Shareware Super Platinum 8/Shareware Super Platinum 8.iso/mac/PROGTOOL/JTXT.ZIP;1/EXTERNAL.TXT

Text File | 1992-12-08 | 27.6 KB | 919 lines

                Using J with External Data:  two examples

                           Donald B. McIntyre
                         Luachmhor, Church Road
                        Kinfauns, Perth PH2  7LD
                             Scotland - U.K.
                         Telephone:  [phone]

    Introduction:

    J is a powerful dialect of APL created by Kenneth Iverson and
    Roger Hui [1].   At the APL91 Conference at Stanford, California,
    I gave an account of my experience with the language [2].   Some
    of the difficulties I met when first trying to use J were due to
    features that have been modified or dropped from the language;
    for example, the manner in which one function inherited the rank
    of another.   Although there was ample theoretical justification
    for the original way rank was inherited (and I gave an example of
    this in [2]), my experience helped to persuade the designers to
    change the rules.   Because J is developing fast, it is essential
    to specify the version used.   The files  Status.Doc and
    Xenos.Doc, provided with the system, document changes from
    earlier versions and should be examined closely.  The examples
    given here were executed with Version 4.1x2

    The special issue of the IBM Systems Journal, commemorating 25
    years of implemented APL, contains several examples of the same
    function, or verb, written in conventional APL, Direct
    Definition, and J [3].   These may be helpful to those already
    familiar with APL.   In an earlier paper in Vector I have
    provided an introduction to J with examples from elementary
    arithmetic [4].   The standard references written by Iverson are,
    of course, essential reading [5-9].   References to some of the
    more technical papers will be found in reference [4].

    Dictionaries like The Oxford English Dictionary and Lewis and
    Short's Latin Dictionary not only define words but include
    phrases illustrating their use by standard authors.   It would be
    impractical to attempt to learn a language from such sources
    alone.   It is necessary to read literature written in the
    language at an appropriate level, and preferably dealing with a
    topic already understood by the reader.   This paper is intended
    as a further contribution to the literature for beginning users
    of J.



                                   -1-





    I always find that, like my students, I benefit most from
    examples in a context to which I can relate.   In the context of
    two practical examples, I give here examples of features of J,
    including hooks and forks, rank, cut, raze, ravel items, format,
    do (execute), copy (compress), box, open, under (dual), amend,
    and the reading and writing of ASCII files.   I hope that others
    will find these helpful as they learn J.

    In every definition only necessary parentheses are used.   It is
    helpful to experiment by starting with fully parenthesised
    expressions and gradually reducing the number of parentheses
    included.   The successive definitions should be checked to see
    whether they give the same result as before, and when they do not
    they should be displayed in boxed and tree form to see how they
    have been interpreted by the J system [4].

    The First Problem Stated:

    A friend, who was starting to investigate J, had monthly total
    rainfall measurements for a period of eight years.   He had used
    a word processor to put the values into an ASCII file and he
    wondered how he could use J to compute various statistics.
    Missing observations were indicated by a letter.   This is how
    typical data appear in a file named data.in:

      3.02    0.88    6.69    1.78    3.48    3.37    3.81    1.57
      4.33    1.59    2.72    1.74    0.86    0.65    X       0.87
      0.22    2.77    2.66    2.08    1.65    2.48    X       3.41
      1.74    5.33    2.44    1.89    2.91    1.93    0.48    3.72
      0.25    0.09    2.33    1.67    1.40    1.54    2.37    4.18
      2.28    2.23    0.77    4.01    1.48    2.88    1.47    0.13
      0.94    X       2.72    5.11    X       2.46    1.54    0.56
      2.03    0.99    3.27    2.35    3.29    3.55    0.98    0.78
      1.26    2.07    4.02    1.09    1.84    0.37    2.30    1.32
      3.82    2.06    2.33    7.00    3.10    0.66    4.28    2.14
      3.08    1.54    0.88    2.60    4.20    2.52    3.82    1.22
      2.28    4.09    1.19    0.76    2.39    4.94    2.23    3.06











                                   -2-




    Input from ASCII to J:

    J has been criticised for having too few system functions, but
    this is a misunderstanding.   APL's system functions are not
    really part of the language;  in J they are implemented with the
    "foreign" conjunction (!:).   As the name suggests, conjunctions
    are dyadic;  they join together two arguments producing verbs
    (functions) or adverbs (monadic operators).   Thus 1!:1 is file
    read, and the argument of the resulting verb is the boxed
    specification of the file.   Similarly 1!:2 is file write, a
    dyadic verb, whose left argument is the character string to be
    written, and right argument the boxed file specification.   It is
    convenient to name these as follows:

       read=. 1!:1 @ <     NB.  x=. read 'input.fil'

       write=. [ 1!:2 <@]  NB.  'string' write 'output.fil'

    In my experience it is rather easy, even for a beginner, to think
    in terms of a fork, and the verb write is defined here as a fork.
    When I began to work with J I thought that both forks and hooks
    were abstruse concepts unlikely to be useful to someone at my
    level.   Very quickly, however, I found forks everywhere.   For a
    long time I thought that hooks were much rarer.   The reason is
    that every hook can be written as a fork, and it easier for the
    novice to use the fork form;  as indeed I have done here.

    When a fork consists of the composition of a dyadic verb, a
    monadic verb, and either a left ([) or a right (]) verb, then we
    can use the hook form.   In the dyadic case (as in the verb
    write):
            x (g h) y   is the same as   x g (h y)

    which implies that g is dyadic and h is monadic.   Thus:

       write=. 1!:2 <        NB.  Write to a file.   Hook

    I shall draw attention to similar examples as they occur.

    Execution of the following sentence causes the data to be read as
    a character string named y.

       y=. read 'data.in'






                                   -3-




    y is the name of a character string containing line-feed, new-
    line, and end-of-file codes which are as follows (where a. is the
    Atomic Vector, or list of the 256 ASCII codes, and { is the verb
    from used by J instead of a[13] for indexing):

       nl=. 13 { a. [ lf=. 10 { a. [ eof=. 26 { a.


    Replacement of the Character X to Characters _1:

    The indexes of the character X in the string are given by:
       ]i=. u # i. # u=. y = 'X'
    130 202 450 474

    Note that where # is preceded by i. it is the monadic tally;
    where it is preceded by u it is the dyadic copy (equivalent to
    compress or replicate in older APL).

    Because rainfall is never negative, we can replace the letter X
    with the pair of characters _1 as a step in converting the
    character string into numbers amenable to arithmetic.   The
    indexes of all positions to be changed are found by using the
    table adverb (/) to produce the outer product.
       ,i +/0 1
    130 131 202 203 450 451 474 475

    The information to be entered at these positions is:
       (+:#i)$'_1'
    _1_1_1_1
    Substitution is then made by amend.  For further examples see [2,
    p.270], but note that amend was changed in Version 4 [10].
       z=. ((+:#i)$'_1') (,i +/0 1)}y

    Defining verbs to accomplish this, we can proceed as follows:
       h=. 'X'&=@] # i.@#     NB.  Fork
       h y
    130 202 450 474

       g=. ,@(+/&0 1)@h
       g y
    130 131 202 203 450 451 474 475

       f=. $&'_1' @ (+:@#@h)






                                   -4-




       f y
    _1_1_1_1

    Then:
       z=. (f y) (g y)} y

    The explicit form is straightforward:
       amend=. '(f y.) (g y.)} y.' : ''
       z-: amend y
    1

    It was simple to amend a matrix with earlier versions of J using
    desk calculator mode [2, p.270].   The syntax was x i} m, where i
    gave boxed indexes.   But a definition in tacit form cannot be
    written easily.   Four hours after discussing this case with Ken
    Iverson and Roger Hui, they telephoned back to say that they had
    extended J (Version 4) to permit definition of the following
    fork:
      amend=. f g@]} ]
       z-: amend y
    1

    This is dramatic testimony to the superior design of the
    language, the flexibility of the implementation, and the response
    and skill of both designer and implementer!


    Conversion from Character String to Numeric Table:

    z is a string of numbers, represented in character form and
    containing printer-control characters.   Reshape it into a 12 by
    8 table after converting from character representation to
    numeric, and after printer-control characters have been removed:
       d=. 12 8 $ ". z #~ -. z e. lf,nl,eof

    We can, however, make this table without explicitly specifying
    its shape.   Take a simple example to illustrate the procedure:
       s=. '1 2 3',lf,'4 5 6',lf,'7 8 9',lf

       cut=. <;._2
    The conjunction cut will box items delimited by the last item (2)
    -- in this case lf -- and exclude (_2) the delimiters from the
    intervals:






                                   -5-




       cut s
    ⁄ƒƒƒƒƒ¬ƒƒƒƒƒ¬ƒƒƒƒƒø
    ≥1 2 3≥4 5 6≥7 8 9≥
    ¿ƒƒƒƒƒ¡ƒƒƒƒƒ¡ƒƒƒƒƒŸ

    Raze the result:
       ; cut s
    1 2 34 5 67 8 9

    Ravel Items:
       ,. cut s
    ⁄ƒƒƒƒƒø
    ≥1 2 3≥
    √ƒƒƒƒƒ¥
    ≥4 5 6≥
    √ƒƒƒƒƒ¥
    ≥7 8 9≥
    ¿ƒƒƒƒƒŸ

    Open:
       > cut s
    1 2 3
    4 5 6
    7 8 9

    Do, or execute (".)  under (&.) open (>) ;  i.e., open each box,
    convert to numeric, and then close each box again:
       $ ".&.> cut s
    3
    Open the result of this operation and show that we have produced
    the required numeric table:
       execute=. > @ (".&.> @ <;._2)
       1 + execute s
    2 3  4
    5 6  7
    8 9 10

    To apply execute to our data, begin by eliminating the new-line
    and end-of-file characters while leaving the line-feeds as
    delimiters:

       d -: data=. execute z#~ -. z e. nl,eof
    1






                                   -6-




       data
    3.02 0.88 6.69 1.78 3.48 3.37 3.81 1.57
    4.33 1.59 2.72 1.74 0.86 0.65   _1 0.87
    0.22 2.77 2.66 2.08 1.65 2.48   _1 3.41
    1.74 5.33 2.44 1.89 2.91 1.93 0.48 3.72
    0.25 0.09 2.33 1.67  1.4 1.54 2.37 4.18
    2.28 2.23 0.77 4.01 1.48 2.88 1.47 0.13
    0.94   _1 2.72 5.11   _1 2.46 1.54 0.56
    2.03 0.99 3.27 2.35 3.29 3.55 0.98 0.78
    1.26 2.07 4.02 1.09 1.84 0.37  2.3 1.32
    3.82 2.06 2.33    7  3.1 0.66 4.28 2.14
    3.08 1.54 0.88  2.6  4.2 2.52 3.82 1.22
    2.28 4.09 1.19 0.76 2.39 4.94 2.23 3.06


    Replace Negative Values by Row Means:

    Substituting the most reasonable values for missing data is a
    matter that requires knowledge of the subject matter.   In this
    case the columns are years (8) and the rows, or items, are months
    (12).   We can replace missing values by the means for the row
    (month) over all years for which we have data.

    We need the indexes of missing values (represented by _1).  First
    write a directly executable expression and then define a verb to
    give the same result:
       (,u)#i.#,u=. 0>data
    14 22 49 52

       h=. 0&>@,
       g=. (] # i.@#)@h    NB.  A fork that can be written as a hook

    Because g consists of a dyadic verb (#), a monadic verb (i.), and
    the verb right (]), it can be written as a hook.
       g=. (# i.@#)@ h     NB.  Hook
    Fix g as ir (Indexes in Ravel) so that the names g and h can be
    reused.
       ir=. g f.
       ir data    NB.  Index in Ravel of matrix
    14 22 49 52

