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CSE3302 Programming Languages

Explore the significance of programming languages, their principles, historical context, and efficiency factors, including language design, computation mechanisms, and adoption. Uncover the development of Fortran and key imperative programming concepts.

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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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