Unit 3 principles of programming language

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1. By-Garima Jain 2.  Sequence control with expressions Conditional Statements, Loops Exception Handling Subprogram definition and activation Simple and…
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  • 1. By-Garima Jain
  • 2.  Sequence control with expressions Conditional Statements, Loops Exception Handling Subprogram definition and activation Simple and Recursive Subprogram Subprogram Environment
  • 3. Control of the order of execution of the operationsboth primitive and user defined.Implicit : determined by the order of the statementsin the source program or by the built-in executionmodelExplicit : the programmer uses statements to changethe order of execution (e.g. uses If statement)
  • 4. Expressions: How data are manipulated usingprecedence rules and parentheses.Statements: conditional and iteration statements changethe sequential execution. Declarative programming: an executionmodel that does not depend on the order of thestatements in the source program.Subprograms: transfer control from one program toanother.
  • 5. What is the sequence of performing the operations?How is the sequence defined, and how is it represented?Functional composition : Basic sequence-controlmechanism:Given an operation with its operands, the operands maybe:· Constants· Data objects· Other operations
  • 6. Example 1: 3 * (var1 + 5)operation - multiplication, operator: *, arity - 2 operand 1: constant (3) operand 2: operation addition operand1: data object (var1) operand 2: constant (5)
  • 7. Example 2: 3* var1 +5Question: is the example equivalent to the above one?Example 3: 3 + var1 +5Question: is this equivalent to (3 + var1) + 5, or to 3 + (var1 + 5) ?
  • 8. Precedence concerns the order of applyingoperationsAssociativity deals with the order of operations ofsame precedence. Precedence and associativity are defined when thelanguage is defined - within the semantic rules forexpressions.
  • 9. Linear representation of the expression tree: Prefix notation· Postfix notation· Infix notationPrefix and postfix notations are parentheses-free.
  • 10.  Machine code sequence Tree structures - software simulation Prefix or postfix form - requires stack, executed by aninterpreter.
  • 11. Eager evaluation - evaluate all operands beforeapplying operators.Lazy evaluation
  • 12. Side effects - some operations may change operands ofother operations.Error conditions - may depend on the evaluationstrategy (eager or lazy evaluation)Boolean expressions - results may differ depending onthe evaluation strategy.
  • 13. if expression then statement1 elsestatement2 if expression then statement1 a choice among many alternatives nested if statements case statementsImplementation:  jump and branch machineinstructions, jump table implementation for casestatements
  • 14. Simple  repetition (for loop) Specifies a count of the number of times to execute a loop: perform statement K times; for loop -Examples: for I=1 to 10 do statement; for(I=0;I<10; I++) statement;
  • 15. while expression do statement;Evaluate expression and if true execute statement, thenrepeat process. repeat statement until expression; Execute statement and then evaluate expression.Repeat if expression is not true.C++ for loop functionally is equivalent to repetitionwhile condition holds
  • 16.  Multiple exit loops Exceptional conditions Do-while-do structureSolutions vary with languages, e.g. in C++ - breakstatement, assert for exceptions.
  • 17. Exception Handlers are subprograms that are not invoked by explicit callsSpecial situations, called exceptions: Error conditions Unpredictable conditions Tracing and monitoring
  • 18. Exception handlers typically contain only: • A set of declarations of local variables • A sequence of executable statementsException Handlers can be - predefined in the language - programmer defined
  • 19. Languages provide methods for raising (throwing) and testing for exceptions.  try { statement1; statement2; …  if badCondition throw ExceptionName; }  catch ExceptionName{ ……….// do something for exception…….}
  • 20. Operating system exceptions - raised directlyby hardware interrupts.Programmer defined -the translator inserts code to handle theexceptions.
  • 21. Subprogram Control :interaction among subprogramshow subprograms pass data among themselves
  • 22. Simple subprogram call return Copy rule view of subprograms: the effect of a call statement is the same as if the subprogram were copied and inserted into the main program.
  • 23. • Subprograms cannot be recursive• Explicit call statements are required• Subprograms must execute completely at each call• Immediate transfer of control at point of call• Single execution sequence
  • 24. CALL RETURN
  • 25. Execution of subprograms Subprogram definition. Subprogram activation.
  • 26. The definition is translated into a template, usedto create an activation each time a subprogram iscalled.
  • 27. a code segment (the invariant part) - executable code and constants,an activation record (the dynamic part) - local data, parameters. created a new each time the subprogram is called, destroyed when the subprogram returns.
  • 28. • Current-instruction pointer – CIPaddress of the next statement to be executed• Current-environment pointer – CEPpointer to the activation record.
  • 29.  An activation record is created Current CIP and CEP are saved in the created activation record as return point CEP is assigned the address of the activation record. CIP gets the address of the first instruction in the code segment The execution continues from the address in CIP
  • 30.  The old values of CIP and CEP are retrieved.  The execution continues from the address in CIPRestrictions of the model: at most one activation of any subprogram
  • 31. Allocate storage for a single activation record staticallyas an extension of the code segment.Used in FORTRAN and COBOL.The activation record is not destroyed - only reinitializedfor each subprogram execution.Hardware support - CIP is the program counter,CEP is not used, simple jump executed on return.
  • 32. The simplest run-time storage management techniquecall statements : push CIP and CEPreturn statements : pop CIP and CEP off of the stack.Used in most C implementationsLISP: uses the stack as an environment.
  • 33. Specification Syntactically - no difference Semantically - multiple activations of the same subprogram exist simultaneously at some point in the execution.E.G. the first recursive call creates a secondactivation within the lifetime of the first activation.
  • 34. Stack-based -CIP and CEP are stored in stack, forming adynamic chain of links.A new activation record is created for each calland destroyed on return.The lifetimes of the activation records cannotoverlap - they are nested.
  • 35. Data control features determine the accessibility of data atdifferent points during program execution.Central problem:the meaning of variable names, i.e. the correspondencebetween names and memory locations.
  • 36. Two ways to make a data object available as an operandfor an operationDirect transmissionReferencing through a named data object
  • 37. A data object computed at one point as the result ofan operation may be directly transmitted to anotheroperation as an operandExample: x = y + 2*z;The result of multiplication is transmitted directly asan operand of the addition operation
  • 38. A data object may be given a name when it iscreated, the name may then be used to designate itas an operand of an operation.
  • 39. VariablesFormal parametersSubprogramsDefined typesDefined constantsLabelsException namesPrimitive operationsLiteral constants
  • 40. Association: binding identifiers to particular dataobjects and subprogramsReferencing environment: the set of identifierassociations for a given subprogram.Referencing operations during program execution:determine the particular data object or subprogramassociated with an identifier
  • 41. Subprogram Environment The set of associations created on entry to a subprogram formal parameters, local variables, and subprograms defined only within that subprogram.Non-local referencing environment The set of associations for identifiers • used within a subprogram • not created on entry to itGlobal referencing environment:associations created at the start of execution of the main program, availableto be used in a subprogram.Predefined referencing environments:predefined associations in the language definition.
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