# Lets Read: Structure and Interpretation of Computer Programs

Chapter 1: Part 1

2022-02-09

This is the second entry in the *Letâs Read
SICP*
series.

## Contents

- 1 Building Abstractions With Procedures
- 1.1 The Elements of Programming
- 1.2 Procedures and the Processes They Generate
- 1.2.2 Tree Recrusion
- 1.2.3 Orders of Growth
- 1.2.4 Exponentiation
- 1.2.5 Greatest Common Divisors
- 1.2.6 Example: Testing for Primality
- Summary

## 1 Building Abstractions With Procedures

Chapter 1 here we go!

## 1.1 The Elements of Programming

Here we lay the foundations of programming and define a lot of terminology.

One tricky concept is the difference between applicative-order evaluation vs.Â normal-mode evaluation, which are two different ways that interpreters can evaluate code and which seems to me like a complicated concept to include with other content here such as âa variable is an abstraction of a hard coded value.â

The way I understand it is this.

Applicative-order evaluation, such as is done with Scheme, evaluates a normal procedure by first evaluating the operator and its operands, and then applying the operator to all of its operands.

In normal-mode evaluation, evaluation of the operands is deferred until they are needed, which is after the expression has been reduced to nothing but primative operators and data. I think this is what is called âlazy evaluationâ.

You can test out which method your interpreter uses with the following test.

```
define (infinite) (infinite)) ; start an infinite loop
(
define (test x y)
(if (= 0 x)
(0
y)
0 (infinite)) (test
```

Applicative-order will evaluate all of the operands immediately and start an infinite loop.

Normal-mode will defer evaluating the operands until they are needed,
which in this case is never, because the `if`

statement
evaluates to true, so the expression evalutes to `0`

, and y
is never expanded.

## 1.2 Procedures and the Processes They Generate

So there is no syntax for looping (that I know of) in Scheme. Iteration is done through recursion.

One neat thing that Iâve never thought of is that a recursive
procedure can generate a recursive or an interative
*process*.

```
;; Factorial 1: Recursive procedure, recursive process
define (factorial n)
(if (= n 1)
(1
* n (factorial ( - n 1 )))))
(
;; Factorial 2: Recursive procedure, iterative process
define (factorial n)
(define (fac-iter product count max-count)
(if (> max-count count)
(
product* product count)
(fac-iter (+ 1 count)
(
max-count)))1 1 n)) (fac-iter
```

Factorial 1 is a recursive process that defers procedure calls to the stack.

Factorial 2 is an iterative process produced by a recursive
procedure. It doesnât push function calls onto the stack. It doesnât use
the stack at all. It relies on no state in the environment. Which means
that if you needed to for some reason, you could stop the process and
resume it later because all of the state is held in
`product`

, `count`

, and `max-count`

.
You canât halt the first version and resume it mid-process because the
state isnât held in the program. Itâs held on the stack.

This is also where we get into tail-recursion.

Given these two examples (assume `inc`

and
`dec`

exist in the environment and increment and decrement a
value by one):

```
;; version 1
define (+ a b)
(if (= a 0)
(
b+ (dec a) b))))
(inc (
;; version 2
define (+ a b)
(if (= a 0)
(
b+ (dec a) (inc b)))) (
```

You can visually confirm that version 2 is an iterative process because it uses tail call optimization by making sure that the last expression in the procedure is a call to itself.

Version 1 calls itself in the middle of the final expression, meaning it must iterate recursively. It is not in tail call form.

## 1.2.2 Tree Recrusion

Okay I feel like the authors made a leap here, started talking about
steps vs space required for tree recursion, and they never explained
what that *is*. I get what steps are. But what is space, the
amount of memory needed to store the data structure?

This went from âdraw a circleâ to âdraw the rest of the owlâ really fast. I can kind of keep up with the text, but the exercises are veering wildly toward complex mathematical proofs that just donât interest me. I want to learn about how to organize programs, not proving that Fib(n) is the closest integer to phi to the n over the square root of five where phi equals 1 + the square root of five over two. (That is real, that is exercise 1.13.)

## 1.2.3 Orders of Growth

This is where the authors start talking about big O notation without ever calling it big O notation.

They instead describe a function R with a domain n equal to the size of the problem which returns the amount of resources needed for a process of size n.Â R(n) has a rate of growth equal to theta of R(n). And it is theta of n that is equal to O(n), which I guess has become the common manâs simplified version of theta of n because Iâm not writing this in LaTeX and I donât know how to write a theta, or any other kind of mathematical formula.

## 1.2.4 Exponentiation

Theta aka Big O is demonstrated here by comparing different methods for calculating exponents.

