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Unit 3 principles of programming language
The document discusses concepts related to sequence control and subprograms in programming languages. It covers conditional statements, loops, exception handling, subprogram definition and activation, and subprogram environments. Key points include implicit and explicit sequence control using statements, precedence and associativity rules for expressions, stack-based implementation of subprogram calls, and static versus dynamic scoping of identifiers through referencing environments.
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Unit 3 principles of programming language
- 1.
- 2. Sequence control with expressions Conditional Statements, Loops Exception Handling Subprogram definition and activation Simple and Recursive Subprogram Subprogram Environment
- 3. Control of theorder of execution of the operations both primitive and user defined. Implicit : determined by the order of the statements in the source program or by the built-in execution model Explicit : the programmer uses statements to change the order of execution (e.g. uses If statement)
- 4. Expressions: How dataare manipulated using precedence rules and parentheses. Statements: conditional and iteration statements change the sequential execution. Declarative programming: an execution model that does not depend on the order of the statements in the source program. Subprograms: transfer control from one program to another.
- 5. What is thesequence of performing the operations? How is the sequence defined, and how is it represented? Functional composition : Basic sequence-control mechanism: Given an operation with its operands, the operands may be: · 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 +5 Question: is the example equivalent to the above one? Example 3: 3 + var1 +5 Question: is this equivalent to (3 + var1) + 5, or to 3 + (var1 + 5) ?
- 8. Precedence concerns theorder of applying operations Associativity deals with the order of operations of same precedence. Precedence and associativity are defined when the language is defined - within the semantic rules for expressions.
- 9. Linear representation ofthe expression tree: Prefix notation · Postfix notation · Infix notation Prefix and postfix notations are parentheses-free.
- 10. Machine codesequence Tree structures - software simulation Prefix or postfix form - requires stack, executed by an interpreter.
- 11. Eager evaluation -evaluate all operands before applying operators. Lazy evaluation
- 12. Side effects -some operations may change operands of other operations. Error conditions - may depend on the evaluation strategy (eager or lazy evaluation) Boolean expressions - results may differ depending on the evaluation strategy.
- 13. if expression then statement1 else statement2 if expression then statement1 a choice among many alternatives nested if statements case statements Implementation: jump and branch machine instructions, jump table implementation for case statements
- 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 dostatement; Evaluate expression and if true execute statement, then repeat process. repeat statement until expression; Execute statement and then evaluate expression. Repeat if expression is not true. C++ for loop functionally is equivalent to repetition while condition holds
- 18. Multiple exitloops Exceptional conditions Do-while-do structure Solutions vary with languages, e.g. in C++ - break statement, assert for exceptions.
- 19. Exception Handlers aresubprograms that are not invoked by explicit calls Special situations, called exceptions: Error conditions Unpredictable conditions Tracing and monitoring
- 20. Exception handlers typicallycontain only: • A set of declarations of local variables • A sequence of executable statements Exception Handlers can be - predefined in the language - programmer defined
- 21. Languages provide methodsfor raising (throwing) and testing for exceptions. try { statement1; statement2; … if badCondition throw ExceptionName; } catch ExceptionName { ……….// do something for exception…….}
- 22. Operating system exceptions- raised directly by hardware interrupts. Programmer defined - the translator inserts code to handle the exceptions.
- 23. Subprogram Control : interaction amongsubprograms how subprograms pass data among themselves
- 24. 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.
- 25. • Subprograms cannotbe recursive • Explicit call statements are required • Subprograms must execute completely at each call • Immediate transfer of control at point of call • Single execution sequence
- 26. CALL RETURN
- 27. Execution of subprograms Subprogram definition. Subprogram activation.
- 28. The definition istranslated into a template, used to create an activation each time a subprogram is called.
- 29. 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.
- 30. • Current-instruction pointer– CIP address of the next statement to be executed • Current-environment pointer – CEP pointer to the activation record.
- 32. 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
- 33. The oldvalues of CIP and CEP are retrieved. The execution continues from the address in CIP Restrictions of the model: at most one activation of any subprogram
- 34. Allocate storage fora single activation record statically as an extension of the code segment. Used in FORTRAN and COBOL. The activation record is not destroyed - only reinitialized for each subprogram execution. Hardware support - CIP is the program counter, CEP is not used, simple jump executed on return.
- 35. The simplest run-timestorage management technique call statements : push CIP and CEP return statements : pop CIP and CEP off of the stack. Used in most C implementations LISP: uses the stack as an environment.
- 36. 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 second activation within the lifetime of the first activation.
- 37. Stack-based - CIP andCEP are stored in stack, forming a dynamic chain of links. A new activation record is created for each call and destroyed on return. The lifetimes of the activation records cannot overlap - they are nested.
- 38. Data control featuresdetermine the accessibility of data at different points during program execution. Central problem: the meaning of variable names, i.e. the correspondence between names and memory locations.
- 39. Two ways tomake a data object available as an operand for an operation Direct transmission Referencing through a named data object
- 40. A data objectcomputed at one point as the result of an operation may be directly transmitted to another operation as an operand Example: x = y + 2*z; The result of multiplication is transmitted directly as an operand of the addition operation
- 41. A data objectmay be given a name when it is created, the name may then be used to designate it as an operand of an operation.
- 42. Variables Formal parameters Subprograms Defined types Definedconstants Labels Exception names Primitive operations Literal constants
- 43. Association: binding identifiersto particular data objects and subprograms Referencing environment: the set of identifier associations for a given subprogram. Referencing operations during program execution: determine the particular data object or subprogram associated with an identifier
- 44. 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 it Global referencing environment: associations created at the start of execution of the main program, available to be used in a subprogram. Predefined referencing environments: predefined associations in the language definition.