The College of Saint Rose CIS 433 – Programming Languages David Goldschmidt, Ph.D. from Concepts of Programming Languages, 9th edition by Robert W. Sebesta, Addison-Wesley, 2010, ISBN

 Syntax is the expected form or structure of the expressions, statements, and program units of a programming language  Syntax of a Java while statement:  while ( )  Partial syntax of an if statement:  if ( )

 Semantics is the meaning of the expressions, statements, and program units of a given programming language  Semantics of a Java while statement  while ( )  Execute zero or more times as long as evaluates to true

 Together, syntax and semantics define a programming language  Syntax errors are detected and reported by a compiler  Errors related to semantics are defects in program logic that cause incorrect results or program crashes

 Terminology to describe syntax:  A sentence is a string of characters over an alphabet of symbols  A language is a set of sentences  A lexeme is the lowest-level syntactic unit of a language (e.g. +, *, sum, while ) ▪ One step above individual characters  A token is a set of lexemes ▪ e.g. identifier, equal_sign, integer_literal, etc.

 A language recognizer reads an input string and determines whether it belongs to the given language  This is the syntax analysis part of a compiler or interpreter input strings (source code) language recognizer language recognizer accept or reject each input string

 A language generator produces syntactically acceptable strings of a given language  Not practical to generate all valid strings  Instead, inspect generator rules (the grammar) to determine if a sentence is acceptable for a given language language generator language generator valid strings of the language

 In the mid-1950s, linguist Noam Chomsky (born 1928) developed four classes of generative grammars  Context-free grammars (CFGs) are useful for describing programming language syntax  Regular grammars are useful for describing valid tokens of a programming language

 In 1960, John Backus and Peter Naur developed a formal notation for specifying programming language syntax  Backus-Naur Form (BNF) is nearly identical to Chomsky’s context-free grammars  Syntax of an assignment statement in BNF: ▪  = ;

 Syntax of an assignment statement in BNF:  BNF rule or production defining :  = ; abstraction being defined definition of  The definition consists of other abstractions, as well as lexemes and tokens

 begin end  | ;  =  a | b | c | d | e  + | -  | literal-integer-value a vertical bar indicates an OR a token, which is simply a grouping of lexemes  Write a sentence that conforms to this grammar

 A derivation is a repeated application of rules  Start with a start symbol and end with a sentence => begin end => begin = end => begin b = end => begin b = + end => begin b = c + end => begin b = c end  Many possible (often infinite) derivations  begin end  | ;  =  a | b | c | d | e  + | -  | literal-integer-value

 A leftmost derivation is one in which the leftmost abstraction is always the next one expanded  Write both a leftmost and rightmost derivation to obtain this sentence: begin d = 10 - a end  Why is the leftmost derivation important?  begin end  | ;  =  a | b | c | d | e  + | -  | literal-integer-value

 Given this simple grammar:  Which of the following sentences are generated by this grammar? ▪ baaabbccc ▪ abc ▪ bbaabbaabbaabbaac ▪ aabbbbccccccccccccccccccccc   a | a  b | b  c | c

 Write BNF for the following constructs from your favorite programming language:  Assignment statement ▪ Include operators +, -, *, /, %, ++, --  Complete while and if statements  Class header for Java/C++/C#  etc.

 Given this grammar:  Show both leftmost and rightmost derivations for the following sentences: ▪ A = A * ( B + ( C * A ) ) ▪ B = B * ( (D) + C ) ▪ C = A + B + C * D + A  =  A | B | C | D  + | * | ( ) |

 Use BNF to write a grammar for reverse Polish notation  Use as your start symbol  Valid sentences include: ▪ * ▪ * / ▪ * / * 1 – ▪ 9 9 * 8 7 * * 5 5 * * 4 -

 Read and study Chapter 3  Do Exercises at the end of Chapter 3