The dangling-else ambiguity

Previously discussed
- The Java compiler (translator) consider white space characters (i.e., SPACE, TAB and New line) as insignificant (See: click here )

Ambiguous syntax

Ambiguous syntax

Example: English (in fact, all human languages)

An American History professor

can mean:

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

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

Commonly held goal in the design of a programming language:

A programming language should have an unambiguous syntax
In other words:

Historical facts about programming languages:

Older programming languages does have ambiguous syntax
These languages include:
Modern programming languages are unambiguous
Examples:

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)

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

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
Write a Java program than reads in:
and prints the shipping cost

Algorithm:

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

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

Explanation:

The first way is to associate the keyword else with the first keyword if
See the corresponding structure diagram
The second way is to associate the keyword else with the second keyword if
See the corresponding structure diagram

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

Note:

Syntactically speaking, both interpretations are equally correct
For our algorithm to be correct, we need the first interpretation
However, it is conceivable that for another algorithm to be correct, the second interpretation is needed.
It is not possible to interpret the same syntactical construct differently in a computer language (because a computer is a dumb machine)

Resolving the dangling-else ambiguity syntactically:

Java (and C, C++) imposes the following syntax rule:

Result:

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 state code: "); state_code = in.nextInt(); // Read in integer 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); } }

Sample execution:

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

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

Example Program: (Demo above code)
- Prog file: click here
How to run the program:

Writing unambiguous computer programs

It is very easy to avoid the dangling-else ambiguity:

Example:

if ( country_code == 1 ) { if ( state_code == 50 ) { cost = 10.0; // Hawaii } } else { cost = 20.0; // Outside US }

There is no ambiguity in this program because:

It is not syntactically possible to associate the keyword else with the second keyword if.

If we try, we will get a syntax error:

if ( country_code == 1 ) { if ( state_code == 50 ) { cost = 10.0; // Hawaii } } <--- This extraneous } will cause a syntax error else { cost = 20.0; // Outside US }

Example Program: (fixing the dangling-else ambiguity)
- Prog file: click here
How to run the program:
Sample execution: