Pohon BBS: recent posts

Excercise 1.8

2026-09-14T14:41:55+00:00

This one was just implementing the formula and then
%%replacing the old one in the "improve" procedure%%

%%(define (improve guess x)%%
%%(/%%
%%(+%%
%%(/ x (square guess))%%
%%(* guess 2))%%
%%3)%%

PHATCATTTT

2026-09-14T02:07:37+00:00

the tolerance I specified was kinda wide, which made the big number calculation off by a good amount (oops!). After reducing the tolerance to 0.000000000001 (rest of the code kept the same) i get the same result as my calculator (Casio fx-991EX). Dr racket seems to have rounded the small square roots output to 1e-7, im not sure if this is a fault in my code or just computer stuff (can anyone confirm?)

%% also i noticed that if im just comparing the guesses,i dont need to square them and then divide. I also had redundant (abs ) functions in the code so heres an improvement%%

%%(define (good-enough? guess x)%%
%%(< %%
%%(abs %%
%%(- %%
%%(/ %%
%%guess%%
%%(improve guess x)) 1)) 0.000000000001))%%

(sqrt 0.00000000000001) ;very small number test
(sqrt 1000000000000000) ;very large number test

outputs:
1e-7
31622776.601683907

New reply

2026-09-14T01:53:51+00:00

%%where 0.001 was the tolerance to see if the guess was good enough%%

Excercise 1.7

2026-09-14T01:50:23+00:00

The full code is VERY long and has multiple things needing to be defined so ill only post what was changed from the procedure defined in SICP. My thought process was:
need to compare guesses, so need to know guess-1 and guess-2
modify the improve definition to "save" the old guess? (oh no...)
OR
%%just use (improve guess x) itself as the guess 2%%

then
%%;modified from 1.1.7 for Exercise 1.7%%
%%(define (good-enough? guess x)%%
%%;(< (abs (- (square guess) x)) 0.001));old/original code [from SICP]%%
%%(< ;check if guess is within tolerance%%
%%(abs ; stops it from breaking if it gives a negative number%%
%%(- ;small difference between subsequent guesses => fraction ~1, so fraction-1 ~0%%
%%(/%%
%%(abs (square guess))%%
%%(abs (square (improve guess x)))) 1)) 0.001))%%
...
(sqrt 0.00000000000001) ;very small number test
(sqrt 1000000000000000) ;very large number test

answers were correct

HomePage revision

2026-09-13T16:35:36+00:00

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New reply

2026-09-10T08:23:05+00:00

%%testing different implementation of spoilers%%
%% test %%%
%%test test%%
%% %%
%% test%%

exercise 1.3

2026-09-09T20:55:03+00:00

%%%
[(define (square a)
(* a a))

(define (smallest x y z)
(if (< x y)
(if (< x z) x z)
(if (< y z) y z)))

(define (procedure x y z)
(- (+ (square x)(square y)(square z))(square (smallest x y z))))

(procedure 7 10 10)]
%%%

New reply

2026-09-09T16:32:09+00:00

%%%
test spoiler again

one two

three four
%%%

New reply

2026-09-09T16:31:09+00:00

%%%
test spoiler

test
%%%

epic phail

2026-09-09T15:35:08+00:00

Fixed the fix

%%#lang sicp%%

%%(define (square x) (* x x))%%

%%(define (sicp_ex_1.3 x y z)%%
%% (+%%
%% (if (= x y z) 0%%
%% (if (< x y) (if (< x z) 0 (square x)) (square x))))%%
%% (if (< y x) (if (< y z) 0 (square y)) (square y))%%
%% (if (< z y) (if (< z x) 0 (square z)) (square z)))%%

%%(sicp_ex_1.3 1 2 3)%%
%%(sicp_ex_1.3 2 2 2)%%
%%(sicp_ex_1.3 2 1 0)%%

fix ex 1.3

2026-09-09T15:27:28+00:00

summed all 3 squares if x y z were the same. added a check to just force one variable to zero if all the numbers were equal so theres probably a more elegant solution :P. Code:

%%#lang sicp%%

%%(define (square x) (* x x))%%

%%(define (sicp_ex_1.3 x y z)%%
%% (+%%
%% (if (= x y) 0 (if (= x z) 0%%
%% (if (< x y) (if (< x z) 0 (square x)) (square%% %%x))))%%
%% (if (< y x) (if (< y z) 0 (square y)) (square y))%%
%% (if (< z y) (if (< z x) 0 (square z)) (square z))))%%

%%(sicp_ex_1.3 1 2 3)%%
%%(sicp_ex_1.3 2 2 2)%%
%%(sicp_ex_1.3 2 1 0)%%

corrected exercise 1.3

2026-09-09T15:22:45+00:00

%%%
Updated to catch PhatCat's fix -- if x, y, and z are the same
value, the code does not work

