The dangling else ambiguity

Ambiguity in programming language constructs

A construct in a programming language is ambiguity, if the construct can be interpreted in more than 1 way
Some programming languages (C, C++, Java) have an if statement that has an ambiguity problem (this problem was discovered around 1960 in the Algol language):

Suppose we want to determine the shipping rate to different countries of the world.

Within the US, we charge $5, but for the State of Hawaii (which is very remote), we charge $10.

Outside the US, we charge $20.

The following is an if-statement that is used to compute the "shippingCharge":

Example: an ambiguious program (can be read in more than one way...)
double shippingCharge; shippingCharge = 5.00; // Charge is at least $5 if ( country is equal to "USA") ) if ( state is equal to "HI" ) shippingCharge = 10.00; // Hawaii is $10 else shippingCharge = 20.00; // Outside USA is $20

See how this program can be read: click here

Resolving the Dangling-Else ambiguity

The more elegant way to resolve this Dangling-Else ambiguity problem is to impose different syntax rules for the IF-statement
One solution (used in the programming language Modula) is to close the IF-statement with a new key-word:

The above IF-statement is written as:

double shippingCharge; shippingCharge = 5.00; // Charge is at least $5 if ( country is equal to "USA") ) if ( state is equal to "HI" ) shippingCharge = 10.00; // Hawaii is $10 fi else shippingCharge = 20.00; // Outside USA is $20 fi

There is ONLY ONE WAY to read this program segment (try it !)

Java, C and C++ decided to resolve the Dangling-Else ambiguity uses the following rule:

an else always associates with the closest if that does NOT cause a syntax error

So, the C/C++ compiler will read this program as:
double shippingCharge; shippingCharge = 5.00; if ( country.equals("USA") ) if ( state.equals("HI") ) shippingCharge = 10.00; else shippingCharge = 20.00;

How to force else to associate with the first if:

So, the proper way to write the program is:
double shippingCharge; shippingCharge = 5.00; if ( country.equals("USA") ) { if ( state.equals("HI") ) shippingCharge = 10.00; // Shipping to Hawaii is $10 } else shippingCharge = 20.00; // Shipping to outside USA is $20

Associating the else with the second if will cause a syntax error !!!