Computer Science Logo Style volume 1: Symbolic Computing 2/e Copyright (C) 1997 MIT


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Brian Harvey
University of California, Berkeley

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By introducing variables in Chapter 3, we made it possible for a procedure to operate on different data each time you invoke it. But the pattern of what the procedure does with the data remains constant. We can get even more variety out of our procedures if we can vary the instructions that the procedure executes. We need a way to say, "Sometimes do this; other times do that."

True or False

One helpful metaphor is this: When you invoke a command, you're giving the computer an order. "Now hear this! Print such-and-such!" But when you invoke an operation, you're asking the computer a question. "What is the first member of such-and-such?"

In real life we single out as a special category yes-or-no questions. For example, these special questions form the basis of the game Twenty Questions. The corresponding category in Logo is the predicate. A predicate is an operation whose output is always either the word true or the word false.

For example, listp (pronounced "list-pea") is a predicate that takes one input. The input can be any datum. The output from listp is true if the input is a list, false if the input is a word.

Wordp is another predicate that takes one input. The input can be any datum. The output from wordp is true if the input is a word, false if the input is a list. (This is the opposite of the output from listp.)

Emptyp is also a predicate with one input. The input can be any datum. The output from emptyp is true if the input is either the empty word or the empty list; if the input is anything else, the output is false.

You'll have noticed by now that predicates tend to have names ending in the letter p. This is not quite a universal rule, but almost. It's a good idea to follow the same convention in naming your own predicates.*

*Many versions of Logo use a question mark at the end of names of predicates, instead of a p. For example, you may see list? instead of listp. Berkeley Logo accepts either form, but I prefer the p version.

As I'm describing primitive predicates, you might want to try them out on the computer. You can do experiments like this:

? print wordp "hello
? print wordp [hello]
? print emptyp []
? print emptyp 0

Of course, most of the time you won't actually want to print the output from a predicate. You'll see in a few moments how we can use a predicate to control the instructions carried out in a procedure.

But first here are a few more primitive predicates. Numberp takes one input, which can be any datum. The output from numberp is true if the input is a number, false otherwise.

Equalp takes two inputs, each of which can be any datum. The output from equalp is true if the two inputs are identical or if they're both numbers and they're numerically equal. That is, 3 and 3.0 are numerically equal even though they're not identical words. A list is never equal to a word.

? print equalp 3 3.0
? print equalp "hello [hello]
? print equalp "hello first [hello]
? print equalp " []
? print equalp [] butfirst [hello]

The equal sign (=) can be used as an infix equivalent of equalp:

? print "hello = first [hello]
? print 2 = 3

As I mentioned in Chapter 2, if you use infix operations you have to be careful about what is grouped with what. It varies between versions of Logo. Here is an example I tried in Berkeley Logo:

? print first [hello] = "hello

Among current commercial implementations, Object Logo and Microworlds give the same answer f. But here is the same example in Logowriter:

? print first [hello] = "hello

You can avoid confusion by using parentheses. The following instructions work reliably in any Logo:

? print (first [hello]) = "hello
? print first ([hello] = "hello)

Memberp is a predicate with two inputs. If the second input is a list, then the first can be any datum. If the second input is a word, then the first must be a one-character word. The output from memberp is true if the first input is a member of the second input.

? print memberp "rain [the rain in Spain]
? print memberp [the rain] [the rain in Spain]
? print memberp [the rain] [[the rain] in Spain]
? print memberp "e "please
? print memberp "e "plain

Lessp and greaterp are predicates that take two inputs. Both inputs must be numbers. The output from lessp is true if the first input is numerically less than the second; the output from greaterp is true if the first is greater than the second. Otherwise the output is false. (In particular, both lessp and greaterp output false if the two inputs are equal.) The infix forms for lessp (<) and greaterp (>) are also allowed.

Defining Your Own Predicates

Here are two examples of how you can create new predicates:

to vowelp :letter
output memberp :letter [a e i o u]

? print vowelp "e
? print vowelp "g

to oddp :number
output equalp (remainder :number 2) 1

? print oddp 5
? print oddp 8

Conditional Evaluation

The main use of predicates is to compute inputs to the primitive procedures if and ifelse. We'll get to ifelse in a while, but first we'll explore if.

If is a command with two inputs. The first input must be either the word true or the word false. The second input must be a list containing Logo instructions. If the first input is true, the effect of if is to evaluate the instructions in the second input. If the first input is false, if has no effect.

? if equalp 2 1+1 [print "Yup.]
? if equalp 3 2 [print "Nope.]

Here is an example of how if can be used in a procedure. This is an extension of the converse example in Chapter 3:

to talk
local "name
print [Please type your full name.]
make "name readlist
print sentence [Your first name is] first :name
if (count :name) > 2 ~
   [print sentence [Your middle name is] first bf :name]
print sentence [Your last name is] last :name

? talk
Please type your full name.
George Washington
Your first name is George
Your last name is Washington
? talk
Please type your full name.
John Paul Jones
Your first name is John
Your middle name is Paul
Your last name is Jones

Talk asks you to type your name and reads what you type into a list, which is remembered in the variable named name. Your first and last names are printed as in the earlier version. If the list :name contains more than two members, however, talk also prints the second member as your middle name. If :name contains only two members, talk assumes that you don't have a middle name.

»Write a procedure of your own that asks a question and uses if to find out something about the response.

You can use if to help in writing more interesting predicates.

to about.computersp :sentence
if memberp "computer :sentence [output "true]
if memberp "computers :sentence [output "true]
if memberp "programming :sentence [output "true]
output "false

? print about.computersp [This book is about programming]
? print about.computersp [I like ice cream]

This procedure illustrates something I didn't explain before about output: An output command finishes the evaluation of the procedure in which it occurs. For example, in about.computersp, if the input sentence contains the word computer, the first if evaluates the output instruction that is its second input. The procedure immediately outputs the word true. The remaining instructions are not evaluated at all.

»Write past.tensep, which takes a word as input and outputs true if the word ends in ed or if it's one of a list of exceptions, like saw and went.

»Write integerp, which takes any Logo datum as input and outputs true if and only if the datum is an integer (a number without a fraction part). Hint: a number with a fraction part will contain a decimal point.

Choosing Between Alternatives

If gives the choice between carrying out some instructions and doing nothing at all. More generally, we may want to carry out either of two sets of instructions, depending on the output from a predicate. The primitive procedure ifelse meets this need.* Ifelse is an unusual primitive because it can be used either as a command or as an operation. We'll start with examples in which ifelse is used as a command.

*In some versions of Logo, the name if is used both for the two-input command discussed earlier and for the three-input one presented here.

Ifelse requires three inputs. The first input must be either the word true or the word false. The second and third inputs must be lists containing Logo instructions. If the first input is true, the effect of if is to evaluate the instructions in the second input. If the first input is false, the effect is to evaluate the instructions in the third input.

? ifelse 4 = 2+2 [print "Yup.] [print "Nope.]
? ifelse 4 = 3+5 [print "Yup.] [print "Nope.]

Here is an example of a procedure using ifelse:

to groupie
local "name
print [Hi, who are you?]
make "name readlist
ifelse :name = [Ray Davies] ~
     [print [May I have your autograph?]] ~
     [print sentence "Hi, first :name]

? groupie
Hi, who are you?
Frank Sinatra
Hi, Frank
? groupie
Hi, who are you?
Ray Davies
May I have your autograph?

»Write an operation color that takes as input a word representing a card, such as 10h for the ten of hearts. Its output should be the word red if the card is a heart or a diamond, or black if it's a spade or a club.

»Write a conversational program that asks the user's name and figures out how to address him or her. For example:

? converse
Hi, what's your name?
Chris White
Pleased to meet you, Chris.

? converse
Hi, what's your name?
Ms. Grace Slick
Pleased to meet you, Ms. Slick.

? converse
Hi, what's your name?
J. Paul Getty
Pleased to meet you, Paul.

? converse
Hi, what's your name?
Sigmund Freud, M.D.
Pleased to meet you, Dr. Freud.

? converse
Hi, what's your name?
Mr. Lon Chaney, Jr.
Pleased to meet you, Mr. Chaney.

What should the program say if it meets Queen Elizabeth II?

Conditional Evaluation Another Way

The use of ifelse in the groupie example above makes for a rather long instruction line. If you wanted to do several instructions in each case, rather than just one print, the if line would become impossible to read. Logo provides another mechanism that is equivalent to the ifelse command but may be easier to read.

Test is a command that takes one input. The input must be either the word true or the word false. The effect of test is just to remember what its input was in a special place. You can think of this place as a variable without a name. This special variable is automatically local to the procedure from which test is invoked.

