SICP 1.2.4 Solutions

September 28, 2012 11:30

Ex 1.16

The mathematical solution to this one is relatively easy, as it can be based on the existing fast-expt procedure. The iterative one just stores the cumulative result in the a argument instead of backing it out by recursion.

To test the results, we compare the answers we get using the original fast-expt to our iterative version. For most of them, the results come out directly to make sure that both procedures are getting sensible values. Even though fast-expt was already provided, this is necessary to make sure that it wasn’t typed in wrong. At the end, I’ve added a simple equality test, since the number produced would be impossible to check by hand. We only have to check that the result is true (in Dr. Racket this is signified with #t).

The equality test uses the expression (= (fast-expt a b) (expo a b)), which is something a little bit new. Previously we’ve only compared actual numbers or variables, but the structure of Lisp makes it readily understandable. It’s going to evaluate (fast-expt a b) and (expo a b), and then test if the resulting answers are equal.

Ex. 1.17 & 1.18

I provided simple forms of double and halve to use for this exercise. At the time this book was originally written, this exercise may have had slightly more relevance to computers then in use (though it still applies to many embedded processors). Halving would be surely done as a binary operation, bit shifting to the right. This would only be accurate for even integers, where the last digit in binary is 0. Bit shifting to halve in that case is both simpler and faster. Using the division operation as I did here is slower, but for writing in Scheme, it’s a little bit clearer what is going on.

The mathematical solution is again fairly easy to understand. If b is even, it can be halved, and we double that result. If odd, then add a once and subtract one from b. The iterative version (1.18) is derived similarly to the solution for 1.16.

To test it, this time we save the ‘accurate answer’ as the value big-result. I’m using the built-in *, though the book-defined version could also be one to test against. That one is probably slower than the built-in version, but that would only illustrate the reason for big-result. Because it actually might take a little time to calculate it, we can compare it to the result of both procedures without having to recalculate it. Again, all we expect is a true from our equality comparison, with the assumption that the built-in * is correct.

Ex. 1.19

Viewing functions, and eventually computer programs, as transformations will be crucial to understanding some things much later in the book. It’s a good idea to make sure you have a full understanding of what’s going on in this exercise.

We have the transformation Tpq on (a,b) expressed as

abq + aq + ap
bbp + aq

Now we want to find the transformation Tp'q' which is equal to (Tpq)2. To do this, we’ll look at the results of applying Tpq twice and then find the equivalent expressions. In the following, the subscripts denote the result of applying T, i.e. a1 means T is applied once for a.

a1 = bq + aq + ap
b1 = bq + aq

a2 = b1q + a1q + a1p
b2 = b1p + a1q

Substitute to get in terms of a and b:
a2 = (bp + aq)q + (bq + aq + ap)(q + p)
b2 = (bp + aq)p + (bq + aq + ap)q

The transformation for Tp'q' is

abq' + aq' + ap'
bbp' + aq'

And we want to use a2 and b2 to make them equivalent. Using just b2, we can arrange it to get

b2 = b(p2 + q2) + a(2pq + q2)

p' and q' then equal the values in parentheses, since we are using b2 for b.

As a check, we can put these values into the expression for a2:

a2ab(2pq + q2) + ap2 + 2aq2 + 2apq

a2bpq + aqq + bqq + bpq + aqq + apq + aqp + app

a2 = (bp + aq)q + (bq + aq + ap)(q + p)

p' = p2 + q2
q' = 2pq + q2

Using those expressions in the procedure gives us our fast Fibonacci finder. To test it out, there are a few sample values given. These can be verified by any outside reference.

The final value is a quick check of the time taken. This line uses a Racket-specific function :

(time (void (fib 4000000)))

The time function (further details at the Racket reference page) will give us the amount of time that a procedure takes. It reports in a format like this:
cpu time: 4334 real time: 5059 gc time: 27
The time is in milliseconds. Usually only CPU time is relevant. The gc is for garbage collection, which is important to know as it can vary depending on what procedures were run previously. The procedure void is used to wrap the expression because time will make sure to return the value of the expression inside it. However, in this case we don’t want to show it because it would be far too large to deal with. All void does is toss out the result inside it, which makes it handy when we only care about the time taken. In this case, the time function shows that this procedure is indeed quite fast, as this number has on the order of a million digits. If you aren’t using Racket, you can get rid of that check (or try to find the equivalent timing function and see how long it takes to compute this value).

Solutions 1.2.4