Justin Does SICP

Ah that’s pretty cool about the ratio of big Fibonacci numbers approaching \phi

I was looking for other people’s answers for Exercise 1.10. E.g. @AnneB has one here:
And this guy has one:

They used a different notation than I did - like more standard exponent notation but with what I think is some composition of functions. I find it harder to follow. I think AnneB tries to show some of the steps but I didn’t follow them very well.

I think i liked the Rudy Rucker notation I found cuz it matched my intuitions well (about having a “tower of exponentiation” that includes the “bottom” value in the tower of stuff being repeatedly exponentiated). It seems worth maybe trying to think about stuff in different ways though.

having some kinda issue with Exercise 1.11.

so recursive was pretty easy for me (i named it fr cuz the r stands for recursive here)

(define (fr n)
  (if (< n 3) n
       (fr (- n 1))
       (* 2 (fr (- n 2)))
       (* 3 (fr (- n 3))))))

i’m having some trouble translating this into iterative version though.

i made a tree of some example operations with this function’s recursive implementation to understand how it works more ( (f 7) is abbreviated)

I’ve gotten stuck trying to translate from a recursive process to an iterative process. for now, my plan is to finish watching a couple of videos on this material (MIT guys have one, as does Brian Harvey) and then take another stab at it. Also I’m planning on looking carefully at the factorial example in the book (where they implement a function for finding factorials both recursively and iteratively) to see if I can describe a general method of going from a recursive to iterative process or some general principles or whatever. Thoughts welcome. I think it’s kind of interesting that I found the recursive form really intuitive and am finding the iterative form kinda difficult.

I looked at it as writing an iterative process from scratch, not translating a recursive process to an iterative one. Maybe that would help you.

I too find it easier to write Scheme programs recursively than iteratively. I don’t know if that’s because I’ve had more experience doing it that way or because Scheme is somehow set up to make recursion more natural.

figured it out :raised_hands::tada:. What I wrote on this to figure things out is pasted below

Let’s start with (fi 3) (the “i” standing for iterative).

For (fi n) where n = 3:

(fi (- n 1)) = (fi 2) = 2
(fi (- n 2)) = (fi 1) = 1
(fi (- n 3)) = (fi 0) = 0

To figure out (fi 3), we want sum up (fi 2) (or 2), 2 multiplied by (fi 1) (or 2), and 3 times (fi 0) (or 0). We get a total of 4 from doing this.

Suppose we want to figure out (fi 4). To figure out (fi 4), we’ll need the values of (fi 3), (fi 2), and (fi 1). When n = 3 for (fi n), those values are:
for (fi 3), the sum of (fi 2), 2 times (fi 1), and 3 times (fi 0). This is 4.
for (fi 2), the value of (fi (- n 1), which is 2.
for (fi 1), the value of (fi (- n 2), which is 1.

So within (fi 3), we can generate all the values we need for calculating (fi 4).

We could think of there being three key values. For a given n, there is the value of fi one step back (fi (- n 1), and the value of fi two step backs, and the value of fi three steps back ((fi (- n 2)).

We might call these 1ago, 2ago, and 3ago.
For (fi 3), these values are 2, 1, 0.
For (fi 4), these values are 4, 2, 1.
For (fi 5), these values are 11, 4, 2.
For (fi 6), these values are 25, 11, 4.
For (fi 7), these values are 50, 25, 11.

Notice that a new value is generated using the existing values in the first slot (for 1ago), and then that value recurs as we increase n by 1, first as 2ago, then as 3ago.

In order to have an iterative procedure that can solve Exercise 1.11 for arbitrary values, we will want to do a recursive call to that iterative procedure with appropriate transformations of these values.

So, suppose we start at a calculation of (fi 3) as our initial starting place with initial values of 2, 1, 0 for 1ago, 2ago, 3ago. How will we transform these values into a suitable form for calculating (fi 4)?

