The dangling-else ambiguity

Previously discussed

• The Java compiler (translator) consider white space characters (i.e., SPACE, TAB and New line) as insignificant (See: http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/03/intro-java.html)

• All that the Java compiler cares about is syntactical correctness

Ambiguous syntax

• Ambiguous syntax

• A language allows ambiguous syntax is the same sentence can be read in more than one way

Ambiguous syntax (cont.)

• Example: English (in fact, all human languages)

can mean:

• An American History professor

• a History professor who is an American citizen     (i.e.: American History professor)

• a professor of American History. (i.e.: American History professor)

Ambiguous syntax (cont.)

• How humans deal with ambiguity:

• Humans can often use additional information to resolve the ambiguity

(E.g., we have additional information about the specific professor)

• If we cannot resolve the ambiguity, we (humans) can ask further questions to resolve it.

Ambiguity in programming languages ?

• Difference between natural language and programming language:

• A natural language evolves through usage

It's evolution is not controlled

---------------------------------------------------------------

• A programming language is designed by one or a group of humans

It's design is completely controlled

Ambiguity in programming languages ? (cont.)

• Commonly held goal in the design of a programming language:

• A programming language should have an unambiguous syntax

• In other words:

• There is exactly one way to interpret each program statement syntactically

Ambiguity in programming languages ? (cont.)

• Historical facts about programming languages:

• Older programming languages does have ambiguous syntax

These languages include:

• Algol 60           

• Pascal

• C

Ambiguity in programming languages ? (cont.)

• Modern programming languages are unambiguous

Examples:

• Algol 68         

• Ada

• Modula

Ambiguity in programming languages ? (cont.)

• However:

• Because Java (and C++) are derived from the older programming language C (and C has an ambiguous syntax):

(This is very unfortunate for a modern language like Java and C++, but the designers want to keep C's syntax extremely badly)

• Java has inherited the ambiguous syntax in C...

The dangling else ambiguity

• The dangling else ambiguity was first discovered in the programming language Algol 60

• I will use a concrete example to illustrate this famous ambiguity

The dangling else ambiguity (cont.)

• Programming problem: shipping cost

• Country code for US = 1

Cost to ship to a package to a destination in the US = $5.00, except for Hawaii which cost $10.00

The state code for Hawaii = 50

• The shipping cost for a destination outside the US is $20.00

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)

The dangling else ambiguity (cont.)

• Algorithm:

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; // Hawaii else cost = 20.0; // Outside US

The dangling else ambiguity (cont.)

(The algorithm is unnecessarily confusing because I want to show the ambiguous syntax....)

The dangling else ambiguity (cont.)

• There are 2 different yet syntactically correct ways to read the if-statements:

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

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

The dangling else ambiguity (cont.)

• (This is the only ambiguous syntax in the Java programming language which it had inherited from C)

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

The dangling else ambiguity (cont.)

• Example:

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

The dangling else ambiguity (cont.)

cost = 5.0; if ( country_code == 1 ) if ( state_code == 50 ) cost = 10.0; // Hawaii else cost = 20.0; // Outside US System.out.println("Shipping cost = " + cost); } }

The dangling else ambiguity (cont.)

• Sample execution:

Enter country code: 1 (code for US) Enter country code: 40 (not Hawaii) Shipping cost = 20.0 (should be $5 !)

The dangling else ambiguity (cont.)

• The reason is that the Java program is executed as follows:

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