The document discusses the 'dangling else' ambiguity in programming, specifically in Java, illustrating how it can lead to multiple interpretations of conditional statements. It provides an example involving shipping costs based on country and state codes, detailing the algorithm and the execution of a Java program demonstrating the issue. The document emphasizes how Java resolves this ambiguity by associating the 'else' keyword with the nearest 'if' statement.

1. if then else
How to resolvle ambiguity

2. dangling-else ambiguity

3. The dangling else ambiguity (cont.)
• Programming problem: shipping cost
• Country code for INDIA = 91
Cost to ship to a package to a destination in the INDIA = ₹5.00,
except for Andaman & Nicobar and Lakshadweep islands which
cost ₹50.00
The state code for Andaman & Nicobar and Lakshadweep = 33
• The shipping cost for a destination outside the INDIA is ₹200.00

4. The dangling else ambiguity (cont.)
• Write a Java program than reads in:
and prints the shipping cost
• A country code (integer)
• A state code (integer)

5. The dangling else ambiguity (cont.)
• Algorithm:

6. The dangling else ambiguity (cont.)
• Note:
• There is no ambiguity in the algorithm given as a
structure diagram
• The ambiguity will be introduced when we write
the algorithm in the Java programming language !!!
cost = 5.0;
if ( country_code == 1 )
if ( state_code == 50 )
cost = 10.0; // Andaman & Nicobar and
Lakshadweep
else
cost = 20.0; // Outside INDIA

7. The dangling else ambiguity (cont.)
(The algorithm is unnecessarily confusing because I want
to show the ambiguous syntax....)

8. The dangling else ambiguity (cont.)
• There are 2 different yet syntactically correct ways to read
the if-statements:

9. The dangling else ambiguity (cont.)
• Explanation:
• The first way is to associate the keyword else with the
first keyword if
See the corresponding structure diagram
• The first if is an if-else-statement
• The second if is an if-statement that comprises
the then-part of the (first) if-else-statement

10. The dangling else ambiguity (cont.)
• The second way is to associate the keyword else with the
second keyword if
See the corresponding structure diagram
• The first if is an if-statement
• The second if is an if-else-statement that
comprises the then-part of the (first) if-
statement

11. The dangling else ambiguity (cont.)
• (This is the only ambiguous syntax in the Java
programming language which it had inherited from C)

12. The dangling else ambiguity (cont.)
• Resolving the dangling-else ambiguity:
• Java (and C, C++) imposes the following rule:
• The keyword else is associated to the nearest
keyword if that makes a syntactically correct
statement

13. The dangling else ambiguity (cont.)
• Example:

14. The dangling else ambiguity (cont.)
• Java program:
import java.util.Scanner;
public class DanglingElse01
{
public static void main(String[] args)
{
int country_code, state_code;
double cost;
Scanner in = new Scanner(System.in); // Construct Scanner object
System.out.print("Enter country code: ");
country_code = in.nextInt(); // Read in integer
System.out.print("Enter country code: ");
state_code = in.nextInt(); // Read in integer

15. The dangling else ambiguity (cont.)
cost = 5.0;
if ( country_code == 1 )
if ( state_code == 50 )
cost = 10.0; // Andaman & Nicobar and
Lakshadweep
else
cost = 20.0; // Outside INDIA
System.out.println("Shipping cost = " + cost);
}
}

16. The dangling else ambiguity (cont.)
• Sample execution:
Enter country code: 1 (code for INDIA)
Enter country code: 40 (not Andaman &
Nicobar and Lakshadweep)
Shipping cost = 20.0 (should be ₹5 !)

17. The dangling else ambiguity (cont.)
• The reason is that the Java program is executed as follows:

18. The dangling else ambiguity (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/06/Progs/
DanglingElse01.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac DanglingElse01.java
• To run: java DanglingElse01