As our new 1ago argument, we’ll want to provide (+ 1ago (* 2 2ago)(3 3ago)). This is 4.
As our new 2ago argument, we’ll want to provide our current 1ago argument, which is (fi 2) or 2.
As our new 3ago argument, we’ll want to provide our current 2ago argument, which is (fi 1) or 1.

With these values provided, our procedure will be able to calculate (fi 4) by engaging in the appropriate transformations of them per the rule specified in the exercise.

I went awry partially cuz I was doing unnecessary operations because I hadn’t thought in sufficient detail about which operations were necessary. He’s some mistaken code that that I wrote:

;bad wrong code; not the answer!

(define (fi n)
  (if (< n 3) n
      (fi-helper 2 1 0 3 n)))

(define (fi-helper 1ago 2ago 3ago current max)
  (if (= current max)
      (+ 1ago (* 2 2ago) (* 3 3ago))
       (+ 1ago 2ago 3ago) ;WRONG!
       (* 2 1ago)   	  ;WRONG!
       (* 3 2ago)         ;WRONG!
       (+ current 1)

Multiple problems here.
One is that, for some reason, I’m doing a different operation on the numbers in my recursive call to fi-helper (specifically, (+ 1ago 2ago 3ago)) than I do as the consequent for the if (specifically, (+ 1ago (* 2 2ago) (* 3 3ago)). It was only after going through and writing all the stuff above that I realized clearly that these should be the same operation, and specifically that they should be (+ 1ago (* 2 2ago) (* 3 3ago)).
The second issue is that I am unnecessarily multiplying the 1ago and 2ago values by 2 and 3, respectively, in providing them as arguments for my recursive call. Going through the above analysis helped me realize that these values should be left untouched.

Below is a fixed and functioning fi program. Note that I also keep track of which n value we’re currently calculating (using current) and the final n we need to reach (with max).

(define (fi n)
  (if (< n 3) n
      (fi-helper 2 1 0 3 n)))

(define (fi-helper 1ago 2ago 3ago current max)
  (if (= current max)
      (+ 1ago (* 2 2ago) (* 3 3ago))
       (+ 1ago (* 2 2ago) (* 3 3ago))
       (+ current 1)

EDIT: trying to write things out really explicitly and precisely was helpful. i was mixing up what I was supposed to do when and was getting myself confused.

Mindmap helped too but sometimes I just really need to write 500-1k words and see if stuff clicks after doing that.

is it even a coding exercise at all? it seems like they’re asking for more of a math proof

Also, what do u actually need to know math-wise to solve 1.13

No. It’s like maybe late high-school advanced (like calculus) maths level.

quality-of-life link back up to 1.13

Not that much really. The problem tells you to use induction so you start with evaluating the base case (n=1) for Fib(n) = (\phi^n - \psi^n)/\sqrt{5}. Since we need to use the definition Fib(n) = Fib(n-1) + Fib(n-2) we probably need a 2nd base case too, for n=2. Then the goal is to show that the formula works for some n=k assuming that it already works for n=k-1 and n=k-2. I’d lay that out like:

Goal, show (\phi^{k-1} - \psi^{k-1})/\sqrt{5} + (\phi^{k-2} - \psi^{k-2})/\sqrt{5} = (\phi^k - \psi^k)/\sqrt{5}

\begin{aligned} LHS & = \text{<some algebra here>} \\ & = \dots \\ & = RHS \; \blacksquare \end{aligned}

once you show that the base cases work, and that the case n=k works provided that n=k-1 and n=k-2 work, then you’re done with the proof by induction (because if it works for n \in \{1,2\} then it must work for n=3, so it must also work for n=4, … etc).

The last bit of the exercise (which is actually the first sentence) is to show that Fib(n) is the closest integer to \phi^n/\sqrt{5}. Since we know Fib(n) = (\phi^n - \psi^n)/\sqrt{5}, one way to do that is show that -0.5 \le \psi^n/\sqrt{5} \le 0.5 which is fairly straight forward once you write it out. Note that \lim_{n \to \infty} \psi^n = 0 as |\psi| < 1.

