Selections
EECS1021:
Object Oriented Programming: from Sensors to Actuators Winter 2019
CHEN-WEI WANG
Learning Outcomes
● The Boolean Data Type
● if Statement
● Compound vs. Primitive Statement ● Common Errors and Pitfalls
● Logical Operations
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Motivating Examples (1.1)
1 2 3 4 5 6 7 8 9
10 11 12
import java.util.Scanner; public class ComputeArea {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
final double PI = 3.14;
System.out.println(“Enter the radius of a circle:”); double radiusFromUser = input.nextDouble();
double area = radiusFromUser * radiusFromUser * PI; System.out.print(“Circle with radius ” + radiusFromUser); System.out.println(” has an area of ” + area);
} }
● When the above Java class is run as a Java Application, Line 4 is executed first, followed by executing Line 5, . . . , and ended with executing Line 10.
● In Line 7, the radius value comes from the user. Any problems? 3 of 57
Motivating Examples (1.2)
● If the user enters a positive radius value as expected:
Enter the radius of a circle:
3
Circle with radius 3.0 has an area of 28.26
● However, if the user enters a negative radius value:
Enter the radius of a circle:
-3
Circle with radius -3.0 has an area of 28.26
In this case, the area should not have been calculated! ● We need a mechanism to take selective actions :
Act differently in response to valid and invalid input values. 4 of 57
Motivating Examples (2.1)
Problem: Take an integer value from the user, then output a
message indicating if the number is negative, zero, or positive. ● Here is an example run of the program:
Enter a number:
5
You just entered a positive number.
● Here is another example run of the program:
Enter a number:
-5
You just entered a negative number.
● Your solution program must accommodate all possibilities! 5 of 57
Motivating Examples (2.2)
● So far, you only learned about writing programs that are executed line by line, top to bottom.
● In general, we need a mechanism to allow the program to: ○ Check a list of conditions; and
○ Branch its execution accordingly.
● e.g., To solve the above problem, we have 3 possible branches:
1.
2.
If the user input is negative, then we execute the first branch that prints You just entered a negative number.
If the user input is zero, then we execute the second branch that prints You just entered zero.
3. If the user input is positive, then we execute the third branch that
prints You just entered a positive number. 6 of 57
The boolean Data Type
● A (data) type denotes a set of related runtime values.
● We need a data type whose values suggest either a condition
holds, or it does not hold, so that we can take selective actions.
● The Java
● All
Math Symbol
type consists of 2 literal values: true, false have the boolean type.
boolean
relational expressions
Java Operator
Note. You may do the following rewritings:
○ x <= y x > y x != y
○ !(x > y) !(x <= y) !(x == y) 7 of 57
Example (r is 5)
r <= 5 true r >= 5 true r == 5 true
< r<5 false > r>5 false false
x == y !(x != y)
≤
≥
=
<
>
≠ != r != 5
Result
<= >= ==
Syntax of if Statement
if ( BooleanExpression1 ) { Statement1.1; Statement2.1;
}
else if ( BooleanExpression2 ) { /* Optional */
Statement2.1; Statement2.2; }
… /* as many else-if branches as you like */ else if ( BooleanExpressionn ) { /* Optional */
Statementn.1; Statementn.2; }
else { /* Optional */
/* when all previous branching conditions are false */ Statement1; Statement2;
}
/* Mandatory */
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Semantics of if Statement (1.1)
start of if-statement
True
True
BooleanExpression
False
BooleanExpression2
False
…
False
BooleanExpressionn
False
Statementà
Statement2à
Statementà2
Statement2à2
True
S tatement
Statementnà
Statementnà2
S tatement2
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end of if-statement
Semantics of if Statement (1.2)
Consider a single if statement as consisting of:
● An if branch
● A (possibly empty) list of else if branches
● An optional else branch
At runtime :
● Branches of the if statement are executed from top to bottom.
● We only evaluate the condition of a branch if those conditions of its preceding branches evaluate to false.
● The first branch whose condition evaluates to true gets its body (i.e., code wrapped within { and }) executed.
○ After this execution, all later branches are ignored. 10 of 57
Semantics of if Statement (2.1.1)
first satisfying branch executed; later branches ignored.
