The FOLRI.OLB Package for List Processing in Fortran IV
Contents
1. C E G 14 denotes either 0 or 1 if I4 D then the leaves will hang right under their dominating nodes if I4 1 then the leaves are plotted on one line e Eg given the same list as in the above example with I4 0 with I4 1 A A D un Gy n c T 27 Note 1 Files from PLOTLI are written on FOR Z4 DAT so do not confuse this with other output on this file by PRLIST 2 When all structures to be plotted are processed by PLOTLI one should call the CLOSE subroutine in the fortran program in particular CALL CLOSE 4 This is needed to close the file i e add an end of file symbol to it Now we proceed with the list processing functions and subroutines themselves 2 Processing functions a Elementary There are 8 elementary functions and subroutines three whith WRITE information in a field of a cell and three which READ information from a field of a cell READ three integer functions CAR 11 where is a pointer address of a cell in the memory The result is the value of the CAR field of the cell E G Let be A B C then CAR I1 is a pointer to the atom A CDR IA where I is a pointer to a cell in the memory The result is the value of the CDR field of the cell 51811 Let be A B C then CDR I4 is B C Let I1 be A B C 1 then CDRCI1 is B C AFCIT where I1 is a pointer to a cell in th2. 12 Let Le A L110 E L1 BC then L1 in La yields A BEC DE ii In det notation A NIL C B NILJ CD NIL We have the following steps L5 L1 L2 D L1 L3 B L2 C L3 A Finally L5 C CCA BJ CC D 13 Restrictions on list notation a It is not possible to represent circular lists b Whenever an atom appears in a CDR field we have to use dot notation To see more clearly the relation between dot and list notation and the Eraphical representation we discuss the reverse way from dot or list notation back to a graphical representation For this purpose we can use the reversal of the previously used strategy i Try to discover linear lists and give them a name ii Construct the graphical representation for each linear list iii Reconstruct the whole by replacing all the variables by their graphical representations Example CCCA CB NIU CC CD NILDJ CE CF NIL G H NIL NIL J or T D NIL or T3 EZ NIL or E FLY NIL J or 5 Construct LS from L1 and t2 L1 gt UJ L3 I Lem m n L4 1 Cc x L5 CLA B NIL 2 CC ED NIL or N oo 14 L6 from L3 and L4 L6 E CF NIL G CH NIL In the same way we construct graphical representations from S expressions in list notation Example AB COD C EF GH eg
3. 4 FORMAT 1H THE NAME IS b PLOTLI A special program called PLOT has been implemented in cooperation with P Reypens to plot tree structures on the plotter Tree structures are list structures with the following restriction i the first element of the list is an atom because this is the label ii the second element is either an atom or a list as specified in 1 iii there should be at least two elements in a list esg given A BC D z A C The list representation of a tree corresponds exactly to a right labelled bracketing of the string formed by its leaves The user can use the program PLOT which is ready for running on disk after first writing the lists on a special file by means of a subroutine called PLOTLI and then running the program PLOT after his own job is Finished 28 PLOTLI PLOTLI is a subroutine of 4 arguments I1 12 13 14 I1 is a pointer to the list l I2 denotes a value for the size of characters of horizontal lines and the space between the leaves This value is equal to I2 x 0 25 cm So if 12 is set to 1 the size of the characters will be 0 25 om which is more or less the normal size I3 denotes either O or 1 If I3 O then the tree is not centered if I2 1 the tree is centered i e the lines from dominating nodes will end at the middle of the bar connecting the dominated nodes g given ACBC COCEF G with I3 0 with I3 1 EOS B
4. ii OUTPUT a PRLIST is a subroutine with three parameters I1 I2 and I3 I1 is a pointer to a list i e to the first element on a list I2 is an integer value denoting the position on the outputline from where the system should start printing if I2 is a line is left open and the system starts from the first character on the next outputline I3 is the device on which the output must appear If I3 then the output appears on the device with LUN B the line printer by default if I3 1 then the output appears on the device with LUN 4 normally a file FOR 4 DAT on a disk if I3 2 then the output appears on the device with LUN 5 normally a file FOR 5 DAT on a disk The result of PRLIST is that the whole list structure pointed at by is recoded in alphanumeric characters and transferred to ths device 25 NOTES 1 PREIST also handles atoms 2 If list notation is impossible dot notation is used but only at the point where it is necessary e g piven A B C D J this will be printed as ABC D 3 When I2 is greater than one all characters before 12 on the output line are blanks One can use this feature for editing e g Suppose you want the following as output THE NAME IS JOHN where the name is is in the program and John an atom referred to by the variable name then the output can be obtained by the following lines of FORTRAN CALL PRLIST 14 WRITE 8 1
5. one takes cells from this free list and when these cells are no longer needed they are returned to the freelist The creation of this freslist is the task of a special subroutine INIT after the subroutine is called the system is ready to start 2 3 Input output routines i INPUT RLIST is an integer function with 3 parameters I1 a pointer to the position where the reading should start I2 a pointer which results in the final position after execution of the function I3 a code for the device from which the system should read 22 The result of RLIST is that 811 decoding and storing is performed and that a pointer to a list or atom is returned as result The following conventions hold for the arguments 4 If 11 is equal to than a new line of input is consumed but the line is NOT printed out during reading If I1 is equal to 1 a new line of input is consumed and this line is printed on the default output device If I1 is greater than 1 the system starts reading on the latest consumed line Whenever a line is completely processed but more characters are needed the system keeps reading new lines from the input device until a complete list or atom is found 2 I2 is set to the final character used in the RLIST process So with 12 we can keep on reading on the same line if we take this as starting point for the next call of RLIST 3 There are 4 devices normally connected to the system i by default CR ca
6. Then the relevant instruction to the taskbuilder is TKB DK1 PROG MAIN DK1 BLOCK DK1 FORLI 0LB 1 LB The file DK1 BLOCK is a necessary special subroutine filling a commonzone with data Due to memory limitations it is not possible to link the plot rout nes directly to a program Instead the relevant information is written on disk by PLOTLI in a file called DK1 FORZZ4 DAT and this file is used by another program available on the 120 00 disk called PLOT Suppose there was a larger memory for taskbuilding available then the plotting routines can be activated by calling PLOTL with the same param ters as PLOTLI 34 CONCLUSIONS The FORLI library discussed in this paper is used for various linguistic tasks such as parsing systems question answering systems production systems etc The library is constantly growing to cope with the problems we are dealing with At the moment special subroutines are constructed for processing networks e g Although it is undoubtely more appropriate to use such a programming language as LISP we are at the moment quite happy with the library One of the advantages is that the programmer can manipulate the lists in a very powerful way more powerful than in LISP and avoid unefficient use of the store think about garbage collection 35 REFERENCES Mc Cracken D 1965 A guide to FORTRAN IV Programming Academic Press New York Waite W 1973 Implementing so
7. execution each atom of I2 is added to 11 iff it is not present yet e g Let I1 ABC and I2 CBA then after CALL ADD 2 11 I1 A B C Let I1 C ABC and I2 DEF then after CALL ADD I2 11 Ti ABCOE F 2 ATTACH I2 11 read attach I2 to IT is a subroutine with 2 parameters 17 is a list After execution a copy of all elements of all elements of 12 is added to e g L t I4 ABC and I2 CBA then after CALL ADD 11 12 I4 ABCCBA Note that I2 is available for further processing afterwards 2 APPEND 11 12 13 is a subroutine that i creates a new cell ii hangs it on the CDR of iii puts I2 in the CAR of and iv sets I3 equal to the new cell APPEND is especially useful in the construction of list structures because one can walk further with a pointer whi e hanging new cells each time to the list 4 COPY I1 is an integer function that creates a completely new sequence of celis identical in structuring and with the same atoms on the same places as in the argument list I1 Copy returns as value a pointer to the newly made list 32 d Manipulating the property list With each atom there corresponds a property list for short p list on which the user can store pa s of properties and values and consult afterwards whether a certain property is present and if it is present what property it is i PROP 11 12 13 is a subroutine which stor
