The h Bar

4:10 PM

@blue A Java program isn't executed line by line.

The compiler goes through the code compiling it to byte code, and the compiler makes several passes. So it won't attempt to compile the private Node first = null; until after it has processed the code defining the Node class.

It's really no different to the fact that function declarations can come after the first call to that function in the source code.

Anonymous

@JohnRennie Ah. I didn't know that! Thanks

Anonymous

Interesting. I haven't thought about this before

dmckee

@Blue There are languages designed to support a single-pass compiler (and therefore possible to comprehend on a straight read-through). Pascal for one.

But such languages have to add extra syntax to allow programmers to break-open closed cycles in type structures and calling trees.

John Rennie

Compiler design is at the nerdy end even even for computer nerds :-)

dmckee

E.g. `forward declaration'.

John Rennie

4:16 PM

Well C has forward declaration ...

Anonymous

Oh. I've heard about Pascal. I'll read about it. :)

dmckee

@JohnRennie I can't off the top of my head think of any features of c that require more than one pass. Though I don't know of an example of a 1-pass c compiler.

Anonymous

But I'm now stuck with another java stuff:

Anonymous

John Rennie

@Blue: see

One-pass compiler

In computer programming, a one-pass compiler is a compiler that passes through the parts of each compilation unit only once, immediately translating each part into its final machine code. This is in contrast to a multi-pass compiler which converts the program into one or more intermediate representations in steps between source code and machine code, and which reprocesses the entire compilation unit in each sequential pass. This refers to the logical functioning of the compiler, not to the actual reading of the source file once only. For instance, the source file could be read once into temporary...

Anonymous

4:18 PM

Anonymous

I can't understand how they are getting N+(2+4+8+...) and how it is approximately 3N

Anonymous

Using GM series summation it should be something of $2^N$ order

Leaky Nun

@Blue no, $2^{1+\log_2N} = 2N = 2+(2+4+8+\cdots+N)$

dmckee

Crenshaw's Let's Build a Compiler walks the reader through writing a one-pass, recursive descent compiler for a simplified pascal-like language without most of the theory typically found in the first few chapters of a compilers textbook.

Anonymous

@LeakyNun Wait, how did you get that?

Anonymous

4:20 PM

Could you explain?

Leaky Nun

@Blue $2+4+8+16+32+64 = 126 = 128-2 = 2\times64-2$

you only add to $N$, so you only have $\log_2N$ terms

Anonymous

Umm, N is not the number of terms? We are going to add upto $N$? i.e. $2+4+8+...+2^x$ where $2^x \leq N$

Leaky Nun

@Blue yes

2 + 4 + 8 + ... + N is what is written

not $2 + 4 + 8 + ... + 2^N$

Anonymous

@LeakyNun I thought N terms

Anonymous

:P

Leaky Nun

4:23 PM

if your stack has 128 items, would you need 2^128 spaces?

Anonymous

Okay, second problem solved. Now the first problem

Anonymous

I can't understand how they got N+(2+4+8+....)

Anonymous

I'm trying to explain my point of view

Anonymous

Say, we do N push operations

Leaky Nun

Is Java your first language?

Anonymous

4:25 PM

@LeakyNun Ah, BASIC was my first language. But I guess that doesn't count

Anonymous

Then Java

Leaky Nun

so you only know BASIC and Java?

Anonymous

@LeakyNun Yes

Anonymous

I'm learning C

Leaky Nun

ok, continue

vzn

4:29 PM

@Blue hey whats up are you still learning QM?

Anonymous

Uh, sorry. Had to go for a while

Anonymous

@vzn Yea

Anonymous

Suppose we have the array $\{1\}$

Anonymous

Then we double it $\{1,2\}$

Anonymous

So, upto this step we had 1 array access for copy

Anonymous

4:31 PM

And another array access for push

Leaky Nun

yes

Anonymous

Say we denote it like $1|1$

vzn

@Blue what did you mean youre a "high school passout"? what country are you in?

Anonymous

@vzn I just joined uni

vzn

@Blue did you graduate HS?

Anonymous

4:32 PM

India

Anonymous

@vzn How can one get into uni without passing HS :P

Anonymous

Umm, so then we again double it

vzn

@Blue just not sure what you meant by "passout"... apparently "graduate"... eg in our language "dropout" means "not graduated"

Anonymous

$\{1,2,3,4\}$

Balarka Sen

generally high school makes me pass out

so sure

Anonymous

4:34 PM

So, $(1+2)|(1+2)$ (push|copy)

vzn

← probably enjoyed school at least as much as BaS

@Blue so are you studying physics? a class? does it cover QM? what is your major?

Anonymous

Then, again we double it $\{1,2,3,4,5,6,7,8\}$

Anonymous

$|(1+2+4)|(1+2+4)|$

vzn

what about you @BalarkaSen any interest in QM?

Balarka Sen

← Just trying to do this cool self-arrow thing for no good reason

4

Anonymous

4:36 PM

So, I getting something of the form $2(1+2+4+...N)$ @LeakyNun

Balarka Sen

Not particularly. I borrowed a copy of Griffth's QM once. It mostly seemed like badly parsed mathematics instead of physics to me. It's interesting, for sure.

vzn

@BalarkaSen congratulations you succeeded! :P

Leaky Nun

@Blue you need $N$ operations to push $N$ items

Anonymous

@vzn Electronics is my major.

vzn

@BalarkaSen (lol) do you have any tolerance for alternative approaches?

Anonymous

4:37 PM

Yeah, we do have physics courses

Anonymous

But not rigorous QM (Just the basics)

Anonymous

I'm studying QM on my own mostly

vzn

@Blue are you taking a physics course right now? is it reqd for your major?

John Rennie

I need a pair of young eyes to help me read some text off a really blurry image. Any volunteers?

Anonymous

@vzn Yes. Solid State Physics, Thermodynamics and Classical Mechanics are my current courses

vzn

4:39 PM

96 refs to "quantum" by BaS :) chat.stackexchange.com/search?user=103383&q=quantum

Balarka Sen

@vzn Alternative approach to which?

Anonymous

@LeakyNun But I've already included the push operations in my calculations. Am I missing something?

Leaky Nun

@JohnRennie post it?

vzn

@BalarkaSen am working on a new interpretation of QM. want to run it by some smart but "unconditioned" audience... planning to post a question on it maybe tonite...

Balarka Sen

isee.winrar

vzn

4:41 PM

huh?

Leaky Nun

@Blue consider $9$ elements

John Rennie

If you look at this URL then the fourth picture along shows the label on the underneath of the laptop. I need the Service Tag off that label.

0celo7

@BalarkaSen I was just told 2 papers in good journals is necessary for a good grad school

djsmiley2k

@vzn i dont know much more than a layman, if u want me to look

vzn

@djsmiley2k (thx) any physics bkg? yeah the more the merrier :)

John Rennie

4:42 PM

If you click on the fourth picture along then hover the mouse over the image it magnifies it. But just not quite enough for me to read the service tag off the label.

Balarka Sen

@vzn It's a passing meme which is overused by a group of people in the math chat, nevermind. I'd like to see the question, but I can't promise I'll read it carefully. I don't really know any physics.

@0celo7 Better start writing then

djsmiley2k

@vzn highschool physics + plus a interest

Leaky Nun

@JohnRennie SERVICE TAG(S/N) 4JKRM12

vzn

@djsmiley2k ok cool. the prj is to try to look at classical analogs of the wavefn. it turns out much of the mathematics shows up in classical physics but it seems almost nobody really points this out much. am looking to tighten that connection.

Leaky Nun

@JohnRennie 1903537?62 EXPRESS SERVICE CODE

John Rennie

4:45 PM

@LeakyNun Thanks, but it isn't that. If you open this link it looks up the service tag on the Dell site. But the tag you've suggested doesn't match.

djsmiley2k

@JohnRennie AJKR and then 1i +c2an2't 301ee

and a 1 yr old is attacking my keyboard :D

John Rennie

@djsmiley2k :-)

Leaky Nun

@JohnRennie how can I know if I read it correctly?

John Rennie

@LeakyNun follow the link to the Dell web site and you'll see that 4JKRM12 is the tag for a E5440 not an E6440 as the post describes.

0celo7

@BalarkaSen what's an easy area of math

Leaky Nun

4:47 PM

@JohnRennie ok

0celo7

Is there a journal for topoi?

John Rennie

@LeakyNun I've spent ages trying to guess what the tag is and putting it into the Dell site to see if it matches, but with no luck so far.

djsmiley2k

hmmm

+ok that broke imgur XD

Balarka Sen

@0celo7 no idea

everything looks too hard

try some topological robotics; fresh branch

Anonymous

@BalarkaSen What's that?

Balarka Sen

4:55 PM

Oh it's something I heard about recently, very cool stuff

Say you have a, uh,

linkage. Right, that's the word.

Anonymous

Sounds like a topic for CS guys :P

Anonymous

Go on

Balarka Sen

It's actually math.

