1 / 87

CSE 3302

CSE 3302. Pascal (Using slides of Tom Rethard). Algol. Successful, so … Use the ideas in other languages Algol-like languages List processing String manipulation Systems programming Artificial Intelligence Upgrades to Algol itself. PL/I.

rajah-decker
Télécharger la présentation

CSE 3302

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CSE 3302 Pascal(Using slides of Tom Rethard)

  2. Algol • Successful, so … • Use the ideas in other languages • Algol-like languages • List processing • String manipulation • Systems programming • Artificial Intelligence • Upgrades to Algol itself

  3. PL/I • A block-structured COBOL/FORTRAN union (IBM, 1967) • Algol block structure • COBOL file manipulation • FORTRAN syntactic style • “Everything for Everyone” • “The Only Programming Language You’ll Ever Need” • Basically, a Swiss Army knife

  4. PL/I • Characterized by Dijkstra as “a fatal disease” and “a programming language for which the defining documentation is of a frightening size and complexity. Using PL/I must be like flying a plane with 7000 buttons, switches, and handles to manipulate in the cockpit”

  5. Extensible Languages • Roll-your-own (sort of) • Just a language kernel • But capable of adding to it if necessary • MAD (Michigan Algorithm Decoder) • McIlroy’s “syntax macros”

  6. Simple Kernel • Turned out to be a good idea • Frequent choice was an Algol subset with more general data structuring abilities • Allowed generalization to an application area, built on a common foundation.

  7. Types of Extensions • Operator Extension • Define new, application-oriented operators • Example: symmetric difference (x # y) operator 2 x # y; value x, y; real x,y; begin return abs(x-y) end; • The “2” is the precedence of the operator.

  8. Types of Extensions • Syntax macros real syntax: sum from I = lb to ub of elem;value lb, ub;integer I, lb, ub; real elem;begin real s; s := 0; for I := lb step 1 until ub do s := s + elem; return send; • Total := sum from k = 1 to N of Wages[k];

  9. Issues with Extensible languages • Usually inefficient • Tough to write a compiler for a language that is always changing! • Poor Diagnostics • The compiler really doesn’t understand what’s going on.

  10. Pascal • Designed by Niklaus Wirth • Previously designed • Algol-W (a proposed extension to ALGOL with C. A. R. Hoare) • Euler • PL360

  11. Pascal Goals • The language should be suitable for teaching programming in a systematic way. • The implementation of the language should be reliable and efficient, at compile-time and run-time, on available computers.

  12. History • Development started in 1968 • First working compiler in 1970 • Pascal-70 Report was 29 pages (cf. Algol’s 16) • P-Code based system in 1973 • Spread quickly on microcomputers in the 70s & 80s

  13. Example Program AbsMean (input, output);const Max = 900;type index = 1 .. Max;var N: 0 .. Max; Data: array [index] of real; sum, avg, val: real; i: index;…

  14. Example (con’t) beginsum := 0;readln (N);for i := 1 to N do begin readln (val); if val < 0 then Data[i] := val else Data[i] := val end;for i := 1 to N do sum = sum + Data[i];avg := sum/N;writeln (avg); end.

  15. Enumerations • Old Way begin integer today, tomorrow;integer Sun, Mon, Tue, Wed, Thu, Fri, Sat;Sun := 0; Mon := 1; Tue := 2; Wed := 3;Thu := 4; Fri := 5; Sat := 6;…today := Tue;tomorrow := today + 1;…

  16. Enumerations • Pascal Way Type DayOfWeek = (Sun, Mon, Tue, Wed, Thu, Fri, Sat); Month = (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec);var today, tomorrow: DayOfWeek;begin…today := Tue;tomorrow := today + 1;…today = Jan; /* type error …

  17. The Enumeration ADT • Operations • := • succ • pred • = • <> • < • > • <= • >= • What is succ(Sat)? • Undefined • What is pred(Nov)? • Oct

  18. Enumeration Characteristics • High Level and Application Oriented • Efficient • Storage • Secure • Does not allow meaningless operations

  19. Subrange Types var DayOfMonth 1 .. 31; • Restricts the range of values for DayOfMonth to the integer subrange of 1..31 • Can also use in enumerations: Type WeekDay = Mon .. Fri

  20. Sets • Set of <ordinal type> (enumeration type(char,Boolean), subrange type) • Var S, T: set of 1..10; • S := [1, 2, 3, 5, 7]; • T := [1 ..6]; • If T = [1, 2, 3, 5] then …

  21. Set Operations • = • <> • <= subset or equal • >= • But: no < or > !

  22. Arrays • Any upper or lower bound • Can also use enumeration types as array indices • Examples (note base type in #2) var A: array [1 .. 100] of real; var HoursWorked: array [Mon .. Fri] of 0 .. 24;

