12. Reading and Writing

Translating to and from a known language provides useful
beginning exercises in learning a new one. The following provides

Cover the right side of the page and make
a serious attempt to translate the sentences on the left
to English; that is, state succinctly in English what the verb
defined by each sentence does. Use any available aids,
including the dictionary and experimentation on the computer:

f1=: <: Decrement (monad); Less or equal
f2=: f1&9 Less or equal 9
f3=: f2 *. >:&2 Interval test 2 to 9 (inclusive)
f4=: f3 *. <. = ] In 2 to 9 and integer
f5=: f3 +. <. = ] In 2 to 9 or integer
g1=: %&1.8 Divide by 1.8
g2=: g1^:_1 Multiply by 1.8
g3=: -&32 Subtract 32
g4=: g3^:_1 Add 32
g5=: [email protected] Celsius from Fahrenheit
g6=: g5^:_1 Fahrenheit from Celsius
h1=: >./ Maximum over list (monad)
h2=: h1-<./ Spread. Try h2 b with the parabola: b=:(-&2 * -&3) -:i.12
h3=: [email protected]][email protected][*[email protected]]%<:@[ Grid. Try 10 h3 b
h4=: h3 <:/ ] Barchart. Try 10 h4 b
h5=: {&' *' @ h4 Barchart. Try 10 h5 b

After entering the foregoing definitions, enter each
verb name alone to display its definition, and learn to
interpret the resulting displays. To see several forms of display,
first enter 9!:3 (2 4 5) .

Cover the left side of the page,
and translate the English definitions on the right
back into J.


12.1   These exercises are grouped by topic and organized
like the section, with programs that are first to be read
and then to be rewritten. However, a reader already familiar
with a given topic might begin by writing.

A. Properties of numbers

pn=: >:@i. Positive numbers (e.g. pn 9)
rt=: pn |/ pn Remainder table
dt=: [email protected] Divisibility table
nd=: +/@dt Number of divisors
prt=: [email protected] Prime test
prsel=: prt # pn Prime select
N=: >:@pn Numbers greater than 1
rtt=: ,@(N */ N) Ravelled times table
aprt=: [email protected](N e. rtt) Alternative test and selection
(primes do not occur in the times table for N)
apsel=: aprt # N  
pfac=: q: Prime factors
first=: p:@i. First primes

B. Coordinate Geometry

Do experiments on the vector (or point) p=: 3 4
and the triangle represented by the
table tri=: 3 2$ 3 4 6 5 7 2

L=: %:@:(+/)@:*:"1 Length of vector (See rank in the dictionary
or in Section 20 of the Introduction)
LR=:L"1 Length of rows in table
disp=: ] - 1&|. Displacement between rows
LS=: [email protected] Lengths of sides of figure
sp=: -:@(+/)@LS Semiperimeter (try sp tri)
H=: %:@(*/)@(sp,sp-LS) Heron’s formula for area
det=: -/ . * Determinant (See dictionary)
SA=: [email protected](,.&0.5) Signed area. Try [email protected]|.
sa=:[email protected](],.%@[email protected]<:@#) General signed volume; try on the tetrahedron
tet=:?4 3$9 as well as on the triangle tri .