(define (square x)
(* x x))
(define (sum-squares x y)
(+ (square x) (square y)))

(define (sum-greater-squares x y z)
(if (> x y)
(if (> y z)
(sum-squares x y)
(sum-squares x z))
(if (> x z)
(sum-squares y x)
(if (= x y z)
(sum-squares x x)
(sum-squares y z)))))
%%%

Spoiler hack (temporary)

2026-09-09T05:15:06+00:00

I am too lazy to make a proper "spoiler field" (similar to markdown's
code fences) to spoiler-ify multiple lines of text right this second, so if
you need to make an entire post a spoiler post, do it exactly like this:

%%%
The text you want to hide across multiple
lines -- the entire body of a post
%%%

That's 3 %'s at the very start of a post,
immediately press enter to break the line,
everything else,
enter,
3 %'s again with nothing following.

Later I'll probably just add a [ ] Spoiler checkbox but I've been up for
something like 30 hours and now is not the time for that. Be patient!
%%enjoy and use responsibly!%%

New reply

2026-09-09T02:00:21+00:00

%%%
%%%
test test
test
test
test
%%%
%%%

New reply

2026-09-09T01:58:36+00:00

%%%
Testing the laziest
possible implementation
of multi-line spoilers:
first line is EXACTLY %%%, newline,
last line is EXACTLY %%%, nothing else
%%%

New reply

2026-09-09T01:57:31+00:00

%%%
test
%%%

Excercise 1.3

2026-09-09T01:43:46+00:00

My attempt at exercise 1.3. it took me a long time as most of my attempts made code that excluded anything that wasn't the maximum number, so i guess finding the "middle" number is what makes this challenging. my first attempts were checking each variable against the others individually (mistake). if x>y then output x otherwise 0, if x>z output x otherwise 0 then multiply both outputs, repeat for each variable (rather silly in retrospect). After this i decided to only compare to the third variable if the first check failed and directly compute the square if the variable wasn't the minimum. Code below:

%%#lang sicp%%
%%(define (sicp_ex_1.3 x y z)%%
%% (+%%
%% (if (< x y) (if (< x z) 0 (* x x)) (* x x))%%
%% (if (< y x) (if (< y z) 0 (* y y)) (* y y))%%
%% (if (< z y) (if (< z x) 0 (* z z)) (* z z))))%%
%% %%
%%(sicp_ex_1.3 1 2 3)%%
%%result was 13 so it worked%%

exercise 1.3

2026-09-08T21:50:10+00:00

%% (define (square x) %%
%% (* x x)) %%
%% (define (sum-squares x y) %%
%% (+ (square x) (square y))) %%
%% %%
%% (define (sum-greater-squares x y z) %%
%% (if (> x y) %%
%% (if (> y z) %%
%% (sum-squares x y) %%
%% (sum-squares x z)) %%
%% (if (> x z) %%
%% (sum-squares y x) %%
%% (sum-squares y z)))) %%
%% %%
%% (sum-greater-squares 1 2 3) %%
%% (sum-greater-squares 4 2 3) %%
%% (sum-greater-squares 5 1 0) %%

New reply

2026-09-08T21:48:46+00:00

test

%%%
test
test
test
%%%

New reply

2026-09-08T20:11:17+00:00

Famous article by a guy who used Lisp to become a billionaire :

> Graham, Paul. "The Roots of Lisp." (2001) 15pp
https://wiki.eecs.yorku.ca/course_archive/2014-15/W/6339/_media/jmc.pdf

discusses how 7 primitives became the meta language

some sample lisp code

2026-09-08T19:51:10+00:00

(define pi 3.14159265)
(define (square x) (* x x))

(define (volume-of-cone radius height)
(* pi (square radius) (/ height 3)))

(define my-cone-volume (volume-of-cone 1 2))

my-cone-volume
=> 2.09

outputs color based on stats (hopefully)

2026-09-08T19:37:11+00:00

(define (clouds_color strength intelligence Defense)
(define red strength)
(define blue intelligence)
(define green defence)
(list red green blue))) % gives RGB color (idk how to/
if its even possible for lisp to output a color)

Chapter 5

2026-09-08T19:30:51+00:00

> Computing with Register Machines

https://mitp-content-server.mit.edu/books/content/sectbyfn/books_pres_0/6515/sicp.zip/full-text/book/book-Z-H-30.html#%_chap_5

Chapter 4

2026-09-08T19:29:38+00:00

> Metalinguistic Abstraction
https://mitp-content-server.mit.edu/books/content/sectbyfn/books_pres_0/6515/sicp.zip/full-text/book/book-Z-H-25.html#%_chap_4

Chapter 3

2026-09-08T19:28:36+00:00

> Modularity, Objects, and State

https://mitp-content-server.mit.edu/books/content/sectbyfn/books_pres_0/6515/sicp.zip/full-text/book/book-Z-H-19.html#%_chap_3