Robin Jones

Sets and Lists

Having suggested in Chapter 5 that you can do pretty much anything with the tools we already have, now would be a good time to demonstrate it. We’ll build a group of functions that have applications in set theory. I’ve chosen this example partly because there is such a convenient link between a set and a list (or, at any rate, a flat list). To some extent, we are reinventing the wheel, in that several of these functions are already present in Common Lisp, but it’s still a useful exercise to see how they might be written. For our purposes, it will be sensible to define a set as a flat list with non-repeating elements. So (A B C D) is a set, but (A B C B D) is not.

Arrays als Variable

In Sprachen sind manche Strukturen schon eingebaut andere mus man ihnen selbst geben. BASIC besitzt nur eine einzige feste Datenstruktur, die im Englischen “array” heist. Im Deutschen kann das wiedergegeben werden durch “Feld” oder “Tabelle” oder “Matrix”. Immer handelt es sich um Variable. Wir verwenden, wenn Variablenmengen gemeint sind, den Ausdruck “Array”.

Arrays, Strings and Structures

We have not yet exhausted Lisp’s internal data types. In this chapter, I’ll introduce a few more.

Some Basic Ideas

There are those who will tell you that LISP is an acronym for LISt Processor and others who insist that it stands for Lots of Infuriatingly Silly Parentheses. Both camps have good arguments to back them up. Lisp is a language which deals almost exclusively in list structures, and there are a great many brackets in a typical Lisp program. Paradoxically, Lisp derives much of its power as a programming language from the fact that it is limited in this way, and, as we shall see, this philosophy leads automatically to the proliferation of similar symbols (which just happen to be brackets) which so incense Lisp’s detractors.

More About Program Control

Lisp has a rich set of mechanisms for handling program control, very few of which we have met so far. We’ll remedy this oversight now, looking first at some more conditional functions. To date we’ve only seen one, cond. There is no absolute need for any others, but there are times when code can be made clearer using other, less general, functions. Common Lisp provides the following.

ABC Intepreter: Execution

In this final chapter we’ll look at an example to show how the interpreter executes a command. I’ll also write down the full code, and describe a short interactive session using the interpreter. But first, let me state in more detail the subset of ABC that the interpreter can handle.

A Look Back (and Forward)

I could argue that, with the exception of the way LISP interfaces with the outside world, we have now met all its main features. It is possible, as I have already hinted, to build any function from head, tail, append and cons. However, the results would be pretty cryptic (especially if head and tail are replaced with the more conventional car and cdr.

A Pot-Pourri of Features

As we have now seen, Common Lisp has a range and power exceeded by no other popular language. These very features, however, put the novice programmer in a quandary. How best should a particular function be used? What optional keyword arguments are there that might simplify the code? Is there a more appropriate function anyway? Even the experienced programmer can sometimes feel like a child in a toy supermarket—simply spoilt for choice.

ABC Interpreter: Scanner

An interpreter for any language (including Lisp) must have a read → translate → execute loop which takes the user’s keyboard input and analyses it to determine what the user wants, translates it into executable form, and finally does the work. This chapter deals with the read → translate stage; the next handles the actual execution.

Implementing Turtle Graphics...

Turtle Graphics were invented by Seymour Papert in the 1960’s and form the best known part of his introductory programming language, Logo. The idea is that lines can be drawn on the screen by an (imaginary) turtle which happens to be carrying a pen around with it. It can be commanded to head off in any given direction, travel a specific distance, lower the pen so that it leaves a track as it goes, or raise it so that no trace is left. In some implementations the turtle is a robot, and the name is more appropriate, but the side effects on carpets can be undesirable.