    The row indexes are:
       <.8%~ir data
    1 2 6 6





                                   -7-




       (<.@(%&8)) ir data
    1 2 6 6

    The column indexes are:
       8|ir data
    6 6 1 4

    To find the indexes we made explicit use of the number of columns
    (8), but this number is given by:
       {:$ data
    8

    Define verbs row and column depending solely on the data:
       row=. <. @ (ir % {:@$)      NB.  Fork
       row data
    1 2 6 6

       column=. {:@$ | ir          NB.  Fork
       column data
    6 6 1 4

       f=. row ,. column           NB.  Fork
       f data
    1 6
    2 6
    6 1
    6 4

    Note that row, column, and f are forks.   Now box the items
    (rows):
       ix=. <"1 @(row ,. column)
       ]i=. ix data
    ⁄ƒƒƒ¬ƒƒƒ¬ƒƒƒ¬ƒƒƒø
    ≥1 6≥2 6≥6 1≥6 4≥
    ¿ƒƒƒ¡ƒƒƒ¡ƒƒƒ¡ƒƒƒŸ
       ]z=. i{data
    _1 _1 _1 _1
       z-: ({~ ix) data     NB.  Hook
    1
       z-: (ir data){,data
    1
       z-: (ir { ,) data    NB.  Fork
    1






                                   -8-




    Compute the means of the rows with negative values.
       mean=. +/%#                            NB.   Fork
       ]m=. mean"1 (row data){data
    1.47 1.78375 1.41625 1.41625

    Note that (row data){ data is a candidate for a monadic hook:
       m-: mean"1 ({~ row) data
    1

    But these means were computed over the eight values in the rows,
    and these include the _1, which is only a flag indicating a
    missing observation.

    We need a special mean that will be based only on positive or
    zero values:

       mean n=. 1 _9 2 3 _1 4 0 5
    0.625
       (>:&0 # ]) n           NB.  Fork
    1 2 3 4 0 5

    Every hook can be written as a fork, and the fork is often easier
    to grasp than the hook that could replace it.   Remember that in
    the hook (g h), h is always monadic and g is dyadic [4].
    Arrange the two verbs in the order, from left to right, Dyadic,
    Monadic.   If the dyadic function is not commutative, it may be
    necessary, as in this case, to use the cross adverb (~).

       h=. >:&0               NB.  Monadic
       h n
    1 0 1 1 0 1 1 1
       g=. #~                 NB.  Dyadic
       n g (h n)
    1 2 3 4 0 5
       (g h) n                NB.  Hook
    1 2 3 4 0 5

       (#~ >:&0) n            NB.  Hook
    1 2 3 4 0 5

    Mean over positive and zero values:
       pzmean=. mean @ (#~ >:&0)      NB.  Hook
       pzmean n
    2.5





                                   -9-





       ]m=. pzmean"1 ({~ row) data
    1.82286 2.18143 2.22167 2.22167

    Amend the data by inserting these values.   Note that the indexes
    (i) are with respect to the ravel of the matrix (data).
       ]x=. m i}data
    3.02    0.88 6.69 1.78    3.48 3.37    3.81 1.57
    4.33    1.59 2.72 1.74    0.86 0.65 1.82286 0.87
    0.22    2.77 2.66 2.08    1.65 2.48 2.18143 3.41
    1.74    5.33 2.44 1.89    2.91 1.93    0.48 3.72
    0.25    0.09 2.33 1.67     1.4 1.54    2.37 4.18
    2.28    2.23 0.77 4.01    1.48 2.88    1.47 0.13
    0.94 2.22167 2.72 5.11 2.22167 2.46    1.54 0.56
    2.03    0.99 3.27 2.35    3.29 3.55    0.98 0.78
    1.26    2.07 4.02 1.09    1.84 0.37     2.3 1.32
    3.82    2.06 2.33    7     3.1 0.66    4.28 2.14
    3.08    1.54 0.88  2.6     4.2 2.52    3.82 1.22
    2.28    4.09 1.19 0.76    2.39 4.94    2.23 3.06

       amend=. ir@]}
       x-: m amend data
    1


    Catenate Month and Year Means:

