Programming Languages Chapter 9

Basic Expressions

Literals (constants) and identifiers

Complex expression

Built up recursively from the basic expressions by the application of operators and functions

Parentheses

Group symbols

Operator

Predefined, written in infix form (if binary), with associativity and precendence

Function

Either predefined or user-defined, written in prefix form, operands are viewed as arguments

New Design

Allow programmers to overload built-in operators. Define new infix operators with arbitrary associativity and precedence

Applicative Order Evaluation/Strict Evaluation

The most common rule in programming languages. Correspond to a bottom-up reduction of the syntax tree to a value.

Operands

Applicative Order Evaluation evaluates ____ first and then apply them to operators

Arguments

Applicative Order Evaluation evaluates ____ first and then apply them to functions

Natural Order

Follow the predefined precedence of operators

Left to right

For operators of the same precedence

Infix Notation

Correspond to inorder traversal of the syntax tree

Unary Operator - Infix Notation

Write the operator first followed by the operand

Binary Operator - Infix Notation

Write the 1st operand followed by the operator and then the 2nd operand

Prefix Notation

Correspond to preorder traversal of the syntax tree

unary operator - Prefix Notation

Write the operator first followed by the operand

binary operator - Prefix Notation

Write the operator followed by the 1st operand and then the 2nd operand

Postfix Notatiion

Correspond to postorder traversal of the syntax tree

Unary Operator - Postfix Notion

Write the operand first followed by the operator

Binary Operator - Postfix Notion

Write the 1st operand followed by the 2nd operand and then the operator

GoTo Controversy

Provide a very flexible control. May result in spaghetti code

Modern Syle

Group code into logical blocks. Avoid explicit jumps. Cannot jump into the middle of block or function

Conditional Branch, loop, and function call/return

Modern style avoids explicit jumps except

Boolean test or logical test

Make __ before entering a sequence of statements

EBNF

if-statemnt → if(expression) statement [else statement]

Statement

Can be a single statement, a null statement, or a sequence of statements surrounded by braces

Dangling Else

The ambiguity between the association of an if and an else

Most Closely Nested Rule

The else is to be associated with the closest prior if that doesn’t have an else part

bracketing keyword

Ada dangling-else problems

Loops

Used to perform repetitive operations

Exception

An unusual event. Usually unpredictable. Detectable by software or hardware, required special processing

Variable or class

C++ an exceptional event is represented by

An Exception Handler

A section of program codes executed when a particular exception occurs

Catches

One part of the program _____ and processes the exception that another part of the program throws

Throw (raise)

To signal the fact that an exception has occurred

Exception Handler Selection

Examines error types in exception handlers

North-South

Exception Handler Selection Order

catch-all

Ellipse parameter (…) is a wild card

User-defined classes (or structures)

Use ___ as exception types

Nonlocal Exception Handlers

It is more common to throw an exception inside a function called by a try-block