I think maybe this is what you meant by evaluating base cases but I’m not sure. Feel free to correct me

Fib(1) = 1 per our definition of Fib(n)
So we want to show that
\frac{\phi^1 - \psi^1}{\sqrt{5}} = 1
Step 1: \frac{\phi^1 - \psi^1}{\sqrt{5}}
Step 2: \frac{1}{\sqrt{5}}(\frac{1 + \sqrt{5}}{2} - \frac{1 - \sqrt{5}}{2})
Step 3: \frac{1}{\sqrt{5}}{(\frac{2\sqrt{5}}{2})}
Step 4: 1

So Fib(2) = 1 per our definition of Fib(n).
So we want to show that:
\frac{\phi^2 - \psi^2}{\sqrt{5}} = 1

Step 1: \frac{\phi^2 - \psi^2}{\sqrt{5}}
Step 2: \frac{(\phi - \psi)({\phi + \psi})}{\sqrt{5}}
Step 3: \frac{1}{\sqrt{5}}{(\phi - \psi)({\phi + \psi})}
Step 4: \frac{1}{\sqrt{5}}{( \frac{1 + \sqrt{5}}{2} - \frac{1 - \sqrt{5}}{2})({\frac{1 + \sqrt{5}}{2} + \frac{1 - \sqrt{5}}{2}})}
Step 5: \frac{1}{\sqrt{5}}{(\frac{2\sqrt{5}}{2})({\frac{2}{2}})}
Step 6: 1

Yup, exactly. Nice finding the difference of two squares, too (I just expanded it, which is messier). :+1:

btw i noticed that Discourse seems to strip the mathjax-indicator $ signs when you quote math stuff for a reply; wonder if there’s an easy way to fix that on an automated basis

hmm not really sure how to do this step. spent some time trying playing around with algebra but idk what to do yet

btw I did part of this quote as an image cuz i didn’t want to fix the mathjax syntax

If you want a hint


  1. \phi^2
  2. \phi + 1
  3. \psi^2
  4. \psi+1

I googled for a hint (it’s tricky without knowing the above). after you know those bits it was much more straight forward to show LHS = RHS.

Yeah, good idea.

Well I tried for a while but couldn’t figure it out.

I’m looking at answers. I’m trying to understand AnneB’s at the moment, at the point where she says:

replace one ɸ * ψ with -1 in each of the two middle terms of the top half of the first fraction, since ɸ * ψ = -1:

I notice that for the middle values in the left fraction’s numerator, the difference in the exponent increased by 1 (going from (n-1) to (n-2)) compared to the previous step. I’m not crystal clear on why that’s happening.

My soln:

Goal: given the inductive hypothesis, show that

\begin{align} \frac{\phi^{k-2}-\psi^{k-2}}{\sqrt{5}} + \frac{\phi^{k-1}-\psi^{k-1}}{\sqrt{5}} & = \frac{\phi^{k}-\psi^{k}}{\sqrt{5}} \\ \text{} \\ \text{i.e.,}\;\;\phi^{k-2}-\psi^{k-2} + \phi^{k-1}-\psi^{k-1} & = \phi^{k}-\psi^{k} \end{align}


\begin{align} \phi^2 & = (\frac{1 + \sqrt{5}}{2})^2 \\ & = \frac{6 + 2\sqrt{5}}{2^2} \\ & = \frac{3 + \sqrt{5}}{2} \\ & = 1 + \frac{1 + \sqrt{5}}{2} \\ & = 1 + \phi \end{align}


