FPChris

FPChris

The Ray Tracer Challenge: Page 37 4x4 cofactor?

On Page 37. How to calculate the cofactor for a 4x4 matrix is never shown. Only the 3x3.
How is the author getting the cofactor(A,0,0) = 690 section? and then the determinant -4071?

Marked As Solved

FPChris

FPChris

I finally resolved this. The book is really unclear. Terrible in fact.
If anyone needs hep on this. Here is my solution below.
This YT video was a great help Finding the determinant of a 3x3 matrix method 2 | Matrices | Precalculus | Khan Academy - YouTube

//-----------------------------------------------------------------------------------
function RCCalcCofactor4x4(A,row,col){

let B = RCGetSubmatrix4x3(A,row,col); //4x4 > 3x3
let s,a;
s = RCGetSubmatrix3x2(B,0,0); //3x3 to 2x2
a = B[0][0] * RCCalcDeterminant2x2(s);
s = RCGetSubmatrix3x2(B,0,1);
a += -B[0][1] * RCCalcDeterminant2x2(s); //Note: -B
s = RCGetSubmatrix3x2(B,0,2);
a += B[0][2] * RCCalcDeterminant2x2(s);

if(isOdd(row + col)){ return -a; } //odd so negate
return a; //even - leave alone. just return it

}
//-----------------------------------------------------------------------------------
function isOdd(num) { return num % 2;}
//-----------------------------------------------------------------------------------
function RCCalcDeterminant4x4(A){
// Ch 3 - page 37
/*
A[0][0] = -2; A[0][1] = -8; A[0][2] = 3; A[0][3] = 5;
A[1][0] = -3; A[1][1] = 1; A[1][2] = 7; A[1][3] = 3;
A[2][0] = 1; A[2][1] = 2; A[2][2] = -9; A[2][3] = 6;
A[3][0] = -6; A[3][1] = 7; A[3][2] = 7; A[3][3] = -9;
*/

let d0 = RCCalcCofactor4x4(A,0,0); //690
let d1 = RCCalcCofactor4x4(A,0,1); //447
let d2 = RCCalcCofactor4x4(A,0,2); //210
let d3 = RCCalcCofactor4x4(A,0,3); //51

let d = (A[0][0] * d0) + (A[0][1] * d1) + (A[0][2] * d2) + (A[0][3] * d3); //-4071

return d;

}

Also Liked

FPChris

FPChris

Thanks. I think I struggled mostly throughout with being presented with a test and not realizing they would fail initially and that solutions where sometimes pages away. I eventually caught on to this and began reading through several more pages before coding a new topic.

Where Next?

Popular Pragmatic Bookshelf topics Top

New
iPaul
page 37 ANTLRInputStream input = new ANTLRInputStream(is); as of ANTLR 4 .8 should be: CharStream stream = CharStreams.fromStream(i...
New
jdufour
Hello! On page xix of the preface, it says there is a community forum "… for help if your’re stuck on one of the exercises in this book… ...
New
AleksandrKudashkin
On the page xv there is an instruction to run bin/setup from the main folder. I downloaded the source code today (12/03/21) and can’t see...
New
jeremyhuiskamp
Title: Web Development with Clojure, Third Edition, vB17.0 (p9) The create table guestbook syntax suggested doesn’t seem to be accepted ...
New
adamwoolhether
I’m not quite sure what’s going on here, but I’m unable to have to containers successfully complete the Readiness/Liveness checks. I’m im...
New
brunogirin
When running tox for the first time, I got the following error: ERROR: InterpreterNotFound: python3.10 I realised that I was running ...
New
oaklandgit
Hi, I completed chapter 6 but am getting the following error when running: thread 'main' panicked at 'Failed to load texture: IoError(O...
New
hazardco
On page 78 the following code appears: <%= link_to ‘Destroy’, product, class: ‘hover:underline’, method: :delete, data: { confirm...
New
mert
AWDWR 7, page 152, page 153: Hello everyone, I’m a little bit lost on the hotwire part. I didn’t fully understand it. On page 152 @rub...
New

Other popular topics Top

PragmaticBookshelf
Free and open source software is the default choice for the technologies that run our world, and it’s built and maintained by people like...
New
dasdom
No chair. I have a standing desk. This post was split into a dedicated thread from our thread about chairs :slight_smile:
New
AstonJ
We have a thread about the keyboards we have, but what about nice keyboards we come across that we want? If you have seen any that look n...
New
Exadra37
Oh just spent so much time on this to discover now that RancherOS is in end of life but Rancher is refusing to mark the Github repo as su...
New
Margaret
Hello everyone! This thread is to tell you about what authors from The Pragmatic Bookshelf are writing on Medium.
1147 29994 760
New
AstonJ
Biggest jackpot ever apparently! :upside_down_face: I don’t (usually) gamble/play the lottery, but working on a program to predict the...
New
hilfordjames
There appears to have been an update that has changed the terminology for what has previously been known as the Taskbar Overflow - this h...
New
sir.laksmana_wenk
I’m able to do the “artistic” part of game-development; character designing/modeling, music, environment modeling, etc. However, I don’t...
New
mindriot
Ok, well here are some thoughts and opinions on some of the ergonomic keyboards I have, I guess like mini review of each that I use enoug...
New
PragmaticBookshelf
Use advanced functional programming principles, practical Domain-Driven Design techniques, and production-ready Elixir code to build scal...
New

Sub Categories: