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Pascal (Using slides of Tom Rethard)
CSE 3302 Pascal (Using slides of Tom Rethard)
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Algol Successful, so … Use the ideas in other languages
Algol-like languages List processing String manipulation Systems programming Artificial Intelligence Upgrades to Algol itself
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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
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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”
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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”
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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.
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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.
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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 s end; Total := sum from k = 1 to N of Wages[k];
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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.
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Pascal Designed by Niklaus Wirth Previously designed
Algol-W (a proposed extension to ALGOL with C. A. R. Hoare) Euler PL360
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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.
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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
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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; …
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Example (con’t) begin sum := 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.
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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; …
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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 …
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The Enumeration ADT Operations What is succ(Sat)? := Undefined succ
pred = <> < > <= >= What is succ(Sat)? Undefined What is pred(Nov)? Oct
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Enumeration Characteristics
High Level and Application Oriented Efficient Storage Secure Does not allow meaningless operations
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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
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Sets Set of <ordinal type> Var S, T: set of 1..10;
(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 …
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Set Operations = <> <= subset or equal >=
But: no < or > !
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Arrays Any upper or lower bound
Can also use enumeration types as array indices Examples (note base type in #2) var A: array [ ] of real; var HoursWorked: array [Mon .. Fri] of ;
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Arrays Var day: Mon .. Fri; TotalHours: ; begin TotalHours := 0; for day := Mon to Fri do TotalHours := TotalHours + HoursWorked[day];
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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 …
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More Complex Arrays var M: array [1..20] of array [ ] of real; or var m: array [ , ] of real;
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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
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Type problems Can write But cannot write: Sum(W)
type vector = array [ ] 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)
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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.
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Record Types Heterogeneous data Multiple components Various types
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Records type person = record name: string; age: ; salary: ; sex: (male, female); birthdate: date; hiredate: date; end; string = packed array [1 ..30] of char; date = record mon: month; day: ; year: ; end; month = (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec);
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Using a Record To use a record: var newhire: person; just like any other type
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Getting to the Components
var newhire: person; today: date; … newhire.age := 25; newhire.sex := female; newhire.date := today;
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More Possibilities if newhire.name[1] = ‘A’ then …
type employeeNum = ; var employees: array [employeeNum] of person; EN: employeeNum;
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Making it Simpler with newhire begin age := 25; sex := female; date := today end;
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Storage Groupings Homogeneous Heterogeneous Arrays
All elements are the same type Computed (dynamic) selector (subscript or index) Heterogeneous Records Elements (components) may be of different types Static selector
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Variant Records Sometimes records vary from one record type to another. Think of this as a primitive form of subclassing
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Variant Records type plane = record flight: ; kind: (B727, B737, B747); status (inAir, onGround, atTerminal); altitude: ; heading: ; arrival: time; destination: airport; location: airport; runway: runwayNumber; parked: airport; gate: ; departure: time; end; {plane}
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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.
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Variant Records type plane = record flight: ; kind: (B727, B737, B747); case status: (inAir, onGround, atTerminal); inAir:( altitude: ; heading: ; arrival: time; destination: airport); onGround: ( location: airport; runway: runwayNumber); atTerminal: ( parked: airport; gate: ; departure: time); end; {plane}
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altitude heading arrival destination
Implementation flight kind status altitude heading arrival destination location runway parked gate departure
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The Dreaded Pointer There is a problem with pointers and strong typing! Pascal solves this problem by typing pointers
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Typed Pointers (notPascal)
var p: pointer; x: real; c: char; begin new(p); p^ := ; c := p^; end
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Typed Pointers (Pascal)
var p: ^real; x: real; c: char; begin new(p); p^ := ; c := p^; {illegal} end
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Pointers with Records var p: ^plane; begin … p^.plane.parked[1] … … end;
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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
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Interpretations for equivalency
Structural equivalence Structural descriptions of the types be the same Name equivalence Names must be same
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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
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But… Consider this type person = 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!
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Name Equivalence Is actually ambiguous,
var x: 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; var x: T00029; y: T00030;
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Name Equivalence Issues
type age = ; 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
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Comparison Name Equivalence generally safer
More restrictive Name Equivalence is easier to implement Simply a string comparison Structural equivalence requires a recursive function ISO Pascal specifies name Equivalence
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Name Structures Pascal provides six types Constant bindings
Type bindings Variable bindings Procedure and function bindings Implicit enumeration bindings Label bindings
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Constant bindings const MaxData = 100;
MaxData can be used almost anywhere All declarations Executable statements (for loops, etc.) Expressions BUT, not in other const declarations! const MaxDataMinus1 = MaxData –1; is not allowed
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Constructors Record constructors Procedure/Function
The major scope defining construct
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Procedures procedure <name> (<formal arguments>); <declarations> begin <statements> end;
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A Problem procedure P (...); ... begin ... Q(...) ... end;
procedure Q (...); ... begin P(...) ... end;
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A Problem Solved procedure Q(...) forward;
procedure P (...); ... begin ... Q(...) ... end; procedure Q (...); ... begin P(...) ... end;
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Procedure Construction
procedure <name> (<formal arguments>); <label declarations> <const declarations> <type declarations> <var declarations> <procedure and function declarations> begin <statements> end;
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Pascal eliminates the block
Simplifies name structure Complicate efficient use of memory
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Control structures Reflects structured programming ideas
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For Loop for <name> := expression { to | downto } <expression> do <statement> Simplified comparing with Algol (overreact to second generation languages) Bounds of the loop are computed once, at loop entry => definite iterator
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While Loop Also a “Leading Decision Indefinite Iterator”
while <condition> do <statement> Checks at top of loop Can use “while true do....” for a loop exiting from the middle (Mid-Decision Iterator)
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Repeat Loop Also “Trailing Decision Indefinite Iterator”
repeat <statement> until <condition> Checks at bottom of loop
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Unlabeled Case Statement
NOT Pascal – modeled according to Fortran computed goto case <expression> of <statement>, <statement>, <statement> end case;
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case I of begin ... S1 ... end; begin ... S23 ... end;
begin ... S end; begin ... S4 ... end; end case; No labels provided.