Anonymous

Okay

Balarka Sen

So a linkage is like a bunch of axes with the end of one axis attached to the front of another.

And the end of the very first axis is fixed to the origin.

Like, a thing like "V" is a linkage.

Leaky Nun

4:57 PM

@JohnRennie 4QKPYZ1

Balarka Sen

Where the first vertex of the "V" is fixed.

So it's free to move with some constraints like those, you see

The angle between the two hands of "V" is allowed to vary

It's like our hand

John Rennie

@LeakyNun well that is an E6440, but the second letter of the tag is surely a J

Anonymous

Oh. I think I am getting a rough idea of it.

Leaky Nun

@JohnRennie does it matter?

John Rennie

@LeakyNun yes, because I want the manufacture date of the laptop. The tag uniquely identifies the specific laptop so it tells you when it was made and when the warranty runs out.

Leaky Nun

5:00 PM

@JohnRennie alright

Balarka Sen

@Blue So given a linkage, which can consist of as many hands as you want, you can ask what is it's moduli space. That is basically a thing which keeps track of how much degree of freedom the linkage has.

John Rennie

@LeakyNun That's the nice thing about Dells. If you're buying secondhand the tag can be used to check all the details and even check that it hasn't been reported stolen.

Anonymous

So....it's like constraint analysis

Anonymous

(Hope that sounds right)

Balarka Sen

A linkage with one hand can rotate 360 degrees, so the moduli space is the circle, or as fancy mathematicians call it, "S^1". A linkage with two hands, well, each hand is allowed to rotate 360 degrees, so the moduli space is S^1 x S^1, which is a torus. And so on.

John Rennie

5:02 PM

@LeakyNun I suspect it's a lost cause. I've e-mailed the seller to ask them the tag and I'll just have to wait and see if they reply.

Anonymous

@BalarkaSen I see. Pretty interesting

Leaky Nun

@JohnRennie what's your best guess of the express service code?

Balarka Sen

@Blue You can also ask the more straightforward question of what are the possible motions you can get from a planar mechanical linkage.

John Rennie

@LeakyNun 4JKHM12 but that isn't correct

Leaky Nun

@JohnRennie I mean, the 10-digit number

Balarka Sen

5:08 PM

Stuff like that can happen. In fact any planar algebraic curve can be obtained from motion of a linkage.

John Rennie

@LeakyNun I can't read that at all ...

Anonymous

@BalarkaSen I think I heard that before....somewhere

Anonymous

Yeah, that's true

Anonymous

There is a proof also iirc

Balarka Sen

Yes, of course there is a proof. It's all math :P

I think it's nontrivial to obtain linear motion from linkages.

Anonymous

5:11 PM

But, is this really a new branch? Constraints have been studied for a long time

Anonymous

I'm not sure though

Balarka Sen

Topological robotics is, stuff like studying configuration/moduli spaces of linkages.

There's a lot of open questions and potentials in there.

Anonymous

@BalarkaSen Huh, I agree

Balarka Sen

At least, that's what someone I met in the workshop who is interested in this told me.

Anonymous

How were the other students you met in the workshop?

Balarka Sen

5:14 PM

Some were pretty cool.

The one who told me about this was actually an engineering student, who switched to math in 2010 or something and is now a grad student in TIFR

He knows his stuff

There were a few grad students from TIFR, one I met is kind of a demigod of a sort in geometry and is now going to Princeton

Anonymous

wow

Anonymous

cool stuff

Balarka Sen

There was a big CMI crowd

Anonymous

@BalarkaSen Of course. All the math nerds go to CMI :P

Anonymous

BTW this year I came to know a very very weird math nerd

Balarka Sen

5:18 PM

I had a nice lunchtime chat with a guy from CMI who smokes weed and watches movies

Anonymous

He was an IMO medalist

Anonymous

But he joined Medical

Anonymous

:P

Anonymous

X'D

Balarka Sen

vOv

Anonymous

5:19 PM

He was in the top 100 in JEE last year

Anonymous

Phew

Anonymous

He seems to be completely crazy

Anonymous

A doctor who is a math-nerd. Weird combo.

Anonymous

@BalarkaSen Well, that's not uncommon

Anonymous

Many people in colleges smoke weed

Anonymous

5:21 PM

Even in very good colleges

0celo7

I need to make an advertisement

Balarka Sen

I haven't seen a random college dude who knows Tarkovsky in the night sky

0celo7

"Selling soul for analysis writing credit"

John Rennie

@Blue I have to say I found cannabis destroys your ability to do maths for 24 hours.