One recurring theme here is that when translating a function to a procedure, the most straightforward expression often includes a recursive process, and is also very expensive in terms of resources. (At least, that seems to be true of the examples they have chosen to show us so far.) And that rewriting them into a less expensive iterative process requires some mathematical intuition or know-how.

For example, exponentiation.

Given the definitions:

`b^n = b * b^(n-1)`

`b^0 = 1`

âŠconsider the following.

```
;; recursive process
;; eloquent, expressive, costly:
;; O(n) steps, O(n) space
define (exp b n)
(if (= n 0)
(1
* b (exp b (- n 1)))))
(
;; iterative process
;; slightly less eloquent, but also less costly:
;; O(n) steps, O(1) space
define (exp b n)
(;; creating an internal "iterator" procedure is a
;; common pattern for turning a recursive process
;; into an iterative one, because it allows you to
;; write the parent procedure in tail call form:
;; the last line is a call to a procedure that does
;; not need to be expanded by the interpreter
define (exp-itr b count product)
(if (= count 0)
(
product
(exp-itr b- count 1)
(* b product))))
(1)) (exp-itr b n
```

But then we get blindsided by mathematical cleverness.

Because we can also use this definition of exponents:

`b^n = (b^(n/2))^2`

where n is even`b^n = b * b^(n-1)`

where n is odd

âŠwhich allows us to write an even *better* procedure.

```
;; recursive process, but not iterating over *every* step
;; O(log n) steps, O(log n) space
define (fast-exp b n)
(cond ((= n 0) 1)
(even? n) (square (fast-exp b (/ n 2))))
((else (* b (fast-exp b (- n 1)))))) (
```

So there. I guess the pattern to learn here is that recursive processes can be very inefficient. Rewriting them as an iterative process can be more effecient. But if you can figure out some way to skip over certain iterations altogether, then that is even better. (Even though doing so may be difficult if, in these contrived examples, you donât have a background in math.)

## 1.2.5 Greatest Common Divisors

Euclidâs Algorithm states that the greatest common divisor of two
numbers `a`

and `b`

is equal to the greatest
common divisor of `b`

and the remainder of `a`

divided by `b`

.

```
define gcd (a b)
(if (= b 0)
(
agcd (b (remainder a b))))) (
```

Then the authors did something funny by saying âIf it takes
*k* steps to compute the GCD of some pair of numbers, then the
smaller number of the pair must be greater than or equal to the
*k*th number of the Fibonnaci sequenceâ, and thus the process
must have an order of growth of O(log n). Although the authors do
actually back up their claim about the Fibonnaci sequence in a footnote
using a mathematical proof, I donât think one should just say stuff like
that as though itâs true, as though itâs common knowledge. This is
becoming a running theme for me in this book so far. âAh, well you see,
as everybody knows, the impact of Euclidâs Algorithm on Lameâs
TheoremâŠâ

Nobody knows what that is. Not anybody in this one-person book club of mine anyway.

## 1.2.6 Example: Testing for Primality

In which *probabilistic* algorithms are introduced.

This is kind of interesting.

Without getting into the details, *Fermatâs Little Theorem*
describes an algorithm that makes a pretty good guess whether a number
is prime or not. For a random number between 1 and n - 1, inclusive, if
f(n) is true, then *n* is *probably* prime. So you just
run *f* a bunch of times with random values between 1 and n - 1,
and if you ever get a false result, then *n* is definitely
*not* prime. But if it evaluates to true for all given values
then *n* is *probably* prime.

This notion of probabilistic algorithms was disturbing to me because
it seems to undermine the *correctness* that we tend to expect
and demand of our computations.

But then the authors were kind of like, deal with it. Thatâs the difference between mathematics and engineering. Implying I guess that âprobablyâ is good enough for engineering. They went on to say that if tested thorougly the likelihood of a probabilistic algorithm returning an incorrect answer is the same as a solar radiation flare moving electrons around on your computer hardware and making an incorrect calculation.

But then they went on to say that there is a whole set of rare
numbers known as *Carmichael numbers* that will always pass
Fermatâs Theorem as true even though they are not prime! False
positives.

So, I donât know about all of that.

Fermatâs Theorem is hella fast, but it canât be assumed to be
*correct* every time, 100% of the time.

Applicative vs Normal mode continue to come up here, so that seems important. Also the idea of subsequent squairing as introdudced in exponentiation. I donât know if thatâs actually an important concept, or whether itâs just the chosen example with witch to talk about logarithmic growth. But I think I need to go back and try to understand that part a bit better.

## Summary

I think this post was kind of long. I might have bit off more than I
expected by trying to do one chapter per post. But now I know. Iâll
probably split chapters up into at least two posts from now on. So stay
tuned for *Chapter 1: Part 2*.

This chapter so far is very challenging because it seems to be all about advanced math, which I think distracts a great deal from the actual teachings. Hope it gets better!