8. AN These functions and subroutines deal with the following aspects i initialization ii input output iii list processing Before discussing the use and functioning of all these subroutines we give some preliminary remarks on the nature of the representation being used 2 1 Representation For the machine representation of list structures we take a data structure already available in FORTRAN the integer declared 2 dimensional array Then we let each cell in the graph representation correspond to a row in the table and the three parts of the cell the AF field the CAR field and the CDR field are the first second and third column of the table respectively So we can address a part of a cell by giving a row and a column This representation may not be the most compact one possible it is undoubtely the best solution as regards the transportibility to other machine configurations Strings of characters e g the name of an atom are represented by coding them into integers and storing these in the same table The user however does not need to worry about the actual representation and the detailed mechanics of the storage and processing of list structures He can think in terms of list structures and atoms instead of integers and table representations In this way it becomes possible to write interesting programs using list processing in FORTRAN without bothering about the complexity of the details As the graphi
9. ANTWERP PAPERS IN LINGUISTICS nr 9 1976 FORLI OLB PACKAGE FOR LIST PROCESSING IN FORTRAN IV USER S MANUAL Luc Steels UNIVERSITEIT ANTWERPEN Universitaire Instelling Antwerpen Departementen GER en ROM Afdeling Linguistiek Universiteitsplein 1 B 2610 Wilrijk Antwerpen ABSTRACT The paper discusses a number of functions and subroutines which 811 deal with list processing and are written in the programming language FORTRAN IV After a brief introduction to the concept of list structures a user s manual is given CONTENTS Preface 4 List Structures 1 1 Lists and atoms 1 2 The representation problem 4 3 Trees 2 List Processing in FORTRAN IV 2 1 Representation 2 2 Initialization 2 3 I O routines 2 4 Processing functions 3 Operating systems Conclusions Preface Good tools area cornerstone in the development and functioning of any exact science and great care is normally taken in their design and use Apart from the machinery involved in acoustic phonetics and related areas in the language sciences there is only one basic instrument and that is a digital computer However digital computers are machines designed for a general purpose hence in order to make them more suitable for a particular task it is necessary to develop some additional components and add them to the machine These components are normally compilers or interpreter systems which accept a part
10. FRIGHTEN NP Det THE N BOY and this is nothing else but the list notation of a list structure The graphical representation of this is 247 HA dex BL 4 ce BR Z 3 TE n e ed te Because of the importance of the relation between trees in graphical and list notation we define explicitly the relationships between the two i Given a tree structure B1 B2 Bn CN N fs with A B1 Bn nonterminal nodes then the equivalent list notation is A 1 B2 Bn ii Given a tree structure A wy with A a nonterminal node and al e a 8 terminal node then the equivalent list notation is A al an Example PaA S NP sss J CAUX ces CVP es NP Ai T l T S NP CN 2 2 jS VYP CVa C NP C Det a ee ie N finally a N M V S NP N sincerity AUX M may sincerity may frighten VP V may NP Det the N boy 2 Det N 19 Reverse Given a list A e d vd with sublists or atoms then the equivalent tree is Example Given A 8C D the equivalent tree is A 20 2 List Processing in FORTRAN IV Fortran IV is not a list processing language that is a language where lists ara available as data structures We will now discuss a number of subroutines that make it nevertheless possible to work with list structures in FORTR
11. H CELIC D B A The following strategy can be followed for the construction of dot notations from graphical structures 1 Consider a list to be linear and whenever a pointer appears introduce a variabie name for the sublist depending from this pointer 2 Similarly construct for each sublist on the same basis a linear list with variables when necessary 3 Replace all variables by their respective dot representations E g Fl A B F2 A B C F3 A B C D F4 F3 E F C G H 3 final result CCA B C D E F G H2 29 E G given L1 We have the following sublists L2 A L5 A M2 L1 L3 D I J L2 M 3 L4 L3 B 0 L4 GB R L5 9 Replacing all the variables yields 32 8 0 3 0 8MHM LA in L3j D2 CB M Y Y Bs LJ L3 in L2 12 CCCE B 02 0 M 2 D I A 5 CA M Finally L1 CC CC B 0 J 0 7M J Jo CD aiI3 oe A L5 in L1 Graphically we had the following sublists Em Although dot notation is an immediate reflection of a graphical structure there is already one sort of list structures that cannot be expressed namely a circular list A circular list is a list where a pointer in some field points to a previous cell of the list Example Clearly a dot notation of this graph would never come to an end The fact that circular listg canno