Iftrue (abbreviation ift) is a command with one input. The input must be a list of Logo instructions. The effect of iftrue is to evaluate the instructions in its input only if the unnamed variable set by the most recent test command in the same procedure is true. It is an error to use iftrue without first using test.

Iffalse (abbreviation iff) is a command with one input, which must be an instruction list. The effect of iffalse is to evaluate the instructions only if the remembered result of the most recent test command is false.

Iftrue and iffalse can be invoked as many times as you like after a test. This allows you to break up a long sequence of conditionally evaluated instructions into several instruction lines:

to better.groupie
local "name
print [Hi, who are you?]
make "name readlist
test equalp :name [Ray Davies]
iftrue [print [Wow, can I have your autograph?]]
iftrue [print [And can I borrow a thousand dollars?]]
iffalse [print sentence [Oh, hello,] first :name]

About Those Brackets

I hope that the problem I'm about to mention won't even have occurred to you because you are so familiar with the idea of evaluation that you understood right away. But you'll probably have to explain it to someone else, so I thought I'd bring it up here:

Some people get confused about why the second input to if (and the second and third inputs to ifelse) is surrounded by brackets but the first isn't. That is, they wonder, why don't we say

if [equalp 2 3] [print "really??]          ; (wrong!)

They have this problem because someone lazily told them to put brackets around the conditionally evaluated instructions without ever explaining about brackets and quotation.

I trust you aren't confused that way. You understand that, as usual, Logo evaluates the inputs to a procedure before invoking the procedure. The first input to if has to be either the word true or the word false. Before invoking if, Logo has to evaluate an expression like equalp 2 3 to compute the input. (In this case, the result output by equalp will be false.) But if the print instruction weren't quoted, Logo would evaluate it, too, before invoking if. That's not what we want. We want the instruction list itself to be the second input, so that if can decide whether or not to carry out the instructions in the list. So, as usual, we use brackets to tell Logo to quote the list.

actual argument expression-->actual argument value
equalp 2 3-->false
[print "really??]-->[print "really??]

Logical Connectives

Sometimes the condition under which you want to evaluate an instruction is complicated. You want to do it if both this and that are true, or if either this or that is true. Logo provides operations for this purpose.

And is a predicate with two inputs. Each input must be either the word true or the word false. The output from and is true if both inputs are true; the output is false if either input is false. (And can take more than two inputs if the entire expression is enclosed in parentheses. In that case the output from and will be true only if all of its inputs are true.)

Or is a predicate with two inputs. Each input must be either the word true or the word false. The output from or is true if either input is true (or both inputs are). The output is false if both inputs are false. (Extra-input or outputs true if any of its inputs are true, false if all inputs are false.)

Not is a predicate with one input. The input must be either the word true or the word false. The output from not is the opposite of its input: true if the input is false, or false if the input is true.

These three procedures are called logical connectives because they connect logical expressions together into bigger ones. (A logical expression is one whose value is true or false.) They can be useful in defining new predicates:

to fullp :datum
output not emptyp :datum

to realwordp :datum
output and wordp :datum not numberp :datum

to digitp :datum
output and numberp :datum equalp count :datum 1

Ifelse as an Operation

So far, we have applied the idea of conditional evaluation only to complete instructions. It is also possible to choose between two expressions to evaluate, by using ifelse as an operation.

When used as an operation, ifelse requires three inputs. The first input must be either the word true or the word false. The second and third inputs must be lists containing Logo expressions. The output from ifelse is the result of evaluating the second input, if the first input is true, or the result of evaluating the third input, if the first input is false.

? print sentence "It's ifelse 2=3 ["correct] ["incorrect]
It's incorrect
? print ifelse emptyp [] [sum 2 3] [product 6 7]

Here is one of the classic examples of a procedure in which ifelse is used as an operation. This procedure is an operation that takes a number as its input; it outputs the absolute value of the number:

to abs :number
output ifelse :number<0 [-:number] [:number]

Expression Lists and Plumbing Diagrams

If and ifelse require instruction lists or expression lists as inputs. This requirement is part of their semantics, not part of the syntax of an instruction. Just as the arithmetic operators require numbers as inputs (semantics), but those numeric values can be provided either as explicit numbers in the instruction or as the result of an arbitrarily complicated subexpression (syntax), the procedures that require instruction or expression lists as input don't interpret those inputs until after Logo has set up the plumbing for the instructions that invoke them.

