Ask HN: Was Apple's Ping Successful? - <AUTHOR>

Of course there was a lot of hate spewed across the interwebs about Apple's ping being a complete #FAIL. But was it successful in terms of Apple's goal for it--to get people to buy more music? <comment><USER2> It may be worthwhile to compare the success of ping in increasing sales to the success of Genius in increasing sales.

I don't have bulk data on this at all, but I can say that personally, I've bought songs and apps that I've seen recommended to me on their respective Genius lists, but I've never bought a song that a friend has recommended.

Is the genius algorithm more meaningful for sales than social connections? In my experience, yes, algorithm beats friends.

<comment><AUTHOR> Right, Netflix has proven that with their algorithm (worth $1M in a developer contest!)</comment></comment>


Heroku dev status: Git push and API access have been temporarily disabled - <AUTHOR> https://status.heroku.com/incidents/484

<comment><USER2> Why do these cloud services seem to have so many more issues than the dedicated servers at my host?

<comment><USER3> Cloud providers have globally facing services, so you hear of the problems. Dedicated server hosts have issues, but unless it directly impacts you, you never hear of it. Additionally, there's so much that PaaS providers do. On a dedicated host, if say, Passenger is misconfigured, that's your issue, not there's, so long as the server is still up and you can SSH in.

</comment></comment><comment><AUTHOR> It's been fixed as of 40 minutes ago: <https://status.heroku.com/incidents/484#update-1502>

</comment><comment><USER4> Argh!!! Panic!!!</comment>


The Price of Paid Vacation - <AUTHOR> http://yglesias.thinkprogress.org/2010/09/the-price-of-paid-vacation/

<comment><USER2> Forget productivity, I see it as humanity. I would rather be paid less an hour if it was made up with PTO days. Maybe don't make paid vacation mandatory but make it mandatory that employees can trade $ per hour for paid vacation. Or at the very least make it mandatory that employees are by law allowed to take up to 1 month off unpaid.

I think letting employees have some time off is a good thing for both the employee and society.

<comment><USER3> I recommend self-employment. You get all the unpaid time off you want! (And, if you're selling software, it is "unpaid" for a very curious value of "unpaid" in which your bank account actually does get fatter on a daily basis.)

Mandating any form of compensation transfers wealth from people who would not choose to avail themselves of that compensation to people who would. I tend to think these transfers are generally unwise and/or unfair, and would prefer to leave them up to negotiation among the parties.

For example, "You can have up to a month off" has real costs for the business. Consider what it does at companies with one owner and one employee, for illustration: the business is mandated by law to virtually shutter itself one month out of the year. It is cheaper to have redundancy in large corporations where everyone is a cog, but it is not free. To the extent it is not free, that cost is implicitly tied to workers, and it competes against other forms of compensation, such as wages, _regardless_ of whether they choose to exercise their vacation options or not.

One could easily imagine a situation in which workers -- say, young single workers -- would prefer wages, or flex time, or "all you can eat Amazon books" over the unexercized option to be single and bored at home.

(Though I'll grant there is one nice bit about everybody having vacation time: it alleviates a bit of a collective action problem, since time off when your friends/family have to work has shockingly lower utility than time off when they do not.)

</comment></comment><comment><USER4> I think the logic is simple: Both workers and employers underestimate the impact that not taking a vacation has on productivity. Forcing them to gain the productivity gains that come through the vacation raises the overall efficiency.

(On the other hand, if people get more vacation _and_ spend half of their day playing Solitaire, that's not a winning proposition.)

<comment><USER5> > Forcing them

It's not a good idea to force someone else to do something you _think_ is good for them, because:

(a) they have a right to do what they want

(b) you could well be wrong

(c) and you would not like them forcing you to do things for your own good

In this case, the belief that "vacations increase productivity" is one of those comfortingly counterintuitive Gladwellian nostrums that collapses under close examination.

Counterintuitive: "But vacation means working less, so how could it mean more productivity? Ah, the vacation increases efficiency during working hours to such an extent that the integral of efficiency over time is greater!"

Collapsing under close examination: "Clearly increasing the vacation proportion p to 1.0 would push the working hours (1-p) to zero, reducing integrated efficiency to zero. Thus if an effect exists[1], it would have (at least one) local maximum between 0 < p < 1\. And it is not obvious what side of this local maximum we are on, nor whether the location of that local maximum is constant from industry to industry & person to person.

Hence it is not at all obvious that forcibly increasing vacation time would raise the overall efficiency. Indeed, it is highly unlikely that the claim holds in anything like the asserted generality."