12. ad be AF of I1 I2 which stores the value I2 in the AF field of the cell pointed at by I1 Unappropriate changing of the contents of the AF fields can however cause serious trouble especially for the output routines A normal user does however not need to change the contents of AF fields 28 0 Storage manipulation There are two special operations to create a new cell and one to give back an already created cell NEW I1 is 8 subroutine that takes a new cell from the free list and sets the pointer I1 equal to this cell It is a sort of initialization that takes place whenever we want to start a new list BACK I1 is a subroutine which is the reverse of NEW it returns one single cell pointed at by I back to the free list NEW and BACK are complementary in the sense that the execution of CALL NEW I1 CALL BACK I1 returns the whole memory structure as it was before the execution Another way of storage manipulation is the pushdownstore feature PUSH K I read push K on I is a subroutine where I is 8 pointer to the first cell of 8 list It pushes the list I down i e creates a new cell on top of I and stores K in the CAR field of the new top of the list E g Suppose 1 2 3 With I 1 gge EXER PI 4 5 8 free list e then after CALL PUSH L I we get with L is the atom D 1 4 2 3 bize 5 6 Note that the pointer to the head of the list does not change POPUP K I read pop
13. cal cells correspond to rows in the table it follows that pointers to lists are actually integers denoting the appropriate row And in the FORTRAN program the name of the list is a variable with as value the row in the table Again the user does not and he does not need to know what the actual values are 21 Note that 1 A pointer to a list is always a pointer to the first cell of the whole list But the routines e g for output consider this a pointer to the whole list E G e jal p a If we call this list in our Fortran program I than the value of I will be 1 aecording to our supposed labelling 2 A pointer to an atom is a pointer to the base cell of the atom 3 In the discussion it is handy to label the cells tn the list structures in order to talk about them These labels are not necessarily those used in the actual processing 4 NIL the null list has the value zero That means that if f is assigned to the variable NIL in a program we can refer to the nullist with NIL We now start the discussion of the functions and subroutines that make it possible to use list structures in FORTRAN 2 2 Initialization As integer type data are used all the time it is useful to start any program or subroutine as follows IMPLICIT INTEGER A W In a list processing system there is normally a so called free list created at the start When in need of a piece of list structured memory
14. e memory The result is the value of the AF field of the cell 9 et I1 be A then AF I1 is 1 le be A then AF I1 is 0 The AF contains either 1 or O and no other values WRITE three subroutines BECAR 11 I2 read be car of 12 This subroutine stores the value I2 in the CAR field of the cell pointed at by I1 E g BECAR 11 12 with A BC and I2 AJ then I1 becomes A B C a FB BECDR I1 I2 read be cdr I1 12 this subroutine stores the value I2 in the CDR field of the cell pointed at by I1 E g CALL BECOR I1 I2 where I1 is A and I2 is A B C results in AABC i e X e F 7 after CALL BECDR I1 12 fal ope Ds In view of the applications of tree structures we add two special functions to it i SON 11 where I1 is a pointer to a node in the tree The result is the leftmost node depending from 11 E G Let I1 be A BCD EFG as 4 tree A B E C D F C then the son of I1 which is the top cell is B C D and the son of this is C ii BRO I1 where I1 is a pointer to a node in the tree The result is a pointer to the first node y on the right of a node x such that x and y are both depending from the same node z When there is no such node the result of BRO is nil eig Given the same tree as in the previous example then BRO SON I1 is EF 6G Note As regards the elementary functions there is also BEAF I1 I2 re