What does that mean? Consider the instruction

ifelse "false ["stupid "list] [print 23]

Even though the second input to ifelse--that is, the first of the two literal lists--makes no sense as an instruction list, this instruction will work correctly without printing an error message. The Logo interpreter knows that ifelse accepts three inputs, and it sees that the three input expressions provided are a literal (quoted) word and two literal lists. It sets up the plumbing without paying any attention to the semantics of ifelse; in particular, Logo doesn't care whether the given inputs are meaningful for use with ifelse. Then, once ifelse starts running, it examines its first input value. Since that input is the word false, the ifelse procedure ignores its second input completely and executes the instruction in its third input.

The use of quotation marks and square brackets to indicate literal inputs is part of the plumbing syntax, not part of the procedure semantics. Don't say, "Ifelse requires one predicate input and two inputs in square brackets." The instruction

ifelse last [true false] list ""stupid ""list list bf "sprint 23

has a very different plumbing diagram (syntax) from that of the earlier example, but provides exactly the same input values to ifelse.

Consider these two instructions:

figure: printfirst

Since the effect of print is easy to observe, it's not hard to see the relationship among the instructions, the plumbing diagrams, and the effects when these instructions are run. Why are brackets used around the first expression in one case but not in the other? Because in one case the expression is how we tell Logo to set up the plumbing diagram, while in the second case we are giving print as input a literal list that just happens to look like an expression. When the context is something like ifelse instead of print, the syntactic situation is really quite similar, but may be harder to see. Consider this instruction:

print ifelse emptyp :a [emptyp :b] [emptyp :c]

Why do we put brackets around two emptyp expressions but not around another similar-looking one? » Draw a plumbing diagram for this instruction, paying no attention to your mental model of the meaning of the ifelse procedure, treating it as if it were the nonsense procedure zot3. You will see that the first input to ifelse is an expression whose value will be the word true or the word false, because Logo will carry out that first emptyp computation before invoking ifelse. The remaining two inputs, however, are literal lists that happen to contain the word emptyp but do not involve an invocation of emptyp in the plumbing diagram. Once ifelse is actually invoked, precisely one of those two list inputs will be interpreted as a Logo expression, for which a new plumbing diagram is (in effect) drawn by Logo. The other input list is ignored.

Stopping a Procedure

I'd like to examine more closely one of the examples from the first chapter:

to music.quiz
print [Who is the greatest musician of all time?]
if equalp readlist [John Lennon] [print [That's right!] stop]
print [No, silly, it's John Lennon.]

You now know about almost all of the primitive procedures used in this example. The only one we haven't discussed is the stop command in the second instruction line.

Stop is a command that takes no inputs. It is only allowed inside a procedure; you can't type stop to a top-level prompt. The effect of stop is to finish the evaluation of the procedure in which it is used. Later instructions in the same procedure are skipped.

Notice that stop does not stop all active procedures. If procedure A invokes procedure B, and there is a stop command in procedure B, then procedure A continues after the point where it invoked B.

Recall that the output command also stops the procedure that invokes it. The difference is that if you're writing an operation, which should have an output, you use output; if you're writing a command, which doesn't have an output, you use stop.

In music.quiz, the effect of the stop is that if you get the right answer, the final print instruction isn't evaluated. The same effect could have been written this way:

ifelse equalp readlist [John Lennon] ~
    [print [That's right!]] ~
    [print [No, silly, it's John Lennon.]]

The alternative form uses the three-input ifelse command. One advantage of using stop is precisely that it allows the use of shorter lines. But in this example, where there is only one instruction after the if, it doesn't matter much. Stop is really useful when you want to stop only in an unusual situation and otherwise you have a lot of work still to do:

to quadratic :a :b :c
local "discriminant
make "discriminant (:b * :b)-(4 * :a * :c)
if :discriminant < 0 [print [No solution.] stop]
make "discriminant sqrt :discriminant
local "x1
local "x2
make "x1 (-:b + :discriminant)/(2 * :a)
make "x2 (-:b - :discriminant)/(2 * :a)
print (sentence [x =] :x1 [or] :x2)

This procedure applies the quadratic formula to solve the equation


The only interesting thing about this example for our present purpose is the fact that sometimes there is no solution. In that case the procedure stops as soon as it finds out.

Don't forget that you need stop only if you want to stop a procedure before its last instruction line. A common mistake made by beginners who've just learned about stop is to use it in every procedure. If you look back at the examples so far you'll see that many procedures get along fine without invoking stop.

Improving the Quiz Program

When I first introduced the music.quiz example in Chapter 1, we hadn't discussed things like user procedures with inputs. We are now in a position to generalize the quiz program:

to qa :question :answer
print :question
if equalp readlist :answer [print [That's right!] stop]
print sentence [Sorry, it's] :answer

to quiz
qa [Who is the best musician of all time?] [John Lennon]
qa [Who wrote "Compulsory Miseducation"?] [Paul Goodman]
qa [What color was George Washington's white horse?] [white]
qa [how much is 2+2?] [5]
Procedure qa is our old friend music.quiz, with variable inputs instead of a fixed question and answer. Quiz uses qa several times to ask different questions.

»Here are a couple of suggestions for further improvements you should be able to make to quiz and qa:

1. Qa is very fussy about getting one particular answer to a question. If you answer Lennon instead of John Lennon, it'll tell you you're wrong. There are a couple of ways you might fix this. One is to look for a single-word answer anywhere within what the user types. So if :answer is the word Lennon, the program will accept "Lennon," "John Lennon," or "the Lennon Sisters." The second approach would be for qa to take a list of possible answers as its second input:

qa [Who is the best musician of all time?] ~
   [[John Lennon] [Lennon] [the Beatles]]

Qa then has to use a different predicate, to see if what the user types is any of the answers in the list.

2. By giving quiz a local variable named score, you could have quiz and qa cooperate to keep track of how many questions the user gets right. At the end the score could be printed. (This is an opportunity to think about the stylistic virtues and vices of letting a subprocedure modify a variable that belongs to its superprocedure. If you say

make "score :score+1

inside qa, doesn't that make quiz somewhat mysterious to read? For an alternative, read the next section.)

Reporting Success to a Superprocedure

Suppose we want the quiz program to give the user three tries before revealing the right answer. There are several ways this could be programmed. Here is a way that uses the tools you already know about.

The general idea is that the procedure that asks the question is written as an operation, not as a command. To be exact, it's a predicate; it outputs true if the user gets the right answer. This asking procedure, ask.once, is invoked as a subprocedure of ask.thrice, which is in charge of allowing three tries. ask.thrice invokes ask.once up to three times, but stops if ask.once reports success.

to ask.thrice :question :answer
repeat 3 [if ask.once :question :answer [stop]]
print sentence [The answer is] :answer

to ask.once :question :answer
print :question
if equalp readlist :answer [print [Right!] output "true]
print [Sorry, that's wrong.]
output "false

You've seen repeat in the first chapter, but you haven't been formally introduced. Repeat is a command with two inputs. The first input must be a non-negative whole number. The second input must be a list of Logo instructions. The effect of repeat is to evaluate its second input, the instruction list, the number of times given as the first input.

The programming style used in this example is a little controversial. In general, it's considered a good idea not to mix effect and output in one procedure. But in this example, ask.once has an effect (it prints the question, reads an answer, and comments on its correctness) and also an output (true or false).

I think the general rule I've just cited is a good rule, but there are exceptions to it. Using an output of true or false to report the success or failure of some process is one of the situations that I consider acceptable style. The real point of the rule, I think, is to separate calculating something from printing it. For example, it's a mistake to write procedures like this one:

to prsecond :datum
print first butfirst :datum

A more powerful technique is to write the second operation from Chapter 2; instead of

prsecond [something or other]

you can then say

print second [something or other]

It may not be obvious from this example why I call second more powerful than prsecond. But remember that an operation can be combined with other operations, as in the plumbing diagrams we used earlier. For example, the operation second can extract the word or from the list as shown here. But you can also use it as part of a more complex instruction to extract the letter o:

print first second [something or other]

If you'd written the command prsecond to solve the first problem, you'd have to start all over again to solve this new one. (Of course, both of these examples must seem pretty silly; why bother extracting a word or a letter from this list? But I'm trying to use examples that are simple enough not to obscure this issue with the kinds of complications we'll see in more interesting programs.)

»If you made the improvements to quiz and qa that I suggested earlier, you might like to see if they can fit easily with a new version of quiz using ask.thrice.

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Brian Harvey,