Only
int i = -4;
if(i < 0) {
System.out.println("i is negative");
}
else if(i < 10) {
System.out.println("i is less than than 10"); }
else if(i == 10) {
System.out.println("i is equal to 10");
}
else {
System.out.println("i is greater than 10"); }
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i is negative
Semantics of if Statement (2.1.2)
first satisfying branch executed; later branches ignored.
Only
int i = 5;
if(i < 0) {
System.out.println("i is negative");
}
else if(i < 10) {
System.out.println("i is less than than 10"); }
else if(i == 10) {
System.out.println("i is equal to 10");
}
else {
System.out.println("i is greater than 10"); }
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i is less than 10
Semantics of if Statement (2.2)
No satisfying branches, and no else part, then nothing is
executed.
if(i < 0) {
System.out.println("i is negative");
}
else if(i < 10) {
System.out.println("i is less than than 10"); }
else if(i == 10) {
System.out.println("i is equal to 10");
}
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int i = 12;
Semantics of if Statement (2.3)
No satisfying branches, then else part, if there, is executed.
if(i < 0) {
System.out.println("i is negative");
}
else if(i < 10) {
System.out.println("i is less than than 10"); }
else if(i == 10) {
System.out.println("i is equal to 10");
}
else {
System.out.println("i is greater than 10"); }
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int i = 12;
i is greater than 10
Two-Way if Statement without else Part
if (radius >= 0) {
area = radius * radius * PI;
System.out.println(“Area for the circle of is ” + area);
}
An if statement with the missing else part is equivalent to an if statement with an else part that does nothing.
if (radius >= 0) {
area = radius * radius * PI;
System.out.println(“Area for the circle of is ” + area);
}
else {
/* Do nothing. */
}
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Multi-Way if Statement with else Part
if (score >= 80.0) { System.out.println(“A”); }
else { /* score < 80.0 */ if (score >= 70.0) {
System.out.println(“B”); } else { /* score < 70.0 */
if (score >= 60.0) { System.out.println(“C”); }
else { /* score < 60.0 */ System.out.println("F");
} }
}
Exercise: Draw the corresponding flow charts for both programs. Convince yourself that they are equivalent.
if (score >= 80.0) { System.out.println(“A”);
}
else if (score >= 70.0) {
System.out.println(“B”); }
else if (score >= 60.0) { System.out.println(“C”);
}
else {
System.out.println(“F”); }
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Multi-Way if Statement without else Part
String lettGrade = “F”; if (score >= 80.0) {
letterGrade = “A”; }
else if (score >= 70.0) { letterGrade = “B”;
}
else if (score >= 60.0) {
letterGrade = “C”; }
In this case, since we already assign an initial, default value “F” to variable letterGrade, so when all the branch conditions evaluate to false, then the default value is kept.
Compare the above example with the example in slide 53.
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Case Study: Error Handling of Input Radius (1)
Problem: Prompt the user for the radius value of a circle. Print an error message if input number is negative; otherwise, print the calculated area.
public class ComputeArea {
public static void main(String[] args) {
System.out.println(“Enter a radius value:”); Scanner input = new Scanner(System.in); double radius = input.nextDouble();
final double PI = 3.14159;
if (radius < 0) { /* condition of invalid inputs */ System.out.println("Error: Negative radius value!");
}
else { /* implicit: !(radius < 0), or radius >= 0 */
double area = radius * radius * PI;
System.out.println(“Area is ” + area); }
} }
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Case Study: Error Handling of Input Radius (2)
The same problem can be solved by checking the condition of valid inputs first.
public class ComputeArea2 {
public static void main(String[] args) {
System.out.println(“Enter a radius value:”); Scanner input = new Scanner(System.in); double radius = input.nextDouble();
final double PI = 3.14159;
if (radius >= 0) { /* condition of valid inputs */ double area = radius * radius * PI; System.out.println(“Area is ” + area);
}
else { /* implicit: !(radius >= 0), or radius < 0 */
System.out.println("Error: Negative radius value!"); }
} }
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One if Stmt vs. Multiple if Stmts (1) Question: Do these two programs behave same at runtime?
