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CSE3302 Programming Languages

CSE3302 Programming Languages. Dr. Carter Tiernan. programming language. A. is. a language that is intended for the expression of computer programs and is capable of expressing any computer program. Why is this interesting?.

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CSE3302 Programming Languages

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  1. CSE3302Programming Languages Dr. Carter Tiernan Programming Languages

  2. programming language A is a language that is intended for the expression of computer programs and is capable of expressing any computer program Programming Languages

  3. Why is this interesting? • Although it’s possible to write any program in any language, it’s not equally easy to do so. • Languages are the tools of the central activity of computer science • The structure of language defines the boundaries of thought • Motivation for and use of modern language facilities Programming Languages

  4. Why is this useful? • Learning language mechanisms can allow you to simulate such things even in a language that does not provide them • Presents the most important principles for the design, evaluation, and implementation of programming languages Programming Languages

  5. Principles • Open the front cover of your textbook. • Yes, right now. Programming Languages

  6. Early computing • Numeric programming with coding of instructions • A “pseudo-code” was a primitive language that implemented the machine code operations with different, and hopefully easier to use, codes • interpreted or • (eventually) compiled Programming Languages

  7. Constraints • Very slow • Very small memory • Computer time cost more than programmer time • For numeric programming - • Significant floating point calculations • Requirement for indexing data Programming Languages

  8. What must a “pseudo-code” (or a programming language) do? Originally based on what the actual machine could do: • Floating point arithmetic and comparisons • Indexing • Transfer of control • I/O Programming Languages

  9. Decisions to make(OK, just a few of them) • Syntax? • How large can addresses be? • How long can instructions be? • How should we code the operations? • and so on… Programming Languages

  10. The Principles (again) • Identify elements most important to good programming language design • Apply with flexibility • Balance among contradictory • Modeled on Strunk and White Programming Languages

  11. Arithmetic vs. Comparison • Calculate values • Use operands • Values created can be used or stored • Test values • Use operands • Test results can be used or stored • Test results may control program flow Programming Languages

  12. Moving, Indexing & Looping[ Addresses ] • Moving • Put a value into a memory address • Indexing • Access a single element from a multiple element structure - an array • Looping • Change the flow of the program back to a previous location Programming Languages

  13. Input and Output • Based on I/O devices and methods of the time • Punched cards • Paper tape • Keyboards • {Not much to say about this one} Programming Languages

  14. Program Structure InterpreterExecution • Declarations • Instructions • Input data • Read instruction • Decode • Execute • Repeat steps with next instruction Programming Languages

  15. Interpreter enhancements • Program tracing • Breakpoints • Data trap • Labels for statements and variables • Beginning of symbol table • Translation Programming Languages

  16. Symbolic Pseudo Code • Syntax • Punch cards led to fixed format fields • Key punches only had upper case • VAR format • Prefix notation Programming Languages

  17. Phenomenology • Ampliative and Reductive • Fascination and Fear • Direct vs. Mediated (transparency) • Focus and Action Programming Languages

  18. Efficiency : Fortran • Inclusion of floating point arithmetic and indexing in hardware exposed the overhead of interpreters • John Backus of IBM recognized that language adoption would be based on : • Use of conventional mathematical notation • Highly efficient (machine) code produced Programming Languages

  19. Adoption and Use of Fortran • Development of a usable Fortran took 2 years (18 person/years) of effort (starting in 1955) • Within a year and a half, approximately half the code being written for the IBM 704 machines was in Fortran • Why? • Exceptionally clear documentation • Very sophisticated optimization techniques • Many versions exist and are in use • Book focuses on ANS Fortran IV (’66) Programming Languages

  20. Fortran structure • Subprograms • Parameters • COMMON blocks • Declarative or Imperative (nonexecutable vs. executable) • Allocate, bind and initialize at declaration • Compute, control flow, or I/O • Stages and phases of compilation Programming Languages

  21. Imperative statements • Assignment (=) is most important and most common computational statement • Control structures developed to direct control to primitive statements • Structures were based on IBM 704 branches • GOTO used to transfer control with IF for selection and iteration • DO loop is only higher level construct • Counted loop with CONTINUE • Can be nested hierarchically Programming Languages

  22. Fortran Control Issues • Static and dynamic structure hard to see and correlate • Confusion of GOTO plethora • Weak typing • DO-loop is optimized with all needed info for execution stated at the top • Procedural abstraction  • SUBROUTINE name (input & output parameters) • FUNCTION name (input parameters) Programming Languages

  23. Subroutines in Fortran • CALL passes parameters and control • RETURN passes control back to caller • Allows modularity and encourages libraries • Parameters passed by reference for efficiency but with side effects • Compare with pass by value-result • Activation records save state Programming Languages

  24. Fortran: Activation Record • Nonrecursive subprogram invocation • Contains all data needed to activate or restart a subprogram • Parameters • Instruction pointer (resumption address) • Dynamic link • Temporary storage Programming Languages

  25. Fortran: Data Structures • Scalars - integers, floating point • Integers, Hollerith constants (strings) • Double precision, complex, logical (boolean) • Representation • Word-based • Appropriate to operations on the type • Mathematical operations • Representation-independent • Overloaded for each type Programming Languages

  26. A(1) A(2) A(3) A(4) Fortran: Data Structures • Arrays • Contiguous memory allocation for indexing • Column-major order • Easy to optimize • Loop controls used array address to start • Subscript format was restricted • Index register increment easy to determine Programming Languages

  27. Fortran: Data Issues • Overloaded operators • Integer type used for integers and character strings (Hollerith constants) • No facilities for character manipulation • Arrays are static • Dimensions must be constants • Limited to 3 dimensions Programming Languages

  28. Fortran: Name structures • Declaration does binding • DATA statement does initialization • Static allocation • Fortran would automatically declare previously-unseen variables • “I through N” names assumed as integer • Major typo problems created • Variable names are local scope • Subprogram names are global scope I J K L M N Programming Languages

  29. COMMON blocks • Data sharing between subprograms • Subprogram explicitly calls out the COMMON block to be used • Aliasing EQUIVALENCE • Share memory within a subprogram Programming Languages

  30. Language Definition • Lexics • The way characters are combined to form words and symbols • Syntax • The way words and symbols are combined to form statements and expressions • Lexical analysis (scanning) • Syntactic analysis (parsing) Programming Languages

  31. Fortran syntax • Fixed-format columns • Free-format statement in columns 7 - 72 • Ignored blanks (ugh!) • No “reserved” words • Allowed (quasi) algebraic notation • Operator precedence • Linear instruction sequence Programming Languages

  32. First Generation Languages 1st • Machine dependence • Especially seen in control structures • And in primitive data types supported • Non-nested and rely on GOTO • One parameter passing mode • Definite loops supported but not recursion • Arrays • Weak typing • Static allocation and disjoint scoping of names • Linear organization Programming Languages

  33. Language Design Issues • Interaction of features • “Higher level” construct allows programmer to state what they want rather than how to do it • “Regular” flow of control • “Cost” of choices, e.g. Fortran efficiency Programming Languages

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