\psi^2 = 1 + \psi

The proof

\begin{align} LHS & = \phi^{k-2}-\psi^{k-2} + \phi^{k-1}-\psi^{k-1} \\ & = \phi^{k-2}(1 + \phi) - \psi^{k-2}(1 + \psi) \\ & = \phi^{k-2}\cdot\phi^2 - \psi^{k-2}\cdot\psi^2 \\ & = \phi^{k} - \psi^{k} \\ & = RHS \;\;\; \blacksquare \end{align}

Yeah, so before that step, the numerator of the first fraction is

\begin{align} & = \phi^n + \phi^{n-1}\psi - \phi\psi^{n-1} - \psi^n \\ & = \phi^n + \phi^{n-2}(\phi\psi) - (\phi\psi)\psi^{n-2} - \psi^n \\ & = \phi^n - \phi^{n-2} + \psi^{n-2} - \psi^n \\ & = (\phi^n - \psi^n) - (\phi^{n-2} - \psi^{n-2}) \\ \end{align}

The second bracketed part then cancels with the numerator of the second fraction.

BTW, I really like Anne’s soln. The extra fact I used makes sense to find because you do some of the operations calculating Fib(2), but I think Anne’s is more creative.

It’s the transition from first line to second line there that I’m not getting.

I think I’m having a similar issue with your solution:

There are some missing lower level steps or something that I’m not following.

I do follow the stuff about \psi and \phi btw. I don’t think that’s the issue (could be wrong tho ofc). My guess is it’s something related to factoring or properties of exponents that’s intuitive enough for other people such that they don’t think it’s worth mentioning but that I need spelled out. Or could be that I’m just like misreading what’s happening.

I don’t expect unlimited help figuring this out btw … saying I’m still confused is partially about my own effort to practice honesty about such things.

Okay, so in Anne’s example (first image) the only thing that’s happening is factorization of the middle two terms. In my example (second image) there is both factorization and slight rearrangement going on. It might be simpler to understand like this:

Consider: \phi^n + \phi^{n-1}\psi - \phi\psi^{n-1} - \psi^n

Let m = n - 2, a = \phi, b = \psi. Thus:

\begin{align} & = \phi^n + \phi^{n-1}\psi - \phi\psi^{n-1} - \psi^n \\ \text{(sub $a$ and $b$)} \;\;\;\;\; & = a^n + a^{n-1}b - ab^{n-1} - b^n \\ \text{(sub $n = m + 2$)} \;\;\;\;\; & = a^{m + 2} + a^{m + 1}b - ab^{m + 1} - b^{m + 2} \tag{1} \\ \text{(factorize out $(ab)$ where possible)} \;\;\;\;\; & = a^{m + 2} + a^{m}(ab) - (ab)b^{m} - b^{m + 2} \tag{2} \\ \text{(sub $m=n-2$)} \;\;\;\;\; & = a^{n} + a^{n-2}(ab) - (ab)b^{n-2} - b^{n} \\ \text{(sub $\phi$ and $\psi$)} \;\;\;\;\; & = \phi^n + \phi^{n-2}(\phi\psi) - (\phi\psi)\psi^{n-2} - \psi^n \end{align}

The point of all the substitution stuff is that I guess it’ll be more straight forward to see what’s going on using normal letters for variables (instead of \phi and \psi) and using indices of the form m + 2 rather than n - 2.

The lines labeled 1 and 2 correspond to the operations/transformations going on in Anne’s thing.

Btw, if anyone wants to see the \LaTeX I used for the above (or whatever else) I think the easiest way to see it is check the edit history of the post. I think you can do that for any post to see what the source code was.


The factoring works for turning a^{m+1}b into a^{m}(ab) cuz a^m \times a^1 = a^{m+1} (I knew that but was having trouble thinking about it before cuz there were other complications getting in the way).

Something related which I think I was looking for earlier is a^m = a^{m - 1 + 1} = a^{m-1} \times a^1

1 Like

Glad I could help! One thing that might have helped you earlier (without the substitutions) is going backwards. Like, you could try this with my example – start with the 2nd line and multiply things out, and try to get to the first line.