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Beginning Scheme

Summary
We consider the purpose of Scheme and the structure of expressions in Scheme.

Algorithms and Programs

Our main objectives in this course are (1) to learn about algorithms, step-by-step methods for solving problems, and (2) to learn how to direct computers to perform such algorithms for us. A programming language, such as Scheme, is a formal notation in which one can express algorithms so exactly that a computer can perform them without any other assistance from human beings. The expression of an algorithm in such a notation is called a program, and the computer is said to be executing the program when it is performing the algorithm as directed.

Although not all of the problems that we’d like computers to solve are arithmetical, the simplest examples belong to that category, and we’ll start with a few of them. Here, for instance, is a program, written in Scheme, that directs the computer to find the answer to the question “what is the square root of 137641?”

(sqrt 137641)

In order to make the Scheme environment answer that question, you need to learn how to work with Scheme. There are many different implementations of Scheme available. We’ll use DrRacket along with a library of procedures that we have designed for this course. In the DrRacket environment, which you will use to develop your programs, you can work interactively with Scheme, typing a bit of code, finding a result, typing another bit of code, finding a result, and so on and so forth.

> (sqrt 137641) 371

Procedure Calls

The full Scheme language contains several hundred primitive procedures – operations, such as finding the square root of a number, for which a Scheme interpreter can use prepackaged algorithms. DrRacket includes implementations of all of those procedures, and many more. Some programmers who are experts on square roots and on the idiosyncrasies of our computers have figured out and written up a step-by-step method for computing the square root of any number, using only the very elementary transformations that the processor can perform. Most Scheme interpreters recognize sqrt as the name of this algorithm and knows where the processor instructions that carry it out are stored. When the interpreter receives a command to compute a square root, it recovers these instructions and arranges for the processor to follow them.

A procedure call is a command that directs the Scheme interpreter to activate a procedure such as sqrt. (Note: sqrt is the name of the procedure, and (sqrt 137641) is the procedure call.) In Scheme, every procedure call begins with a left parenthesis and ends with the matching right parenthesis. Within the parentheses, one always begins by identifying the procedure to be called and then continues by identifying the arguments, the values that the procedure is supposed to operate on. The sqrt procedure takes only one argument, the number of which you want the square root, but other procedures take two or more arguments, and some need no arguments at all.

All arithmetic in Scheme is done with procedure calls. The primitive procedure + adds numbers together, the primitive procedure - subtracts one number from another. Similarly, the primitive procedure * performs multiplication, and the primitive procedure / performs division. The fact that in a procedure call the procedure is identified first makes calls to these procedures look different from ordinary arithmetic expressions: For instance, to tell DrRacket to subtract 68343 from 81722, one gives the command (- 81722 68343). Why someone might want to compute that value is left as an exercise for the reader.

Other Scheme procedures include abs, and expt. The Scheme procedure abs computes the absolute value of its argument. The Scheme procedure for raising a number to some power is expt.

Scheme Syntax

As you may have noted, the appropriate way to write a Scheme expression is (operation operand_1 ... operand_n). That is, you parenthesize the expression (and any nontrivial subexpressions) and you place the operation before the operands.

It is harmless, though unproductive, to try to give a Scheme interpreter ordinary arithmetic expressions, in which the procedure is written between the operands.

However, it is both harmful and unproductive to include extra parentheses. If you write (sqrt (123)), the Scheme interpreter will issue an error message to tell you that 123 is not an operation. Why? The Scheme interpreter assumes that operations come after

Nested Expressions

What if you want to compute something more complex than a simple sum, product, difference, or whatever? Sometimes, you want to combine operations. For example, to find the average of four numbers, we need to both sum the numbers and divide the result by four. One strategy would be to compute the sum, copy that number, and then insert it into an expression that divides by four.

However, Scheme provides a much more sensible solution: You can nest expressions, one inside another. When Scheme encounters nested expressions, it evaluates the inner expressions first. For example, consider the expression (/ (+ 90 80 10 90) 4) tells the Scheme interpreter to first add 90, 80, 10, and 90, and then to divide the result by 4. Similarly, (+ (* 80 0.45) (* 95 0.35) (* 100 0.20)) tells the Scheme interpreter to multiply 80 by 0.45, to multiply 95 by 0.35, to multiply 100 by 0.20, and to sum the results. Interestingly, the Scheme interpreter is free to do the multiplications in any order, as long as it completes all three before it does the sum. For example, it might first multiply 100 by 0.20 or it might first multiply 80 by 0.45. (DrRacket tends to evaluate subexpressions from left-to-right.)

Definitions

DrRacket’s Scheme interpreters, like most versions of Scheme, can also learn new names for things by reading definitions. Here’s what a definition looks like (define name value). For example:

(define days-in-a-week 7)


Like a procedure call, a definition begins and ends with matching parentheses. To distinguish between definitions and procedure calls, the Scheme interpreter looks at what comes immediately after the left parenthesis. In a definition, the keyword define must appear at that point. The keyword define is not the name of a procedure; it is part of the syntactic structure of the Scheme programming language. Its only role is to serve as the mark of a definition.

After the keyword define, a definition contains the name being defined and an expression that identifies the value that the name should stand for. In this example, the name is days-in-a-week. (Notice that in Scheme a name can, and often does, contain hyphens internally.) The value that it names is the number 7. Once DrRacket’s Scheme interpreter has seen this definition, it remembers that days-in-a-week stands for 7.

The value that gets a new name need not be a number; it can be anything, even a computation or the name of a procedure. For example, if you don’t like the name * for the multiplication procedure and would rather call it by the name multiply, just start each sequence of interactions with DrRacket by giving it the definition (define multiply *). (Alternately, place the definition in a file, and load the file in your interactions pane or definitions pane.)

More Procedures

At this point, we hope you’re wondering what other useful and interesting procedures are built into Scheme. Section 6.2.5 of the Revised report on the algorithmic language Scheme contains a list of the ones that are mainly about numbers, and that’s only one section of the full roster of standard Scheme procedures. Fortunately, most of the primitive procedures perform small, simple jobs and are easily learned.

Self Checks

Check 1: Simple Expressions

What do you expect the value of the following expressions to be?

(square 4)
(* (+ 4 2) 2)
(- 1 (/ 1 2))
(/ (- (+ 2 3) 1) 2)
(expt 2 3)


At the beginning of the next class, compare your answers to your lab partner’s.