    The verb ymean is a hook that catenates yearly means to the foot
    of the columns:
       ymean=. ,"2 1 mean
       ymean x

    Catenate the means of rows (months), the means of columns
    (years), and the grand mean (at bottom right corner):















                                  -10-




       ]s=. 6.2":((,"2 1 mean) x),"1 0 (mean"1 x),mean ,x
      3.02  0.88  6.69  1.78  3.48  3.37  3.81  1.57  3.07
      4.33  1.59  2.72  1.74  0.86  0.65  1.82  0.87  1.82
      0.22  2.77  2.66  2.08  1.65  2.48  2.18  3.41  2.18
      1.74  5.33  2.44  1.89  2.91  1.93  0.48  3.72  2.55
      0.25  0.09  2.33  1.67  1.40  1.54  2.37  4.18  1.73
      2.28  2.23  0.77  4.01  1.48  2.88  1.47  0.13  1.91
      0.94  2.22  2.72  5.11  2.22  2.46  1.54  0.56  2.22
      2.03  0.99  3.27  2.35  3.29  3.55  0.98  0.78  2.15
      1.26  2.07  4.02  1.09  1.84  0.37  2.30  1.32  1.78
      3.82  2.06  2.33  7.00  3.10  0.66  4.28  2.14  3.17
      3.08  1.54  0.88  2.60  4.20  2.52  3.82  1.22  2.48
      2.28  4.09  1.19  0.76  2.39  4.94  2.23  3.06  2.62
      2.10  2.16  2.67  2.67  2.40  2.28  2.27  1.91  2.31

    This illustrates the verb format (":), the conjunction rank (")
    applied to catenation (,), and the hook (,"2 1 mean)

    We now have a character array.   Catenate to it the new-line and
    line-feed codes, and ravel to produce an ASCII string that can be
    sent to DOS:
       (,s,"1 nl,lf) write 'data.out'

    The file Data.Out is the required ASCII file.


    Using Box (<) for further Formatting:

       f=. <@(6.2&":)
       g=. f&:(,:@ mean"1) ,: f@ mean @ ,     NB.   Fork
       h=. f , f@ mean                        NB.   Fork
       table =. h ,"0 1 g                     NB.   Fork

















                                  -11-




       ]t=. table x
    ⁄ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ¬ƒƒƒƒƒƒø
    ≥  3.02  0.88  6.69  1.78  3.48  3.37  3.81  1.57≥  3.07≥
    ≥  4.33  1.59  2.72  1.74  0.86  0.65  1.82  0.87≥  1.82≥
    ≥  0.22  2.77  2.66  2.08  1.65  2.48  2.18  3.41≥  2.18≥
    ≥  1.74  5.33  2.44  1.89  2.91  1.93  0.48  3.72≥  2.55≥
    ≥  0.25  0.09  2.33  1.67  1.40  1.54  2.37  4.18≥  1.73≥
    ≥  2.28  2.23  0.77  4.01  1.48  2.88  1.47  0.13≥  1.91≥
    ≥  0.94  2.22  2.72  5.11  2.22  2.46  1.54  0.56≥  2.22≥
    ≥  2.03  0.99  3.27  2.35  3.29  3.55  0.98  0.78≥  2.15≥
    ≥  1.26  2.07  4.02  1.09  1.84  0.37  2.30  1.32≥  1.78≥
    ≥  3.82  2.06  2.33  7.00  3.10  0.66  4.28  2.14≥  3.17≥
    ≥  3.08  1.54  0.88  2.60  4.20  2.52  3.82  1.22≥  2.48≥
    ≥  2.28  4.09  1.19  0.76  2.39  4.94  2.23  3.06≥  2.62≥
    √ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ≈ƒƒƒƒƒƒ¥
    ≥  2.10  2.16  2.67  2.67  2.40  2.28  2.27  1.91≥  2.31≥
    ¿ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ¡ƒƒƒƒƒƒŸ

    t is a table with 2 rows and 2 columns:
       $t
    2 2


    The Second Problem Stated:

    Early versions of J were supplied with tutorial files that could
    be displayed while in J.   In Version 4 of the Dictionary the
    tutorials are provided in an appendix [5].   In the files of one
    preliminary version each new-line code stood alone instead of
    being followed by a line-feed.   Consequently, when these files
    were read lines of text were superposed.   If the last line was
    the longest in the file, then that line was the only one
    displayed.