QED

[1] To be precise, by postulating that an effect exists, we are stipulating that there exists a p' such that E(p') > E(0), as the efficiency is (supposed to be) greater at that p=p' than at p=0. We also note that no matter how large E(p), the integral of it over time at p = 1 is zero as there are no hours worked. These two observations mean that the integrated productivity is increasing at the beginning & decreasing at the end. The classic such smooth function is a parabola; there may be multiple wiggles depending on exactly how efficiency evolves with productivity.

<comment><USER6> > they have a right to do what they want

The first thing the auditors do is go through the HR records, find out who hasn't taken off any vacation time or sick leave for three years. That person is raising purchase orders and signing and counter-signing the approval forms or otherwise doing something so Byzantine that if they leave it for a single day, it will come crashing down on them...

</comment></comment></comment><comment><USER7> It seems to me that work hours, vacation, and pay are some of the few things that are really negotiable before accepting a job offer. What if each person simply negotiated for themselves how much vacation time they felt is reasonable, along with a salary adjusted to the amount of work they plan to do?

<comment><USER8> Extra vacation time is much harder to negotiate than the equivalent bump in salary. Vacation is more visible than salary, and people who want more time off than their coworkers are perceived as slackers or 'not a team player'. Plus, many companies link vacation allotment to seniority, so asking for more vacation threatens their primary reward for loyalty.</comment></comment>


Clever (YC S12) Hires VP Engineering from Square - <AUTHOR> http://techcrunch.com/2015/04/21/clever-hires-a-head-of-engineering-from-square/

<comment><AUTHOR> One of the Clever founders here & I'm very excited to have Ben onboard. We're 55 people today, with 22 engineers.

I'd love to crowdsource some advice from HN. Who's been part of a great VP Engineering transition? Who's been part of a crappy one? We're off to a great first week, but would love to hear thoughts on challenges that might lie ahead :-)</comment>


Free Google Analytics Alternative - <AUTHOR> https://simple-web-analytics.com/ <comment><USER2> Small typo: "All metrics are based on visits, which is a browser [season] at a given day."

<comment><AUTHOR> Thank you very much! Fixed, will be visible once caches are expired.</comment></comment>


Types of MVP - <AUTHOR> http://mlsdev.com/en/blog/50-types-of-mvp <comment><USER2> I would say an MVP is the first viable step in a plan for a more elaborate product in order to test the water etc. Just showing earlier iterations of famous products without a snapshot of the accompanying vision isn't massively useful. Of course every company/product started somewhere, but a first offering isn't the same as an MVP. Did Yahoo intentionally set out to create an MVP or did they just launch with their best idea at the time?

</comment><comment><USER3> I strongly disagree with the premise that the Yahoo! presented in that screenshot was a minimum viable product - that really _was_ the product: the differentiator between Yahoo! and, say, AltaVista was that Yahoo! was a hierarchical list of links rather than an actual indexed search engine. It stayed that way for many years.

</comment><comment><USER4> A wireframe or a mock-up is not a "viable product".

Unless you plan to sell wireframes or mockups, I guess.

Even a "prototype" isn't a "viable product" \-- unless it is.

Does "MVP" actually mean anything anymore?

<comment><USER5> "First, a definition: the minimum viable product is that version of a new product which allows a team to collect the maximum amount of validated learning about customers with the least effort." \- [http://www.startuplessonslearned.com/2009/08/minimum- viable-...](http://www.startuplessonslearned.com/2009/08/minimum-viable- product-guide.html)

A wireframe or a mockup very might as well be a minimum viable product. They're easy to set up, and the right people can often gather valuable information using a mockup only. The point isn't to have a functional product. The point is to gather information and validate your assumptions about customers.

<comment><USER4> Okay, that's not the definition of "minimum viable product" I've heard and used.

I thought it was... well, the minimum viable product. Meaning it's a product. Which is viable.

So, okay, apparently "MVP" doesn't mean much anymore.

I'd call that definition of an "MVP" something else. Like, I dunno, a prototype? Minimum viable prototype? I guess that still has the same acronym...

Even the wikipedia article says "A minimum viable product has just those core features that allow the product to be deployed, and no more." It's the product. Deployed. Not a drawing. A drawing is not the product unless your business is selling drawings.

<comment><USER5> Context: Eric Reis is the writer of "The Lean Startup" and the person who coined the term MVP.</comment></comment></comment></comment>


Why are variables "i" and "j" used for counters? - <AUTHOR> http://stackoverflow.com/questions/4137785/why-are-variables-i-and-j-used-for-counters <comment><USER2> i, j, and k come from Hamilton's Quaternions.

Euler picked i for the imaginary unit.

Hamilton needed two more square roots of -1:

        ii = jj = kk = ijk = -1

Hamilton was really influential, and quaternions were the standard way to do 3D analysis before 1900. By then, mathematicians were used to thinking of (ijk) as a matched set.