0celo7

I wonder what the longest time I've gone without any math is

John Rennie

5:22 PM

I pretty much gave it up after I discovered that.

0celo7

Sometimes I will open a book at 3am and pretend to read

Alex K Chen

@Blue Kapil Pause ?

Balarka Sen

@JohnRennie I have heard that from actual potheads, actually.

Anonymous

@AlexKChen Nah. An Indian guy named Tamoghna Ghosh. He was in the same institute as me. :P

Alex K Chen

@Blue I remember seeing a profile pic of him on quora in FIITJEE uniform... that guy ?

0celo7

5:24 PM

Is quora an Indian website

Anonymous

@AlexKChen Yes. He is in AIIMS Delhi now

Alex K Chen

quora.com/profile/Tamoghna-Ghosh-6

0celo7

Or are Indians that desperate for homework answers

Alex K Chen

How the hell he went to IMO ?! He never went to IMO, but he cleared INMO, though.

Anonymous

@AlexKChen He did.

0celo7

5:25 PM

Aaaaand the chat is about JEE et al

Alex K Chen

@Blue Which year ? 2015 ?

Balarka Sen

lol

Anonymous

@AlexKChen 2016

Secret

My program hell in a nutshell:

2

Q: Hold remainder of shell script commands until PBS qsub array job completes

I am very new to shell scripting, and I am trying to write a shell pipeline that submits multiple qsub jobs, but has several commands to run in between these qsubs, which are contingent on the most recent job completing. I have been researching multiple ways to try and hold the shell script from ...

Alex K Chen

Hi @BalarkaSen, can you give me a tour of the interesting stuffs about algebraic curve ? (Mostly connection to NT and other stuff ?)

Anonymous

5:26 PM

@0celo7 If you wish, I'll stop

0celo7

@Blue in a class

You can continue

Anonymous

@0celo7 That's true

Alex K Chen

@Blue Of course you're (or he is) lying

Anonymous

@AlexKChen Maybe he didn't win a medal

Alex K Chen

@Blue It shows you all the 6 guys from a country, and only 6 contestants from a country can participate, regardless of they get medal or not.

vzn

5:29 PM

@JohnRennie youre familiar with the wave eqn for classical physics right? wondering if anyone has tried to figure out exactly how it differs in QM... seen anything along those lines?

Alex K Chen

@BalarkaSen Did you see my question ?

Balarka Sen

@AlexKChen That's a too general thing to ask for.

Besides, I don't know number theory.

Alex K Chen

@BalarkaSen Oh really ?!?!

Anonymous

@AlexKChen I don't know. I know this much that he went to New Zealand for IMO (coz he was absent for about a month). Either someone gave me wrong information or he lied or the teachers who told me lied.

0celo7

Number theory is pretty boring

There's some interesting ergodic stuff

Alex K Chen

5:32 PM

@Blue You can verify it: There's a Physics SE user who was in the 2016 India team (and was in this year's team too)

Anonymous

@AlexKChen Huh. Who?

Alex K Chen

I forgot the userID, but do remember seeing him on Aops.

Balarka Sen

@0celo7 Ergodicity is good stuff.

DanielSank

Hi, everybody.

Prathyush Poduval

Hello

@Blue IMO was never held in newzealand. Maybe it was APMO

Phase

5:43 PM

@0celo7 favourite Humble meme yet

Balarka Sen

looool

Anonymous

@PrathyushPoduval ah...maybe

Anonymous

That's more probable

Balarka Sen

I can really connect to this meme

Prathyush Poduval

But then I tried a google search and came to know APMO is held in the respective countries :|

Anonymous

5:49 PM

I really don't know the details

Anonymous

Maybe it was a rumour

Anonymous

Created by some guys at fiitjee

Anonymous

I never bothered to check whether he was on the list

dmckee

@Blue BASIC counts: serious programs have been written in it.

Though many BASIC variants instill bad habits and I would encourage people to chose something else for a newbie's first language when you have influence.

Anonymous

@dmckee My fist language was actually Logo

Anonymous

5:53 PM

lol

Anonymous

But I feel too ashamed to mention it

Anonymous

:'D

dmckee

Nothing wrong with LOGO as a language.

Anonymous

Ah, turtles n all :P

dmckee

The hype surrounding it and the pathological focus on the drawing abstraction was over the top and rather nauseating.

Anonymous

5:56 PM

@BalarkaSen Can we continue with Linear Algebra tomorrow afternoon around 3pm-5pm? I want to wrap my CS lesson by tonight or else it keeps getting delayed.