15. ed 88 8 user s manual of the library The first section gives a brief introduction to list structures For a more extended introduction and useful exercises we refer to the text books on LISP e g Weissman 1965 The second section explains all the functions and subroutines and how they should be used As this paper is meant as a reference text for people actually working with the library we assume throughout the text that the reader is familiar with programming and has a working knowledge of FORTRAN IV e g as covered by McCracken 1965 interesting and very helpful textbook during the implementation of the library was Waite s book Waite 1973 on the implementation of software for non numeric applications in general Ts List structures 1 Lists and atoms A data structure is i a set of cells which can contain a certain datum and 11 a relation among the cells a way of organizing them Some data structures are e g a table or a linear array In these two data structures the location of the different cells of the data structure is defined in an implicit way namely on the basis of the horizontal or vertical order We can retrieve a value in one of the cells by addressing the position the cell takes in the data structure Suppose now that we make the structure explicit by drawing arrows if two cells are linked with each other Fa ae OE bp 8 or b A data structure where the rela
16. ee es pare eps 2 139 L1 C0 or L1 eee ee ee 15 Note The reader who is not yet thoroughly familiar with list representations should at this point consult a textbook and make more exercises on the translation from one representation into another one 3 Trees List structures are widely used in any sort of linguistic applications There are two reasons for this 1 easy input and output of strings and easy processing of alphanumeric data and 2 and this is even more important easy representation of structures This last point will be illustrated more explicitiy in this section We will discuss the standard means of representing tree structures as a special case of list structures A typical representation used in linguistics is a labelled plane rooted graph or tree 16 5 NP AUX PS N V NP Det N sincerity may frighten the bay An alternative linear representation of the same information is the so called labelled bracketing C SINCERITY dy up fen MAY aux THE BOY FRIGHTEN DT N ONP yp V S or sup SINCERITY C aux MAY up V FRIGHTEN up THE Cy BOY respectively called right labelled bracketing and left labelled bracketing Now if we take the left labelled bracketing and write all symbols on one line we obtain S NP N SINCERITY AUX M MAY J VP V
17. es the property 12 with the value I3 on the property list of the atom I2 can be a linear list I3 can be any sort of list or an etom ii GET 11 12 13 is a subroutine which checks whether the property I2 18 on the p list of the atom I1 if so I3 is set equal to the value of I2 if not the value of I3 is set equal to NIL If I2 is a list then I3 is returned iff all items on the original list which constitute the property are presented on I2 There may be more elements in I3 and the order is of no importance e Predicates Finally some predicates are available to check whether the contents of the car field of a cell is equal to an atom or 4 list use this predicates they should be initially declared to be of type LOGICAL i ATOM I1 this logical function checks whether the cell addressed by 11 is an atom or not If it is an atom the result of the function is TRUE else the result is FALSE ii LIST I4 this logical function checks whether the cell addressed by I1 is a list or not If it is a list the result is TRUE else FALSE 33 3 Operating systems in relation to RSX The whole library of functions and subroutines is called FORLI OLB 1 and is stored on disk 120 00 Suppose you have a main program calied MAIN which is in a compiled form stored on disk say DK1 and some routines in it are used from the library Let us call the program which should result after taskbuilding PROG
18. ftware for non numeric applications Englewood Cliffs Weissman C 1955 LISP 1 5 Primer Belmont Caiifornia ISSUES OF d 1875 Z 1975 3 1975 4 1976 5 1976 6 4976 7 1978 B 1878 ANTWERP PAPERS IN LINGUISTICS Luc Steels Parsing systems for Regular and Context free languages Johan Van der Auwera Semantic and Pragmatic Presupposition Luc Steels Completion grammars and their applications Georges de Schutter amp Eddy Kockx Meervoudsvorming en vervoeging in het Nederlands Een morfofonologische proeve Luc Steels amp Dirk Vermeir On the formal properties of completion grammars and their related automata Luc Steels Producing natural language from semantic information Georges de Schutter Een semantisch syntak tische beschrijving van adjektieven in het Nederlands Manuel Aguirres Factuality and Modality Special Issue L Steels ed Advances in natural language processing Preprints of a workshop held at the University of Antwerp UIA October 1976