  23. Arrays Var day: Mon .. Fri; TotalHours: 0..120;begin TotalHours := 0; for day := Mon to Fri do TotalHours := TotalHours + HoursWorked[day];

  24. Arrays – of Characters Any finite discrete type for index var Occur: array [char] of integer; …Occur[ch] := Occur[ch] + 1; …if Occur[‘e’] > Occur[‘t’] then …

  25. More Complex Arrays var M: array [1..20] of array [1 .. 100] of real;orvar m: array [1 .. 20, 1 .. 100] of real;

  26. Arrays Issue • There are some problems • Need to be static, not dynamic • Must know types at compile time • Dimensions are part of the array type • Arrays are considered the same type if index types and base types both match

  27. Type problems type vector = array [1 .. 100] of real;var U, V, vector;function sum (x: vector): real; …begin … end {sum}; • Can write var W: array [1 ..75] of real; • But cannot write: Sum(W)

  28. Type Problems • Types of W and of x are not the same because the ranges of the indices are different! • This appears to be a violation of the Abstraction Principle.

  29. Record Types • Heterogeneous data • Multiple components • Various types

  30. Records type person = record name: string; age: 16 .. 100; salary: 10000 .. 100000; sex: (male, female); birthdate: date; hiredate: date;end;string = packed array [1 ..30] of char;date = record mon: month; day: 1 ..31; year: 1900 .. 2100;end;month = (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec);

  31. Using a Record • To use a record:var newhire: person;just like any other type

  32. Getting to the Components varnewhire: person;today: date;…newhire.age := 25;newhire.sex := female;newhire.date := today;

  33. More Possibilities if newhire.name[1] = ‘A’ then … type employeeNum = 1000 .. 9999;var employees: array [employeeNum] of person; EN: employeeNum;

  34. Making it Simpler with newhirebegin age := 25; sex := female; date := todayend;

  35. Storage Groupings • Homogeneous • Arrays • All elements are the same type • Computed (dynamic) selector (subscript or index) • Heterogeneous • Records • Elements (components) may be of different types • Static selector

  36. Variant Records • Sometimes records vary from one record type to another. • Think of this as a primitive form of subclassing

  37. Variant Records type plane = record flight: 0 .. 999; kind: (B727, B737, B747); status (inAir, onGround, atTerminal); altitude: 0 .. 100000; heading: 0 .. 359; arrival: time; destination: airport; location: airport; runway: runwayNumber; parked: airport; gate: 1.. 100; departure: time;end; {plane}

  38. What’s Wrong? • Not all data has meaning at the same time. • Can imply a plane is located at one airport and is parked at another • Violates security principle.

  39. Variant Records type plane = record flight: 0 .. 999; kind: (B727, B737, B747); case status: (inAir, onGround, atTerminal); inAir:( altitude: 0 .. 100000; heading: 0 .. 359; arrival: time; destination: airport); onGround: ( location: airport; runway: runwayNumber); atTerminal: ( parked: airport; gate: 1.. 100; departure: time);end; {plane}

  40. Implementation flightkindstatus altitudeheadingarrivaldestination locationrunway parkedgatedeparture

  41. The Dreaded Pointer • There is a problem with pointers and strong typing! • Pascal solves this problem by typing pointers

  42. Typed Pointers (notPascal) var p: pointer; x: real; c: char;begin new(p); p^ := 3.14159; c := p^; end

  43. Typed Pointers (Pascal) var p: ^real; x: real; c: char;begin new(p); p^ := 3.14159; c := p^; {illegal}end

  44. Pointers with Records var p: ^plane;begin … p^.plane.parked[1] … …end;

  45. Type Equivalence • Not as clear as it could have been • Revised Pascal Report • Specifies assignments are ok if expression and variable have identical type • Does not define identical type

  46. Interpretations for equivalency • Structural equivalence • Structural descriptions of the types be the same • Name equivalence • Names must be same

  47. Structural Equivalence var x: record id: integer; weight: real end; y: record id: integer; weight: real end; • The above are the same because their structure is the same

  48. But… Consider this typeperson = record id: integer; weight: real end;car = record id: integer; weight: real end; • The above are the same because their structure is the same, so:car := person;according to structural equivalency is legal!

  49. Name Equivalence varx: record id: integer; weight: real end;y: record id: integer; weight: real end; • Is actually ambiguous, • Different versions of Name Equivalence Rule differ on this example. • If reinterpreted as follows, then they are different type T00029: record id: integer; weight: real end; T00030: record id: integer; weight: real end; varx: T00029;y: T00030;

  50. Name Equivalence Issues type age = 0 .. 150; var n: integer;a: age; • Is n:= a legal? • Pure name equivalence says no • Logic says yes • Revised Pascal Report says that a subrange of a type is still that type

More Related