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Labeled Case Statement
Major contribution of Pascal case <expression> of <case clause>; <case clause>; <case clause> end case; Designed by C.A. Hoare: the most important of his many contributions to language design
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Labeled Case Statement
case I of 1: begin ... S1 ... end; 2: 3: begin ... S end; 4: begin ... S4 ... end; end case; Some dialects of Pascal add an otherwise case.
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The labeling principles
…
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Parameter Passing Pass by reference Pass by value
Replaces Algol pass by name Intended to allow side effects (ie, I-O parameter usage) Passes only the address Pass by value Intended for input only parameters Side effects not allowed Done by copy-in
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Pass as Constant Original specification contained this instead of pass by value Similar to C const parameter passing Allowed compiler to pass either address or value Called procedure could not modify it Elimination encourages call by reference
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Security advantages of value parameters
Efficiency advantages of reference parameters But there are some security problems … when aliasing
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type vector=array[1..100] of real;
var A:vector … procedure P(x:vextor); \*pass by constant begin writeln(x[1]); A[1]:=0; writeln(x[1]) end; P(A)
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Pass as Constant Two orthogonal issues involved
Should copy its value or use its address? Should be I or I-O parameter Approach violates the Orthogonality Principle
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Write a Pascal fragment that does not use the nonlocal variables, to illustrate the security loophole in parameters passed as constants.
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Procedural Parameters: Security Loophole
Pascal allows passing a procedure or function name as a parameter To restore some of the flexibility lost by omitting name parameters Makes it difficult to determine if the function parameter is properly constructed.
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Procedure difsq (function f:real; x:real):real
begin difsq:= f(x*x) – f(-x*x) end difsq(sin,theta)=sin(theta^2 ) – sin(-theta^2)
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The arguments of a formal procedure parameter shall be specified
Procedure difsq (function f(y:real):real; x:real):real
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Pascal Actually has lived up to its goals Teaching language
Reliability Simplicity Efficiency Wirth: “the principle to include features that were well understood, in particular by implementors, and to leave out those that were still untried and unimplemented, proved to be the most successful single guideline.”
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Extensions Basis for most modern language design Multiple offshoots:
One step past Algol Multiple offshoots: Concurrent Pascal Mesa Euclid Modula-2 (by Wirth)
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Systems Languages Subsets of PL/I PL/S PL/360 PL/M XPL
TI Pascal and MPP
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BCPL Basic CPL B Simplified version of CPL – Cambridge Plus London
Became popular in the early 70s B Ken Thompson, “BCPL squeezed into 8K of memory and filtered through Thompson’s brain” UNIX on PDP-7 Extreme weak typing (only 1 data type!) More like Assembly Language
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From B We Got C PDP-11 arrived
Problem with B’s addressing scheme discovered Dennis M Ritchie started by extending B to contain basic data types (but no type checking) Unions and enumerations added later, but still no type checking Serious problems in porting UNIX to other platforms, so lint (a separate type checker) used to check types
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C Language attempted to maintain compatibility with older B language
The C Programming Language by K&R 1978 ANSI Standard C Began 1983 Approved 1989 Permitted spread within universities and research organizations Also used as the output language for many compilers.
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C Generational Characteristics
1st Generation No nested procedures Poor support for modular programming 2nd Generation Low-level model of arrays and pointers 3rd Generation Hierarchical data structures
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C According to Ritchie “C is quirky, flawed, and an enormous success”
Suggests success attributed to Simplicity Efficiency Portability Closeness to the machine Evolution in an environment in which it was used to write practical programs
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3rd Generation Language
Emphasis on simplicity and efficiency Data structures Shift emphasis from machine to application Application-oriented constructors: sets, subranges, enumerations Name structures Simplification of Algol’s Add new new binding and scope-defining constructs Control structures Simplified, efficient versions of the 2nd generation Some new structures, such as a real case
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Exercises 5-1 5-4 5-8 5-14 5-15 5-17 5-20 5-25 5-26
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