    To create such a file for yourself, first use your word processor
    prepare a text file (let us call it junk) with half a dozen short
    lines without wordwrap;  i.e. press the Enter key at the end of
    each line.   Next use a hex-editor, such as provided by PCTOOLS
    or NORTON, to see the sequence 0D 0A marking new-line and line-
    feed commands.   Now change all occurrences of 0A to 20 in order
    to replace line-feeds by blanks.   The problem then is how to use
    J to reverse the process by inserting a line-feed after every
    occurrence of new-line.   Try reading this file from J by
          ]x=. read 'junk'





                                  -12-





    Observe that each line is overlaid on the previous lines.   Count
    the number of new-lines and line-feeds, verifying that the line-
    feeds have been eliminated:
          +/ x =/ 13 10 {a.

    The verb h cuts the string using the new-line codes as
    terminating delimiters.   We can compose it from 3 verbs as a
    fork;  a monadic verb, a dyadic verb, and the verb right (]).
    Or we can compose it from 2 verbs as a hook.
          h=. =&nl <;.2 ]       NB.  Fork
          h=. <;.2~ =&nl        NB.  Hook

    As in all hooks, the right-hand verb is monadic and the left-hand
    verb is dyadic.   A hook combines one noun with the result of a
    verb that has been applied to a noun, either the same noun as the
    first (in the monadic case) or a different one (in the dyadic
    case).   Use a hook whenever this situation occurs.

    The boolean string produced by equals (=) points to the positions
    of new-line codes.   This is the left argument for cut (< ;. 2).
    The 2 indicates that the delimiters mark the ends of the
    intervals and are to be included in the result [5, p.10-11].
          each=. &.>
          g=. ,&lf each
          edit=. ; @ (g @ h)
          input=. edit @ read

    The definition should be fixed so that input will not be changed
    if the names g and h are reused.
          input=. input f.

    The adverb each opens the boxes produced by h, applies the verb
    (which is here catenate lf to the tail), and then closes the
    boxes again.   After reading the file, input edits it by putting
    in the required line-feed codes.  The file can now be displayed
    without superposed lines:
          ]x=. input 'junk'

    To create a new file with the corrections:
          x 1!:2 <'junk.out'

    The character string x is written to the new file (which must be
    designated by a boxed string).   This file can now be read
    correctly (remember that our read verb boxes the name before
    using it):



                                  -13-




          read 'junk.out'

    Leave J and use your hex-editor to see that line-feeds (0A) have
    been inserted between the new-line (0D) and space (20) codes.

    We have modified a single file, but I had several files to edit.
    Let infiles and outfiles be the lists of boxed names of files for
    input and output:

       infiles=. 'tut10.in';'tut11.in';'tut12.in';'tut13.in'
       outfiles=. 'tut10.out';'tut11.out';'tut12.out';'tut13.out'

    With the power of J we can create all the new files in one step.
    Switching arguments to avoid parentheses, my first successful
    solution was this:

       output=. >@[ 1!:2 ]           NB.  Fork
       outfiles output~"0 input each infiles

    The logic is as follows.  The file names are already boxed.   Use
    each to apply input (edit after read) and give the boxed strings
    ready for the output files.  Let the boxed names of the output
    files be the right argument, and let the left argument be the
    boxed strings that are to go into these files.   Then the rank
    conjunction ("0) can make output handle each file in turn.  The
    syntax would be like this:

    For one file:
                'string'  1!:2  (boxed file name)

    Note the asymmetry:  the right argument must be boxed, but the
    left argument must not be boxed.

