on booleans
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Assume p and q are booleans
There are four basic operations on booleans:
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Negation (“not”):
!p is true if p is false (and false otherwise)
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Conjunction (“and”):
p && q is true if both p and q are true
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Disjunction (“or”):
p || q is true if either of p and q is true
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Exclusive or (“xor”):
p^q
is true if just one of p and q is true
Simpler tests
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A simpler leap-year test:
if (year % 4 == 0 &&
(year % 100 != 0 || year % 400 == 0))
leapYear = true;
else leapYear = false;
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An even simpler leap-year test:
leapYear = year % 4 == 0 &&
(year % 100 != 0 || year % 400 == 0);
The while loop
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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
Flowchart for the while loop
condition?
false
true
statement
Countdown example
seconds = 5;
while (seconds > 0) {
System.out.print(seconds + "...");
seconds = seconds - 1;
}
System.out.println("Blast off!");
Result:
5...4...3...2...1...Blast off!
The End