Redick for C Programmers

Table of Contents

A shameless plagiarism of the best book for learning J: "J for C Programmers" by Henry Rich.

1. Introduction

J programs are usually a fifth to a tenth as long as corresponding C programs, and along with that economy of expression comes coding speed.

C is a computer language; it lets you control the things the computer does. J is a language of computation: it lets you describe what needs to be done without getting bogged down in details.

J will take advantage of your skill in grasping the essence of a problem.

J contains a couple of dozen array-processing primitives and a dozen or so very cleverly chosen pipe-fittings that allow those primitives to be connected together to provide the limitless supply of array-processing functions needed for practical programming.

2. Culture Shock

3. Declarations

3.1. Arrays

Array: the only data type
Rank: number of axes
Shape: rank-1 array indicating length of all axes
Atom: may be of type numeric or character. It is rank 0. Its shape is an empty array of rank-1.

3.2. Cells

The best essay for describing the relation betweens cells and rank, "A Fine Line" was written by Dan Bron.

Cell: rank of elements to be operated on.
Frame: the array of cells.
Negative cell rank: used when you know the frame and want the cell to be whatever is left over.
Item: synonomous with -1 cells. The item of an atom is the atom.

3.3. Array-creating Verbs (and their interrogators)

3.3.1. Shape and ShapeOf

Example: Create a datum with shape (rank-1 5)

csi> (Shape 5 2)
#(2 2 2 2 2)

Example: Create another datum with shape (rank-1 5)

csi> (Shape 5 (rank-1 "ab"))
#(#\a #\b #\a #\b #\a
csi> (array->string (Shape 5 (rank-1 "ab")))
"ababa"

Table: Redick data types with rank and shape
Datum	Rank	Shape
atom	0	'()
list	1	'(m)
table	2	'(m n)
report	3	'(m n p)
empty list	1	0