FORTRAN 04 February 1999 CS 655 FORTRAN Concepts Contributions Binding time Separate compilation Modules Retention data Initialization of data objects Strong weak typing FORTRAN definitely weak Auto declaration Operator overloading Aliasing in common blocks Coercion 04 February 1999 CS 655 FORTRAN Design Considerations Underlying hardware Efficiency time direct translation to machine ops optimization DO loops array subscripting Efficiency space equivalence common flat structure no recursion 04 February 1999 CS 655 FORTRAN Design Principles Abstraction control Defense in depth assigned computed GOTO DO loop Structure goto s Syntactic consistency two goto types Preservation of information DO loop Zero one infinity array dims identifier length Regularity strings are second class 04 February 1999 CS 655 FORTRAN Interesting Problems Array subscripting limit of three dimensions limit on subscript expressions Parameter passing reference as implemented is dangerous expr actuals Computed Assigned GOTO s Syntax space compression combined with no key words DO 10 I 1 10 DO 10 I 1 10 04 February 1999 CS 655 ALGOL60 04 February 1999 CS 655 ALGOL was indeed an achievement It was a significant advance on most of its successors Alan Perlis 04 February 1999 CS 655 ALGOL60 Design by committee US Europe 1957 Goals Machine independence Improve RE FORTRAN s established flaws One standard to end proliferation of languages Model of Computation Static block structure gave additional control over name space Recursion multiple instances of same routine s birth of stack model 04 February 1999 CS 655 ALGOL60 Contributions Declaratives c f FORTRAN named the object Named data objects allocated fixed memory Declared their types provided for initialization Determined storage in activation record or what needed to be done at runtime Bound name to that storage Allowed for initialization Block structure scope visibility hiding Scope range over which name is defined Visibility set of names that have meaning Hiding re use of name in new context 04 February 1999 CS 655 ALGOL60 Contributions 2 Static vs dynamic referencing became an issue Static meaning of non local ref determined by static context Dynamic meaning of non local ref determined by dynamic call chain 04 February 1999 CS 655 ALGOL60 Types Primitive types real integer boolean strings 2nd class no dub precision for machine independence no complex Constructors arrays only arbitrary bounds arbitrary dimensions arbitrary subscripts Zero one infinity includes functions and other array refs dynamic sizing on block entry 04 February 1999 CS 655 ALGOL60 Types more In general declarations were required no auto declarations except procedure formals labels Strong typing The only operations allowed to be performed on a data object are those defined for objects of that type one of many defs for strong typing Others Many loopholes labels and integers specifications and declarations 04 February 1999 CS 655 ALGOL60 Imperatives Imperatives Computational control flow no I O Computational assignment much more general than FORTRAN s fac IF x 1 THEN 1 ELSE x fac x 1 Control flow Conditional had awkward inconsistency IF THEN s1 ELSE s1 can t be cond l violates regularity 04 February 1999 CS 655 ALGOL60 Oddities Conditional booleans odd IF IF IF a THEN b ELSE c THEN d ELSE f THEN g ELSE h k For loop very general FOR i 3 7 11 STEP 1 UNTIl 16 i 2 WHILE i 1 2 STEP i UNTIL 32 DO generates 3 7 1 12 13 14 15 16 8 4 2 1 2 4 8 16 32 user can modify loop indices see Knuth violates localized cost 04 February 1999 CS 655 Switch early case switch statement BEGIN switch status in air take off landing on runway i integer value goto status i out of range treated as fall through in air no ordering req mnts landing can be Switch take off anywhere supports labeling END violates structure security BEGIN switch s L IF j 0 THEN M ELSE N Q i integer value goto s i dest depends on j if i 2 Designational expression END See Knuth for some gems e g p 617 04 February 1999 CS 655 Example from Knuth Switch 04 February 1999 CS 655 Name Parameter Passing PROCEDURE swap x y integer x y BEGIN INTEGER t t x x y y t END swap i j works x i y j swap A i i works x A i y i swap i A i doesn t work x i y A i Similarly Knuth claims you can t write a general successor function Why 04 February 1999 CS 655 Jensen s Device real procedure SUM EXPR INDEX LB UB value LB UB real EXPR integer INDEX LB UB begin real TEMP TEMP 0 for INDEX LB step 1 until UB do TEMP TEMP EXPR SUM TEMP end proc SUM Manifest interface SUM A I I 1 25 SUM SUM B I J J 1 N I 1 M How many calls to SUM 04 February 1999 CS 655 Undesirable Interactions BEGIN INTEGER max m n READ max FOR i 1 UNTIL max DO BEGIN READ m n BEGIN OWN REAL ARRAY a 1 m 1 n Storage operations on a END END END How can we deal with changing bounds while trying to keep a copy of a around between block entries 04 February 1999 CS 655