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Tuesday, February 7, 2012

Syntax and Semantics of a Programming Language

Syntax of a Programming Language

The  Syntax  of  a  Programming  Language  is  the  set  of  rules  that  defined  in  that programming language in order to express how the combinations of symbols are formed.
Syntax is differed into two main categories.
  • Abstract Syntax  -->     Provide definition for the constructs
  • Concrete Syntax  -->       Provide definitions of the form of the constructs
E.g. In Java while loop is defined as
//Statements goes  here

Semantics of a Programming Language

Semantics reveals  the  meaning  of a  syntacticallcorrect  programming  language  in expressions, statements, program units. Semantics of a programming language also can be divided into two different categories.

Static Semantics

For compiled languages static semantics are usually tested at the compile time.

E.g. checking that every identifier is declared before it is used (in languages that require such declarations)

Dynamic Semantics

As the above classification there are three types of Dynamic Semantics. Dynamic Semantics are usually tested  while at the runtime. Dynamic Semantics of a Programming language defines  how and  when the various  constructs of a  language should  produce a program behavior.

e.g In java

Meaning of this statement is derived by executing this program on a computer. (This is what we called operational semantics also in the same category of Dynamic semantics)

Ambiguity of a Programming Language

If a statement constructed by using a particular grammar has more than one parse tree; it is said to be that grammar is ambiguous.

E.g. Simple example

<E> --> <D> 
  <E> --> <E>
  <E> --> <E> + <E> |<E> - <E> |<E> * <E> |<E> / <E>
  <D> --> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
Lets consider statement  1+2*5  for the above grammar

Above grammar is ambiguous since it has two parse trees and the left derivation tree give the answer 11 which is correct and the right derivation tree gives 15 which is incorrect.

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