if(i >= 3) {System.out.println(“i is >= 3”);}
else if(i <= 8) {System.out.println("i is <= 8");}
if(i >= 3) {System.out.println(“i is >= 3”);} if(i <= 8) {System.out.println("i is <= 8");}
Question: Do these two programs behave same at runtime?
if(i <= 3) {System.out.println("i is <= 3");}
else if(i >= 8) {System.out.println(“i is >= 8”);}
if(i <= 3) {System.out.println("i is <= 3");} if(i >= 8) {System.out.println(“i is >= 8”);}
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One if Stmt vs. Multiple if Stmts (2)
int i = 5;
if(i >= 3) {System.out.println(“i is >= 3”);}
else if(i <= 8) {System.out.println("i is <= 8");}
i is >= 3
int i = 5;
if(i >= 3) {System.out.println(“i is >= 3”);} if(i <= 8) {System.out.println("i is <= 8");}
i is >= 3
i is <= 8
Two versions behave differently because the two conditions i >= 3 and i <= 8 may be satisfied simultaneously.
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One if Stmt vs. Multiple if Stmts (3)
int i = 2;
if(i <= 3) {System.out.println("i is <= 3");}
else if(i >= 8) {System.out.println(“i is >= 8”);}
i is <= 3
int i = 2;
if(i <= 3) {System.out.println("i is <= 3");} if(i >= 8) {System.out.println(“i is >= 8”);}
i is <= 3
Two versions behave the same because the two conditions i <= 3 and i >= 8 cannot be satisfied simultaneously.
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Scope of Variables (1)
When you declare a variable, there is a limited scope where the variable can be used.
● If the variable is declared directly under the main method, then all lines of code (including branches of if statements) may either re-assign a new value to it or use its value.
public static void main(String[] args) { int i = input.nextInt(); System.out.println(“i is ” + i);
if (i > 0) {
i = i * 3; /* both use and re-assignment, why? */ }
else {
i = i * -3; /* both use and re-assignment, why? */
}
System.out.println(“3 * |i| is ” + i); }
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Scope of Variables (2.1)
● If the variable is declared under an if branch, an else if branch, or an else branch, then only lines of code appearing within that branch (i.e., its body) may either re-assign a new value to it or use its value.
public static void main(String[] args) { int i = input.nextInt();
if (i > 0) {
int j = i * 3; /* a new variable j */
if (j > 10) { … } }
else {
int j = i * -3; /* a new variable also called j */ if (j < 10) { ... }
} }
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Scope of Variables (2.2)
● A variable declared under an if branch, an else if branch, or an else branch, cannot be re-assigned or used outside its scope.
public static void main(String[] args) { int i = input.nextInt();
if (i > 0) {
int j = i * 3; /* a new variable j */
if (j > 10) { … } }
else {
int k = i * -3; /* a new variable also called j */ if(j
int j = i * 3; /* a new variable j */
if (j > 10) { … } }
else {
int j = i * -3; /* a new variable also called j */ if (j < 10) { ... }
}
System.out.println("i * j is " + (i * j )); }
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within sub-scopes of its declared scope [ e.g., use of i at L4, L8 ]
○ A variable cannot be referred to outside its declared scope.
○ A variable can be used:
within its declared scope [ e.g., use of i at L11 ]
[e.g., illegal use of j at L11]
Primitive Statement vs. Compound Statement
● A statement is a block of Java code that modifies value(s) of some variable(s).
● An assignment (=) statement is a primitive statement: It only modifies its left-hand-side (LHS) variable.
● An if statement is a compound statement:
Each of its branches may modify more than one variables via
other statements (e.g., assignments, if statements).
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Compound if Statement: Example
1 2 3 4 5 6 7 8 9
10 11 12
int x = input.nextInt(); int y = 0;
if (x >= 0) {
System.out.println(“x is positive”); if (x > 10) { y = x * 2; }
else if (x < 10) { y = x % 2; }
else { y = x * x; }
}
else{ /*x<0*/
System.out.println("x is negative");
if(x < -5) { y = -x; } }
Exercise: Draw a flow chart for the above compound statement. 28 of 57
Logical Operators
● Logical operators are used to create compound Boolean expressions.
○ Similar to arithmetic operators for creating compound number expressions.
