1 CMPE 104 Pascal (Modified slides of Tom Rethard)
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CMPE 104
Pascal
(Modified slides of Tom Rethard)
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Pascal Designed by Niklaus Wirth Development started in 1968 First working compiler in 1970 Pascal-70 Report was on 29 pages
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Pascal Goals Goals
It is teaching language Reliability Simplicity Efficiency
Wirth: “the principle to include features that were well
understood, in particular by implementers, and to leave out those that were still untried and unimplemented, proved to be the most successful single guideline.”
1. The language is suitable for teaching programming in a systematic way.
2. The implementation of the language is reliable and efficient, at compile-time and run-time, on all available computers.
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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)beginsum := 0;readln (N);for i := 1 to N do
beginreadln (val);if val < 0 then Data[i] := valelse Data[i] := val
end;for i := 1 to N do
sum = sum + Data[i];avg := sum/N;writeln (avg);
end.
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EnumerationsTypeDayOfWeek = (Sun, Mon, Tue, Wed, Thu, Fri, Sat);Month = (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec);vartoday, tomorrow: DayOfWeek;begin…today := Tue;tomorrow := today + 1;…today = Jan; /* type error …
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Operations := succ pred = <> < > <= >=
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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> (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
var A: array [1 .. 100] of real;var HoursWorked: array [Mon .. Fri] of
0 .. 24;
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Arrays
Var day: Mon .. Fri; TotalHours: 0..120;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 [1 .. 100] of real;
or
var m: array [1 .. 20, 1 .. 100] of real;
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More on Arrays 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 problemstype vector = array [1 .. 100] of real;var U, V, vector;function sum (x: vector): real;
…begin … end {sum};
Can writevar 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!
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Record Types Heterogeneous data Multiple components Various types
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Recordstype
person = recordname: 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);
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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
varnewhire: person;today: date;…newhire.age := 25;newhire.sex := female;newhire.date := today;
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More Possibilities
if newhire.name[1] = ‘A’ then …
typeemployeeNum = 1000 .. 9999;
varemployees: array
[employeeNum] of person;EN: employeeNum;
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Making it Simpler
with newhirebegin
age := 25;sex := female;date := today
end;
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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
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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 Recordstype
plane = recordflight: 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}
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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 Recordstype
plane = recordflight: 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}
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Implementation
flightkind
status
altitudeheadingarrival
destination
locationrunway
parkedgate
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
varp: ^real;x: real;c: char;
beginnew(p);p^ := 3.14159;c := p^; {illegal}
end
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Pointers with Records
varp: ^plane;
begin…p^.plane.parked[1] ……
end;
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Interpretations for equivalency
Structural equivalenceStructural descriptions of the types be the same
Name equivalenceNames must be same
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Structural Equivalence
varx: 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 thistype
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!
Name Equivalencevar
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
typeT00029: record id: integer; weight: real end;T00030: record id: integer; weight: real end;
varx: T00029;y: T00030;
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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
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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 Problemprocedure P (...);
...begin
...Q(...)...
end;procedure Q (...);
...begin
...P(...)...
end;
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A Problem Solvedprocedure Q(...) forward;procedure P (...);
...begin
...Q(...)...
end;procedure Q (...);
...begin
...P(...)...
end;
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Procedure Constructionprocedure <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 Complicates 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>
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 Modeled according to Fortran
computed gotocase <expression> of
<statement>,<statement>,...<statement>
end case;
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case I ofbegin ... S1 ... end;begin ... S2 ... end;
begin ... S3 ... end;begin ... S4 ... end;
end case;
No labels are 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 of1: begin ... S1 ... end;2: 3: begin ... S23 ... end;4: begin ... S4 ... end;end case;
Some dialects of Pascal add an otherwise case.
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Parameter Passing Passing by reference
Replaces Algol pass by name Intended to allow side effects (ie, I-O
parameter usage) Passes only the address
Passing 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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Procedure 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; begin P(A) end;
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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
Procedure
Procedure difsq (function f:real; x:real):realbegin
difsq:= f(x*x) – f(-x*x)end
difsq(sin,theta)=sin(theta^2 ) – sin(-theta^2)
Procedure arguments The arguments of a formal
procedure parameter shall be specified
Procedure difsq (function f(y:real):real; x:real):real
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