# I put my thing down, flip it and reverse itNovember 25, 2016 7:47 AM   Subscribe

Since we all know that the day after Thanksgiving is Math Friday, and we all need to know matrix multiplication for our everyday lives, it's perfect that we now have this lovely tool.
posted by selfnoise (12 comments total) 50 users marked this as a favorite

Okay, that "lovely tool" is REALLY nice. Thanks for posting.
posted by wittgenstein at 7:57 AM on November 25, 2016 [3 favorites]

Man, I needed that lovely tool back in high school calculus or whichever you learn this in. For whatever reason I just could not visualize matrix multiplication very well at all and it limited my advancement (or possibly simply discouraged me) in vector and diffeq.

Watching that thing go a couple of times just made the whole thing click in my brain here almost fifteen years later.
posted by cmoj at 8:10 AM on November 25, 2016 [1 favorite]

Wow. I made it pretty far in math in high school. enough to where in college, I could just opt out. I only very vaguely remember matrices, but I defnitely remember not liking them. But with the tool, I just get it now. It makes sense. I'm a HUGE proponent for visual or physical representation for math concepts, it makes it real
posted by FirstMateKate at 8:33 AM on November 25, 2016

I strongly recommend 3blue1brown's video series on linear algebra if you want to get a visual intuition for why matrix multiplication (and much more) is the way it is. Previously (but since completed)
posted by Jpfed at 9:04 AM on November 25, 2016 [6 favorites]

It's very different from the pencil-and-paper method I learned in school. I'm not convinced it's really practical on paper- just transposing a matrix is kind of a huge pain to do by hand. The method of "dot-producting" the rows and columns from the first link was closest to what I learned. This page has a handy animation of where to put your fingers (very helpful).

You can also arrange your matrices like so, which makes it super obvious where each element of the final matrix (the xs) comes from (I'm pretty sure I'm doing this right, it's been a while...)
```     1 2
4 5
0 1  x x
2 3  x x

1          2
4          5
0 1  0(1)+1(4)  0(2)+1(5)
2 3  2(1)+3(4)  2(2)+3(5)

4  5
14 19
```
Matrix multiplication is much less fun than linear algebra, which is awesome and you should watch this video series.
posted by BungaDunga at 9:09 AM on November 25, 2016 [5 favorites]

jinx, Jpfed!
posted by BungaDunga at 9:09 AM on November 25, 2016

There's also this matrix calculation website which is a little less user-friendly but has other useful operations, and shows step-by-step procedures.
posted by The Pluto Gangsta at 9:26 AM on November 25, 2016

Had Microsoft not made the decision to drop BASIC's MAT commands from their micro versions, and then omit them from their version of the language from then on, we'd be all so-what about matrix manipulation tools. There are a few implementations of ANSI BASIC that kept the faith, though.
posted by scruss at 1:56 PM on November 25, 2016

For some reason, facebook is blocking my attempt to post that last link.
posted by oheso at 4:46 PM on November 25, 2016

I have to remind myself of this about once a year
posted by grobstein at 5:14 PM on November 25, 2016

Nobody's made the obvious xkcd reference yet!
posted by iffthen at 9:11 PM on November 25, 2016 [1 favorite]

For years I've been referring to Matrix multiplication: an interactive micro-course for beginners. Apparently I multiply matrixes by hand so rarely that I am perpetually a beginner.

It doesn't do the calculations for you; it's just a reminder of how the arithmetic works.
posted by foobaz at 12:38 AM on November 27, 2016

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