○ Logical operators can combine Boolean expressions that are built using the relational operators.
e.g.,1 <= x && x <= 10
e.g.,x < 1 || x > 10
● We consider three logical operators:
Java Operator
! && ||
Description
Meaning
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logical negation not logical conjunction and
logical disjunction or
Logical Negation
● Logical negation is a unary operator (i.e., one operand being a Boolean expression).
● The result is the “negated” value of its operand. Operand op !op
true false false true
double radius = input.nextDouble();
boolean isPositive = radius > 0;
if (!isPositive) {/* not the case that isPositive is true */
System.out.println(“Error: radius value must be positive.”); }
else {
System.out.println(“Area is ” + radius * radius * PI);
}
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Logical Conjunction
● Logical conjunction is a binary operator (i.e., two operands, each being a Boolean expression).
● The conjunction is true only when both operands are true. ● If one of the operands is false, their conjunction is false.
Left Operand op1 true
true
false false
Right Operand op2 true
false
true
false
op1 && op2
true
false false false
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int age = input.nextInt();
boolean isOldEnough = age >= 45;
boolean isNotTooOld = age < 65
if (!isOldENough) { /* young */ }
else if (isOldEnough && isNotTooOld) { /* middle-aged */ } else { /* senior */ }
Logical Disjunction
● Logical disjunction is a binary operator (i.e., two operands, each being a Boolean expression).
● The disjunction is false only when both operands are false. ● If one of the operands is true, their disjunction is true.
Left Operand op1 false
true
false
true
Right Operand op2 false
false
true true
op1 || op2
false
true true true
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int age = input.nextInt();
boolean isSenior = age >= 65;
boolean isChild = age < 18
if (isSenior || isChild) { /* discount */ } else { /* no discount */ }
Logical Laws (1)
● The negation of a strict inequality is a non-strict inequality. Relation Negation Equivalence
>
<=
>=
<
<
>=
● e.g.,
i>j !(i j) i j i>=j !(i j) i j
i
if( ){
/* Action 1 */
}
else{ /* !(i>j) */
/* Action 2 */
}
i>j
○ Action 1 is executed when i > j
○ Action 2 is executed when i <= j. 33 of 57
equivalent to
if( ){ /* Action 2 */
}
else{ /* !(i<=j) */
/* Action 1 */
}
i <= j
Logical Laws (2.1)
Say we have two Boolean expressions B1 and B2:
● What does !(B1 && B2) mean?
It is not the case that both B1 and B2 are true.
● What does !B1 || !B2 mean?
It is either B1 is false, B2 is false, or both are false.
● Both expressions are equivalent! [proved by the truth table]
B1 B2 (B1 B2) B1 B2
true true false false true false true true false true true true false false true true
!
&&
!
||
!
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Logical Laws (2.2)
● When is Action 2 executed? ● When is Action 1 executed?
● When is Action 2 executed? ● When is Action 1 executed?
● When is Action 2 executed? Lesson: Be careful not to write
but always evaluate to false. 35 of 57
i < 0 || i > 10
if(0 <= i && i <= 10) { /* Action 1 */ } else { /* Action 2 */ }
if(i < 0 && false) { /* Action 1 */ } else { /* Action 2 */ }
true
false
(i.e., i >= 0 || true)
false
if(i < 0 && i > 10) { /* Action 1 */ } else { /* Action 2 */ }
true (i.e., i >= 0 || i <= 10) branching conditions that use &&
Logical Laws (3.1)
Say we have two Boolean expressions B1 and B2:
● What does !(B1 || B2) mean?
It is not the case that either B1 is true, B2 is true, or both are true.
● What does !B1 && !B2 mean? Both B1 and B2 are false.
● Both expressions are equivalent! [proved by the truth table] B1 B2 (B1 B2) B1 B2
!
||
!
&&
!
true true true false false true false false
false false false true
false false false true
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Logical Laws (3.2)
● When is Action 2 executed? ● When is Action 1 executed?
● When is Action 2 executed? ● When is Action 1 executed?
● When is Action 2 executed? Lesson: Be careful not to write
but always evaluate to true. 37 of 57
0 <= i && i <= 10
if(i < 0 || i > 10) { /* Action 1 */ } else { /* Action 2 */ }
if(i < 0 || true) { /* Action 1 */ } else { /* Action 2 */ }
true
false (i.e., i >= 0 && false)
true
false (i.e., i >= 10 && i < 10) branching conditions that use ||
if(i < 10 || i >= 10) { /* Action 1 */ } else { /* Action 2 */ }
Operator Precedence
● Operators with higher precedence are evaluated before those with lower precedence.
e.g.,2 + 3 * 5
● For the three logical operators , negation (!) has the highest
precedence, then conjunction (&&), then disjunction (||). e.g., true || true && false means
○ true || (true && false), rather than ○ (true || true) && false
● When unsure, use parentheses to force the precedence. 38 of 57
Operator Associativity
● When operators with the same precedence are grouped together, we evaluate them from left to right.
e.g.,1 + 2 – 3means
((1 + 2) – 3)
e.g., false || true || false means ((false || true) || false)
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Short-Circuit Evaluation (1)
● Both Logical operators && and ∣∣ evaluate from left to right.
● Operator && continues to evaluate only when operands so far
evaluate to true.
if (x != 0 && y / x > 2) { /* do something */
}
else {
/* print error */ }
● Operator ∣∣ continues to evaluate only when operands so far evaluate to false.
if (x == 0 || y / x <= 2) { /* print error */
}
else {
/* do something */ }
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Short-Circuit Evaluation (2)
● Both Logical operators && and ∣∣ evaluate from left to right.
● Short-Circuit Evaluation is not exploited: crash when x == 0
if (y / x > 2 && x != 0) { /* do something */
}
else {
/* print error */ }
● Short-Circuit Evaluation is not exploited: crash when x == 0
if (y / x <= 2 || x == 0) { /* print error */
}
else {
/* do something */ }
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Common Error 1: Independent if Statements with Overlapping Conditions
if (marks >= 80) { if (marks >= 80) {
System.out.println(“A”); }}
if (marks >= 70) { else if (marks >= 70) {
System.out.println(“B”); }}
if (marks >= 60) { else if (marks >= 60) {
System.out.println(“C”); }}
else { else {
System.out.println(“F”); }}
/* Consider marks = 84 */ /* Consider marks = 84 */
● Conditions in a list of if statements are checked independently .
● In a single if statement, only the first satisfying branch is executed. 42 of 57
System.out.println(“A”);
System.out.println(“B”);
System.out.println(“C”);
System.out.println(“F”);
Overlapping Conditions: Exercise (1)
● Does this program always print exactly one line?
if(x < 0) { println("x < 0"); }
if(0 <= x && x < 10) { println("0 <= x < 10"); } if(10 <= x && x < 20) { println("10 <= x < 20"); } if(x >= 20) { println(“x >= 20”); }
● Yes, because the branching conditions for the four if-statements are all non-overlapping.
● That is, any two of these conditions cannot be satisfied simultaneously :
○x<0
○ 0 <= x && x < 10 ○ 10 <= x && x < 20 ○ x >= 20
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Overlapping Conditions: Exercise (2)
● Does this program always print exactly one line?
if(x < 0) { println("x < 0"); }
else if(0 <= x && x < 10) { println("0 <= x < 10"); } else if(10 <= x && x < 20) { println("10 <= x < 20"); } else if(x >= 20) { println(“x >= 20”); }
● Yes, because it’s a single if-statement:
Only the first satisfying branch is executed.
● But, can it be simplified?
Hint: In a single if-statement, a branch is executed only if all
earlier branching conditions fail. 44 of 57
Overlapping Conditions: Exercise (3)
● This simplified version is equivalent:
1 2 3 4
if(x < 0) { println("x < 0"); }
else if(x < 10) { println("0 <= x < 10"); } else if(x < 20) { println("10 <= x < 20"); } else { println("x >= 20″); }
● At runtime, the 2nd condition
when the 1st condition at L1 fails
(i.e., !(x < 0), or equivalently, x >= 0).
● At runtime, the 3rd condition at L3 is checked only when the 2nd condition at L2 fails
(i.e., !(x < 10), or equivalently, x >= 10).
● At runtime, the else (default) branch at L4 is reached only when
the 3rd condition at L3 fails
(i.e., !(x < 20), or equivalently, x >= 20). 45 of 57
x < 10
at L2 is checked only
x < 20
General vs. Specific Boolean Conditions (1)
Two or more conditions overlap if they can evaluate to true simultaneously.
e.g., Say marks is declared as an integer variable:
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○ marks >= 80 and marks >= 70 overlap. [why?] Values 80, 81, 82, . . . make both conditions true
marks >= 80 has fewer satisfying values than marks >= 70 We say marks >= 80 is more specific than marks >= 70 Or, we say marks >= 70 is more general than marks >= 80
marks <= 65 has fewer satisfying values than marks <= 75 We say marks <= 65 is more specific than marks <= 75 Or, we say marks <= 75 is more general than marks <= 65
○ marks <= 65 and marks <= 75 overlap. [why?] Values 65, 64, 63, . . . make both conditions true
General vs. Specific Boolean Conditions (2)
Say we have two overlapping conditions x >= 5 and x >= 0:
○ Whatvaluesmakebothconditionstrue?
○ Which condition is more general?
○ If we have a single if statement, then having this order
is different from having this order
○ Sayx is5,thenwehave
What output from the first program?
What output from the second program? [x >= 0, not specific enough!]
○ The cause of the “ not-specific-enough ” problem of the second
program is that we did not check the more specific condition (x >=
5) before checking the more general condition (x >= 0). 47 of 57
if(x >= 5) { System.out.println(“x >= 5”); }
else if(x >= 0) { System.out.println(“x >= 0”); }
[5,6,7,…] [x >= 0]
if(x >= 0) { System.out.println(“x >= 0”); }
else if(x >= 5) { System.out.println(“x >= 5”); }
[x >= 5]
Common Error 2: if-elseif Statement with Most General Condition First (1)
if (gpa >= 2.5) { graduateWith = “Pass”;
}
else if (gpa >= 3.5) {
graduateWith = “Credit”; }
else if (gpa >= 4) { graduateWith = “Distinction”;
}
else if (gpa >= 4.5) {
graduateWith = “High Distinction” ; }
The above program will:
○ Not award a “High Distinction” to gpa == 4.8. ○ Why?
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Common Error 2: if-elseif Statement with Most General Condition First (2)
● Always “sort” the branching conditions s.t. the more specific conditions are checked before the more general conditions.
if (gpa >= 4.5) {
graduateWith = “High Distinction” ;
}
else if (gpa >= 4) {
graduateWith = “Distinction”; }
else if (gpa >= 3.5) { graduateWith = “Credit”;
}
else if (gpa >= 2.5) {
graduateWith = “Pass”; }
else { graduateWith = “Fail”; }
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Common Error 3: Missing Braces (1)
Confusingly, braces can be omitted if the block contains a statement.
single
final double PI = 3.1415926;
Scanner input = new Scanner(System.in); double radius = input.nextDouble();
if (radius >= 0)
System.out.println(“Area is ” + radius * radius * PI);
In the above code, it is as if we wrote:
final double PI = 3.1415926;
Scanner input = new Scanner(System.in); double radius = input.nextDouble();
if (radius >= 0) {
System.out.println(“Area is ” + radius * radius * PI);
}
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Common Error 3: Missing Braces (2)
Your program will misbehave when a block is supposed to execute multiple statements , but you forget to enclose them within braces.
final double PI = 3.1415926;
Scanner input = new Scanner(System.in); double radius = input.nextDouble(); double area = 0;
if (radius >= 0)
area = radius * radius * PI; System.out.println(“Area is ” + area);
This program will mistakenly print “Area is 0.0” when a negative number is input by the user, why? Fix?
if (radius >= 0) {
area = radius * radius * PI; System.out.println(“Area is ” + area);
}
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Common Error 4: Misplaced Semicolon
Semicolon (;) in Java marks the end of a statement (e.g., assignment, if statement).
if (radius >= 0); {
area = radius * radius * PI; System.out.println(“Area is ” + area);
}
This program will calculate and output the area even when the input radius is negative, why? Fix?
if (radius >= 0) {
area = radius * radius * PI; System.out.println(“Area is ” + area);
}
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Common Error 5:
Variable Not Properly Re-Assigned
1 2 3 4 5 6 7 8 9
String graduateWith = “”; if (gpa >= 4.5) {
graduateWith = “High Distinction” ; } else if (gpa >= 4) {
graduateWith = “Distinction”; } else if (gpa >= 3.5) {
graduateWith = “Credit”; } else if (gpa >= 2.5) {
graduateWith = “Pass”; }
The above program will award “” to gpa == 1.5. Why? Possible Fix 1: Change the initial value in Line 1 to “Fail”. Possible Fix 2: Add an else branch after Line 9:
else { graduateWith = “fail” }
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Compare this example with the example in slide 17.
Common Errors 6: Ambiguous else (1)
if (x >= 0)
if (x > 100) {
System.out.println(“x is larger than 100”); }
else {
System.out.println(“x is negative”);
}
● When x is 20, this program considers it as negative. Why?
∵ else clause matches the most recent unmatched if clause. ∴ The above is as if we wrote:
if (x >= 0)
if (x > 100) {
System.out.println(“x is larger than 100”);
}
else {
System.out.println(“x is negative”); }
}
{
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Common Errors 6: Ambiguous else (2) ● Fix?
Use pairs of curly braces ({}) to force what you really mean to specify!
if (x >= 0)
if (x > 100) {
System.out.println(“x is larger than 100”);
}
else {
System.out.println(“x is negative”);
}
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}
{
Common Pitfall 1: Updating Boolean Variable
boolean isEven;
if (number % 2 == 0) {
isEven = true; }
else {
isEven = false;
}
Correct, but simplifiable : boolean isEven = (number%2 == 0); Similarly, how would you simply the following?
if (isEven == false) { System.out.println(“Odd Number”);
}
else {
System.out.println(“Even Number”); }
Simplify isEven == false to !isEven 56 of 57
Index (1)
Learning Outcomes
Motivating Examples (1.1) Motivating Examples (1.2) Motivating Examples (2.1) Motivating Examples (2.2)
The boolean Data Type
Syntax of if Statement Semantics of if Statement (1.1) Semantics of if Statement (1.2) Semantics of if Statement (2.1.1) Semantics of if Statement (2.1.2) Semantics of if Statement (2.2) Semantics of if Statement (2.3)
Two-Way if Statement without else Part 57 of 57
Index (2)
Multi-Way if Statement with else Part Multi-Way if Statement without else Part Case Study: Error Handing of Input Radius (1) Case Study: Error Handing of Input Radius (2) One if Stmt vs. Multiple if Stmts (1)
One if Stmt vs. Multiple if Stmts (2)
One if Stmt vs. Multiple if Stmts (3)
Scope of Variables (1)
Scope of Variables (2.1)
Scope of Variables (2.2)
Scope of Variables (2.3)
Primitive Statement vs. Compound Statement Compound if Statement: Example
Logical Operators
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Index (3)
Logical Operators: Negation Logical Operators: Conjunction Logical Operators: Disjunction Logical Operators: Laws (1) Logical Operators: Laws (2.1) Logical Operators: Laws (2.2) Logical Operators: Laws (3.1) Logical Operators: Laws (3.2) Operator Precedence
Operator Associativity
Short-Circuit Evaluation (1)
Short-Circuit Evaluation (2)
Common Error 1: Independent if Statements with
Overlapping Conditions
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Index (4)
Overlapping Conditions: Exercise (1) Overlapping Conditions: Exercise (2) Overlapping Conditions: Exercise (3) General vs. Specific Boolean Conditions (1)
General vs. Specific Boolean Conditions (2)
Common Error 2: if-elseif Statement with Most General Condition First (1)
Common Error 2: if-elseif Statement with Most General Condition First (2)
Common Error 3: Missing Braces (1)
Common Error 3: Missing Braces (2)
Common Error 4: Misplaced Semicolon
Common Error 5:
Variable Not Properly Re-Assigned
Common Error 6: Ambiguous else (1) 60 of 57
Index (5)
Common Error 6: Ambiguous else (2)
Common Pitfall 1: Updating Boolean Variable
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