    For several files one might try:
           (boxed edited strings) 1!:2"0 (boxed output file names)

    The rank conjunction takes 1 box on the left along with one box
    on the right.   However after the box on the left has been taken
    it must be opened before output begins.   If we open the left
    argument too soon then we defeat the purpose of the rank
    conjunction!   Consequently open (>) must be part of output.
    The other part is, of course,  1!:2

    A fork is then obvious:  open the left argument and don't open
    the right argument of 1!:2




                                  -14-





           output=. >@[ 1!:2 ]           NB.  Fork

    Because this fork (like many others I have written!) consists of
    a monadic verb (open), a dyadic verb (1!:2), and the verb right,
    it can be recast as a hook.   Because there are two distinct
    arguments, the hook is dyadic:

           x (g h) y     is the same as     x g (h y)

    h is the monadic function, therefore we must switch arguments;
    it is the boxed string that is to be opened -- not the boxed file
    name.   The right argument must be the result produced by input
    each infiles -- which is boxed;  the rank conjunction sees to it
    that only one box is taken at a time, and the phrase denoted (h
    y) opens that box.   Along with the rank conjunction, this
    provides the right argument for (1!:2), and the left argument is
    a single boxed file-name.

    In using the hook (instead of the fork) we have no choice about
    which argument is to be on the left and which on the right.
    Because this conflicts with the positions of the arguments
    required for (1!:2), we must cross them (~).   The result is
    therefore:

       (boxed output file names) (g~ h) (boxed edited strings)

    That is, we need the following hook:

        output=. 1!:2~ >               NB.  Hook
        outfiles output"0 input each infiles

    After this line is executed, all files named in outfiles can be
    read correctly.


    Acknowledgements:

    Kenneth Iverson and Roger Hui (Iverson Software Inc.) created the
    system a small facet of which I have described here.   They have
    given unfailing assistance, coaching me by long-distance
    telephone and patiently answering my more naive questions.  In
    response to my reported experience, in a few cases they modified
    the language.   They are, of course, in no way responsible for





                                  -15-




    infelicities in my use and exposition of their language.   I
    claim only that the expressions given here work in the current
    version of the system (J4.1x2, 13 Jan 1992).

    By asking for a solution to the first problem, Dr. Richard
    Fulford directed me into the realm of external communication.
    Graham Woyka and Anthony Camacho were always encouraging when the
    going was rough.


    References


        [1]    J is available from Iverson Software Inc., 33 Major
              Street, Toronto, Ontario, Canada M5S  2K9.   Phone
              [phone]; Fax [phone].

        [2]    Donald B. McIntyre, Mastering J, APL91 Conference
              Proceedings, Stanford, California, August 1991.   APL
              Quote Quad Vol. 21 Number 4 (August 1991), p.264-273.

        [3]    Donald B. McIntyre, Language as an Intellectual Tool:
              From hieroglyphics to APL, IBM Systems Journal, Vol.
              30, Number 4 (1991) p.554-581.

        [4]    Donald B. McIntyre, Hooks and Forks and the Teaching of
              Elementary Arithmetic, Vector, Vol. 8 Number 3 (January
              1992) 101-123.

        [5]    Kenneth E. Iverson, The ISI Dictionary of J, Version 4
              with Tutorials, Iverson Software Inc., Toronto (1991)
              29pp.

        [6]    Kenneth E. Iverson, Programming in J, Iverson Software
              Inc., Toronto (1991) 72pp.

        [7]    Kenneth E. Iverson, Tangible Math, Iverson Software
              Inc., Toronto (1991) 33pp.

        [8]    Kenneth E. Iverson, Arithmetic, Iverson Software Inc.,
              Toronto (1991) 119pp.

        [9]    Kenneth E. Iverson, An Introduction to J, Iverson
              Software Inc., Toronto (1992) 46pp.





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        [10]    Donald B. McIntyre, Using J's Boxed Arrays, Vector,
              Vol. 9 (1992) In Press.














































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