Semantics

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Semantics
Ishmael Surely all this is not without
meaning. Herman Melville, Moby Dick
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Outline

• Motivation
• Expression Semantics
• Program State
• Assignment Semantics
• Control Flow Semantics
• I/O Semantics
• Exceptions Handling Semantics

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Assignment Semantics

• Abstract Syntax
• Multiple assignment
• Assignment statement vs. expression
• Copy vs. reference semantics

• Abstract Syntax
• Assignment Variable target Expression source

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Multiple Assignment

• Example
• a b c 0
• Sets all 3 variables to zero.

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Assignment Statement vs. Expression

• In most languages, assignment is a statement
  cannot appear in an expression.
• In C-like languages, assignment is an expression.
• Examples
• if (a 0) ... // an error?
• while (p q) // strcpy
• while (ch getc(fp)) // read until EOF
• while (p p-gtnext) // traverse list

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Copy vs. Reference Semantics

• Copy a b
• a and b have same value.
• Changes to either have no effect on other.
• Used in imperative languages.
• Reference
• a and b point to the same object.
• A change in object state affects both
• Used by many object-oriented languages.

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Control Flow Semantics

• Turing Completeness A programming language is
  Turing complete if its programs are capable of
  computing any computable function.
• A language is Turing complete if it has
• Assignment statement
• Statement sequencing
• Conditional statement
• Looping statement

Control structures
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Sequence

• s1 s2
• Semantics (in the absence of a branch)
• First execute s1
• Then execute s2
• Output state of s1 is the input state of s2
• Branching statements
• return
• break
• continue
• goto

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Conditional

• IfStatement ? if ( Expresion ) Statement else
  Statement
• Example

if (a gt b) z a else z b
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Conditional

• If the test expression is true,
• then the output state of the conditional is
  the output state of the then branch,
• else
• the output state of the conditional is the
  output state of the else branch.

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Loops

• WhileStatement ? while ( Expression ) Statement
• The expression is evaluated.
• If it is true,
• first the statement is executed,
• and then the loop is executed again.
• Otherwise the loop terminates.

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GoTo

• Widely used in early programming languages
• Controversy stated with ACM letter by Dijkstra
• Started the push for structured programming
• Structure of the program gives a clear guide to
  what it does

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GoTo

• Example from Fortran
• Sums non-negative numbers in an array

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GoTo

• Most modern languages dont have goto
• Reduced reliance on flowchart as a program design
  tool
• Increased focus on structured programming
  techniques
• Many languages do have restricted form of goto
• return, break, continue

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Input/Output Semantics

• Relatively straight forward
• Reading Assignment

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Exceptions

• Unexpected or unusual condition that arises
  during the program execution
• Can not be resolved in the context of the
  operation
• Moves error checking out of the normal flow of
  the program
• Example
• Division by zero
• Causes a hardware interrupt

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Error Handling Techniques

• Ostrich algorithm
• Ignore the problem
• OK for small personal programs not commercial
  code
• Use assert
• Program terminates if the assertion is false
• Abort the program
• Graceful exit Appearance that program worked OK
• Not OK for mission critical applications

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Exceptions

• Throw
• Signal that an exception has occurred
• Catch
• Control has been passed to an exception handler
• Example (division by zero)
• Hardware interrupt routine
• Prints a message and halts the program

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Exceptions

• Occurs at many levels of abstraction
• Hardware Level
• Division by zero
• Floating Point overflow/underflow
• Programming Language Level
• Array index out of bounds
• Read values of wrong types
• Access to a null pointer
• Applications Level
• Pop an empty stack

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Exception Handling 

• Mechanism that allows a program to capture and
  handle the exceptions thrown in your program.
• If exception handling does not exist, the program
  will quit when an exception occurs.

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Exception HandlingWhy?

• Simplify programmers task of developing more
  robust applications
• Robustness
• Recovering from errors in a reasonable or
  graceful way
• Production quality software systems must to be
  robust
• Part of engineering systems
• Need to be fault tolerant

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Exception HandlingWhy?

• Catch errors before they occur
• Can deal with synchronous errors (e.g. division
  by zero)
• Can not deal with asynchnonous errors (disk I/O
  falut)
• Useful when program can not recover must
  gracefully terminate
• Improve fault-tolerance
• Write error processing code
• Specify the types of exceptions to be caught

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Exception HandlingWhy?

• Should not be used for program control
• Not optimized
• May have a negative impact on the performance

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Exception Handling

• Early languages did not provide for exception
  handling
• Pioneered in PL/1
• ON condition
• statement
• Statement executed only when condition occurs
• Most languages since Ada have adapted some
  exception handling mechanisms

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Exception Handling

• Modern languages make the exception handler
  lexically bound
• PL/1 had dynamic binding of the handlers
• Exceptions are propagated
• If not handled by the current block it is moved
  to a parent block
• Using the dynamic link of the stack

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Exception Handling

• Java uses lexically scoped exception handlers
• try
• int anew int12
• a4
• catch (ArrayIntexOutofBoundsException e)
• System.out.println(exception
  e.getMessage())
• e.printStackTrace()
• Dynamic chain is made available through
  printStackTrace

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Exceptions

• PL/1 exceptions did not have a type
• Ada Type exception
• Java Classes
• Hierarchy of exceptions

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Exception Handling

• Unchecked Exceptions
• Any descendent of Error or RuntimeException
• Serious error programmer can do little to fix it
• Checked Exceptions
• Any other descendent of Throwable/Exception
• Error programmer knows how to deal with it

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Announcement

• No Class Monday (9/18)
• Midterm Exam Wednesday (9/20)
• Chapters 1-7

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Exception HandlingImplementation

• Linked Lists
• Dynamically bound handlers
• Maintained at run time
• Slow must be updated for each block of code
• Compile-time table of blocks and handlers
• Table of two fields
• Starting address of block and address of the
  handler
• Locate the handler using a binary search
• Logarithmic in the number of handlers

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Exception HandlingJava

• Each subroutine has a separate exception handling
  table
• Each stack frame contains a pointer to the table

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Exception Handling Figure 6.1
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Java Exception Type Hierarchy Figure 6.2
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Creating a New Exception Class Figure 6.3
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Missing Argument Exception Figure 6.4
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Invalid Input Exception Figure 6.5
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StackUnderflowException Class Figure 6.6
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Throwing an Exception Figure 6.7
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AssertException Class Figure 6.8
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Assert Class Figure 6.9
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Using Asserts Figure 6.10