19. icular programming language designed for a particular sort of problems Our work is less ambituous than the design of a new programming language although it serves the same purpose We started from the given fact that there was only a FORTRAN compiler available at the computer we were suppose to use and that for this machine a PUP 11 and the currently implemented operating system RSX the desired software e g a LISP compiler or interpreter system could not immediately be obtained To fill the gap a project was started concentrating on the development of software for linguistic applications This necessarily includ s i list processing ii recursive programming iii string manipulations iv flexibility in definition of functions for work on semantics It was decided that the capability of doing list processing was the first step For this purpose a library of functions and subroutines was created A first version of the library has been used extensively by many persons over the last few months It was felt to beahandy tool in coping with linguistic problems and a valuable help for people working on small machines with limited supporting software In the second improved version which is documented in this paper some new features e g more flexible I 0 are added also new routines are added and some parts such as dot notation of list structures were removed because they were not being used at all This paper is conceiv
20. in one of its CAR fields then the list starting from such a CAR field is a sublist A list with sublists is called a branched list a list without sublists is known as a linear list 2 The representation problem To have a successful data structure it is not sufficient to have a graphical representation One must be able to write down the graphical representation in a linear way i e algebralcally For tables and vectors we do th s by naming the whole data structure with a symbol say X and the differant cells of the data structure are addressed by subscripts e g X 1 2 denotes the first cell of the second column in a table called X For list structures the solution to the representation problem is not so easy simply because cells cannot be addressed by subscripts on the basis of their location i e by referring to lines and columns The problem is solved by the introduction of S expressions with two particular formats dot notation and list notation DOT NOTATION The dot notation of a list structure is a direct mirror of its graphical representation For each cell we introduce two brackets and one dot EX ixe3 V M V On the right side of the dot we write the CAR and on the left side the CDR If the CAR or CDR contains a pointer then we replace this pointer by the whole sublist depending from this pointer written in dot notation Examples nn Be i E I Cs D ECCE EL eck V B
21. rd reader with logical unit number 1 LP line printer with LUN 6 can be both connected to teletype during taskbuilding ii two files on disk FOR 4 DAT and FORGES DAT i e LUN 4 and 5 respectively Now if I3 then the input device is the card reader i if I3 1 the input device is the FORZZA DAT file on disk if I3 2 the input device is the FOR Z5 DAT file on disk Examples 1 I1 RLIST 1 1 U A list or atom is read from the card reader the input lines are printed 2 I1 RLIST I 1 I2 RLIST I I 1 Two lists or atoms are read from a file FOR 4 DAT the second list or atom comes after the first one possibly on the same line The line is not printed 23 Notes 1 Blanks are ignored if not meaningful 2 Superfluous right brackets on the last card are ignored but if you keep reading on the same line an error message will follow TOO MANY RIGHT PARENTHESES 3 A lack of right brackets will make the system look for further brackets and therefors consume the rest of input cards Then a message will be issued TOO MANY LEFT PARENTHESES So a lack of right brackets is a fatal error in that it is noticed only when all cards have been read 4 the null string can be represented in the input by NIL and NIL is the only atom that is present as soon as the program starts The integer value of NIL is 5 Each character that is given as input is coded directly in
22. t be expressed is however seldom felt as a drawback certainly not in linguistic practice LIST NOTATION Although the dot notation of lists is a very nice way of writing graphs inta a linear format it soon becomes extraordinary complex when the list structures themselves grow Therefore another representation has been designed list notation This goes as follows i A linear list is transferred by writing all the elements of the respective CAR fields right after each other E g EE ETS TS OY emm e Lab Xf CS CT 4101933 dot notation or LIST list notation note that nothing is provided if there is an atomin the CDR field 11 11 As for dot notation as soon as there appears a pointer to a sublist in one of the CAR fields construct the list notation for this sublist and replace the pointer by this sublist Example in dot notation Lb NIL a C I NIL J S NIL C T NIL in list natation CCCLO TI S T The technique for constructing dot notation from graphical structures can also be used here to construct list notation from graphical structures i Consider a list to be linear and whenever a pointer appears introduce a variable name for the sublist depending from this pointer 11 Similarly construct for each sublist on the same basis a linear list with variables when necessary iii Replace all variables by their respective list representations Example
23. tions between the cells are made explicit by drawing links between them is a list structure To locate a data cell in the structure we walk through it till we come to the desired place From the examples it could be seen that in an explicitly linked data structure a cell or box contains two parts These two parts are known as the CAR and the COR pronouns cudder of the cell CAR COR A CDR or CAR field contains either a data item or another pointer i e a link to another celi Ma Compare From now on we will call a datum an ATOM it is considered to be a nondivisible entity The second category in the discussion is the list being a number of cells linked onto each other by their respective pointers Note the slash at the end it denotes the end of the list Another name for E ga the slash is NIL denoting that there is nothing in that part of the cell If a list contains no elements at all then it is also represented as NIL In this case NIL is called the null list So if NIL is placed in a CAR or CDR field then we may assume that a list without any elements is attached to this field Some more definitions consider a b In a we have linked one cell to its successor this is a one way list In b we have a link from one cell both to its successor and its predecessor a two way list From now on we will only deal with one way lists If a list contains a pointer
24. to an integer We list here all the characters and their codings other characters are not accepted by the input and output subroutines If they are given as input a message UNRECOGNIZED CHARACTER will be issued CHAR CODE CHAR CODE CHAR CODE lj 1 M 7 37 A 2 8 38 E 3 P 8 38 I 4 Q 40 5 R 41 U 8 5 x 42 Y 7 T 7 43 B 8 V 44 C 9 W 45 D 10 X _ 48 11 47 G 12 48 H 13 j 48 J 14 50 K 15 1 s 51 L 16 2 52 3 53 4 5 B ey s 6 An important but difficult question is the fact that there is a fundamental distinction between the FORTRAN program and the variables for lists and atoms used therein and the users specification for the atoms and lists a distinction which is not so stringent in LISP e g due to the QUOTE feature Clearly the bridge between the two is the RLIST function _Therefore any atom that is used as an entity in the program should be read in by RLIST E g suppose NOUN is an entity which is being referred to in the program we can write NOUN RLIST 1 1 1 where NOUN is on the card From then on the variable NOUN in the FORTRAN program will refer to the same object as the atom NOUN in input output 7 A special sort of atom called p atoms can be processed by RLIST They are not storsd in the initial atom dictionary but on a special array fof 200 characters length Also all information about p atoms can be removed by one single operation CLEAR
25. up K from I is a subroutine where I is again 8 pointer to the first cell of a list The subroutine takes the contents of the first cell and sets it equal to K then the first cell is removed from the list E g suppose with I 1 sd 2 3 l then after CALL POPUP K I we get freelist Note that I still points to the same cell it is in fact the second cell that has been removed K now points to the atom A Remarks 1 It should be clear that a new element can only be placed on top of the pushdown store AFTER the pushing of the list and an element should be read from the top BEFORE the popping up operation is performed 2 To use a list as pushdown store it should first be initialized by calling NEW When the last element is popped up from the list in a popup operation the cell is automatically returned to the free list Other subroutines to remove information 1 When INIT is called at any point in a program all previous information is lost and the memory contains only the frselist 2 To remove all information as regards p atoms the routine CLEAR should be called 3 To remove a list structure without destroying the atoms one should call the subroutine ERASE I1 where I1 is the address of the first cell in the list 34 c Other list processing routines 1 ADD I2 11 read add I2 to I1 is a subroutine with 2 parameters 1 is a list and I2 is an atom or a linear list of atoms After
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