Anonymous

I'm free all afternoon

Anonymous

If you are free I'll ping you at 3pm sharp

Balarka Sen

@Blue Sure

Anonymous

Thanks. So 3pm-5pm. :)

0celo7

6:25 PM

@BalarkaSen I found a GSM about ergodic theory for Riemannian manifolds. No clue if I'll ever read it

Sid

@WrichikBasu looks like your proposal got deleted

0celo7

My reading list has grown by 10 since last week

It does seem pretty interesting

Leaky Nun

6:42 PM

@JohnRennie are you still here?

EdinburghDruid

Hi guys would appreciate some help if possible!
For a Deuterium molecule, I have that $E_l = \dfrac{\hbar^2l(l+1)}{mR^2}$.What are the allowed values of l if the total spin is 0 or 2? I think it is 0,2,4 since individually the nuclei can only have l=0,2. Would you agree?

Leaky Nun

The barcode reads 4jsbm12[ with checksum 104x1+20x1+74x2+83x3+66x4+77x5+17x6+18x7 = 1294 which leaves a remainder of 59 when divided by 103, corresponding to the character [, which validates my reading.

the interpreted serial tag is 4JSBM12 which does not correspond to anything.

And yes, I spent some time with a table of how to decode barcode.

I guess I'm two hours too late

djsmiley2k

@LeakyNun that.... is amazing

Leaky Nun

edit: 4JSBM12 is valid.

djsmiley2k

:O

@JohnRennie ^^

Ok, that is getting bookmarked, Bravo!

Balarka Sen

7:33 PM

@Avantgarde Holy shit things get more and more dark and (redacted: flipped) up in this album trilogy of Walker.

The Drift is a million times darker than The Tilt, and apparently Bish Bosch reviews says it's complete madness

0celo7

@BalarkaSen I survived the first day of algebra

Balarka Sen

Congrats!

Sid

@0celo7 First day? Linear Algebra?

Anonymous

They are selling radioactive stuff on Amazon

Anonymous

wooooaaaaaauuuwwww

Anonymous

7:45 PM

amazon.com/Images-SI-Uranium-Ore/dp/B000796XXM

0celo7

@Sid Representation theory of finite groups

Basically linear algebra with big words

Anonymous

lol...maybe I should start making a nuclear reactor in my backyard with this XD

0celo7

@Blue that's my specialty

Anonymous

@0celo7 I know I know..your uni is the largest radioactive material importing facility in US or something. You told me. :P

0celo7

Largest nuclear engineering school

Anonymous

7:49 PM

@0celo7 Oh, yes

0celo7

and we have one of two nuclear breeder reactors in the world

Anonymous

I guess you already worked with radioactive stuff. Has any major injury happened in your uni campus till date?

Sid

@Blue Isn't that illegal or something

0celo7

No, HF is the real danger though

@Sid No.

There are probability limits to how much U you can own

Anonymous

@Sid I don't think so. Many kids have already made such stuff

0celo7

7:51 PM

I imagine it's less than the amount needed for criticality

@Blue Fusion reactors, not fission

Making a basic fusion reactor isn't impossible. Fission without explosion is much harder

And you need a LOT of metal

Anonymous

MIT rejected one guy because he made a nuclear reactor in his backyard (safety pedants :P)

Anonymous

@0celo7 Huh, but uranium can't be used for fusion. Isn't it?

0celo7

You use light isotopes for fusion

Anonymous

Maybe they used some other metal

0celo7

I would think a homemade U fission reactor would be pretty damn hard

Anonymous

7:53 PM

The one on Amazon is Uranium

0celo7

Yeah I know

Anonymous

@0celo7 Yup. I know. Below Fe (56)

Mithrandir24601

@Blue Well... If it's not a really, really safe reactor, it's not a good reactor, so he didn't do a good job of it...

0celo7

The Chicago pile used like 1 ton of shielding and everyone involved still got irradiated

Actually no, that one wasn't even shielded

during the war they didn't have time for shielding lmao

At X10 they put uranium sausages into the reactor with poles

They pushed the spent fuel out of the back and into the river for cooling

the whole area is radioactive now

Anonymous

@Mithrandir24601 They could have taught him the safety rules. C'mon, how many high schoolers have that kind of courage and enthusiasm to build such stuff? But ok, that is open to debate.

Anonymous

7:56 PM

@0celo7 wah, sounds bad

0celo7

@BalarkaSen did you get to Section 7.3 yet?

Sid

@0celo7 Chi town is radioactive? :o

0celo7

@Sid X10 was in Tennessee

and yes, Tennessee is radioactive

some of it at least

Chiraq is just a war zone