A Pilot Course in APL

Transcribed from the printed issue; not yet reviewed. Read it in the PDF of the issue, page 20

An APL Pilot Course¶

Put in the APL disc and call up APL with the command *APL. The message CLEAR WS tells us that there is an empty workspace that can be filled with functions and variables.

Check that the Caps Lock light is on. Then hold SHIFT and touch 'I'; the squiggle is called iota; it affects the single number that follows it. Enter ⍳100 and ⍳1000 and any other values you care to try.

Holding SHIFT and pressing 'R' produces the greek letter ρ. It can be used to find how many numbers there are in a list of numbers like the ones you have just created. Try this with: ρ⍳10 ρ1000 2 ρ⍳100 ρ1 4 8 ρ100 100 ρ⍳53.

These APL commands give quick results that are printed on the next line. Some results do not fit onto one line: try

2 5ρ10      (THE 2 5 RESHAPE OF 10)
2 5ρ⍳10     (THE 2 5 RESHAPE OF IOTA 10)
20 1ρ⍳20    (THE 20 1 RESHAPE OF IOTA 20)

To save typing time, parts of commands can be copied with the COPY and arrow keys; the whole of the last command can be repeated by using the right bracket ')'.

Now try 2 5 8ρ⍳50 4 5 8ρ⍳100

Note that if reshape runs out of information to use, it just starts at the beginning again as often as necessary to fill the pattern.

Try 2 3 4 20ρ⍳480

You must be sick of ρ by now.

The left arrow ← is called "gets". Print it with SHIFT 'P' Now enter ...

PAGE2←3 4 20ρ⍳480

Nothing happens until you enter PAGE again. Try typing PAGE and the following commands:

PAGE+2
PAGE+100
PAGE-100

(Note the high minus used to display negative numbers.)

PAGE1←PAGE+5
PAGE2←PAGE-5
PAGE1+PAGE2
ρPAGE
20 18ρPAGE
ρ20 18ρPAGE      (THE SHAPE OF THE 20 18 RESHAPE OF PAGE).

Exercise¶

Experiment with different numbers all through this course. If you have stuck with us so far, you will make a good APL programmer!. All that it takes is the familiarity that comes with practice.

A First Database¶

Make a list of five surnames and enter them as:

LIST1←'SMITH JACKSON DICKSON DUCKWORTH GRAHAM'

Check with LIST1 that the spelling is O.K. Improve it to LIST2 by putting enough spaces in between the names so that there are 12 characters for each name, i.e. 'SMITH' must have 7 spaces before 'JACKSON' starts, which is then followed by 5 spaces and so on.

Now try

5 12ρLIST2 (THE 5 12 RESHAPE OF LIST2)

If all is well, you should get

SMITH
JACKSON
DICKSON
DUCKWORTH
GRAHAM

against the left margin. If not, make LIST2 again counting spaces carefully.

Make this into LIST3 with

LIST3←5 12ρLIST2

The expression 5 1ρ⍳5 makes a column of numbers from 1 to 5. Before we can make a numbered list, we have to change this column of numbers to a column of characters. This is done with the 'format' symbol '⍕' obtained using SHIFT and 'H'. Type in

(⍕5 1ρ⍳5),LIST3

To put spaces between the numbers and names, enter

LIST4←(⍕5 1ρ⍳5),' ',LIST3

and check that all is well again by typing LIST4.

Finally try ,LIST4 . The comma unravels the table (matrix) of characters.

Exercises¶

1. Change SMITH in LIST4 to your surname. Hint: use ,LIST4, change it, call it LIST5 and then reshape it with LIST6←5 12ρLIST5
2. Make a new numbered list with at least three of your friends' names.
3. (More difficult) Can you make a numbered list of your friends' surnames and first names in three columns?

N.B. To see what lists we have made so far in this workspace, enter )VARS.