Vector calculus replaced quaternionic analysis in the 1890s because it was a better way to write Maxwell's equations. But people tended to write vector quantities as like this: (3i-2j+k) instead of: (3,-2,1). So (ijk) became the standard basis vectors in R^3.

Finally, physicists started using group theory to describe symmetries in systems of differential equations. So (ijk) started to connote "vectors that get swapped around by permutation groups," then drifted towards "index-like things that take on all possible values in some specified set," which is basically what they mean in a for loop.

<comment><USER3> What variable did mathematicians use for summation before physicists started using group theory to describe symmetries in systems of differential equations? Isn't that a fairly recent development?

My search for an old paper with a bloody summation symbol found nothing. I did, however, find this OT gem about Newton, written in 1848:

 _From the thick darkness of the middle ages man's struggling spirit emerged as in new birth; breaking out of the iron control of that period; growing strong and confident in the tug and din of succeeding conflict and revolution it bounded forwards and upwards with resistless vigour to the investigation of physical and moral truth; ascending height after height; sweeping afar over the earth, penetrating afar up into the heavens; increasing in endeavour, enlarging in endowment; every where boldly, earnestly out stretching, till, in the Author of the Principia, one arose, who, grasping the master key of the universe and treading its celestial paths, opened up to the human intellect the stupendous realities of the material world, and in the unrolling of its harmonies, gave to the human heart a new song to the goodness, wisdom, and majesty of the all creating, all sustaining, all perfect God._

<comment><USER4> If you want an old paper with a summation symbol, you could do worse than the book where the symbol was first used, by Euler: [http://www.math.dartmouth.edu/~euler/docs/originals/E212sec1...](http://www.math.dartmouth.edu/~euler/docs/originals/E212sec1ch1.pdf)

(He uses x as the index variable there.)

</comment></comment></comment><comment><USER5> In FORTRAN identifiers starting with I,J,K,L,M,N declared as INTEGER by default.

<comment><USER6> It is exactly I was taught in university when I learned FORTRAN. But it is probably influenced from mathematical notation of indexing.

</comment></comment><comment><USER7> I still use 'n' and 'f' every so often.

Reason being, on the ZX Spectrum, it had all the BASIC keywords on each key, and pressing the key would input the keyword itself (depending on where you were etc). The 'N' key had 'NEXT', and the 'F' key had 'FOR'.

So to type 'next n' you just pressed 'N' twice. Typing in 'for f' you pressed 'F' twice.

</comment><comment><USER8> I think this:

[http://en.wikipedia.org/wiki/Summation#Capital- sigma_notatio...](http://en.wikipedia.org/wiki/Summation#Capital- sigma_notation)

<comment><USER9> I agree, it probably comes from maths.

As a side note, the first letters are used for constants, the last letters are for variables; and it was Descartes who popularized this usage: <http://en.wikipedia.org/wiki/Polynomial#Notation>

I believe they used i for index, and also for the reason that it wasn't used by other usages as above.

Similarly, the integration sign is actually a long s, standing for sum. <http://en.wikipedia.org/wiki/Integral_symbol>

</comment></comment><comment><USER10> Next question: what other single character variable names do people use? I've always wanted to work up a master list somewhere.

Speaking to my own usage, I use i and j for indices, x, y, and z for Cartesian coordinates, n for "number", k and v for "key" and "value" when iterating through a hash, and r for a variable I'm using for a return value later on.

<comment><USER11> I use a description of what I;m iterating over. If it's a list of people, I use 'person'. If the keys are session IDs, I use 'sessionid'. I do this because it's easier for me and I think I'm being nicer to the next person to come along.

<comment><USER10> I do that if I'm doing collection.each in Ruby or something, but if I'm in C or even if I'm doing collection.each_with_index, I use "i" for the index.

</comment></comment></comment><comment><USER12> I use 'i' for 'index' / 'iterator', j because it follows i, and k because it follows j. The first time I wrote a double-deep loop, the only ones I could recall having seen had used x and y, because they mapped to x,y coordinates.

So why don't I use x and y where it's not coordinate based? Weaker fingers. 'X' is inherently much harder to hit than 'i', 'i' serves as a mnemonic as it's usually referring to the index being accessed, and 'y' takes a larger movement than 'j'.

</comment><comment><USER13> Fortran is the culprit

<comment><USER14> TODAY IS EVERYBODY TALK LIKE FORTRAN DAY

<comment><USER15> You didn't begin in column 7 but wisely avoided the continuation character.

</comment></comment></comment><comment><USER16> When I first learned programming, the tradition was to use 'f' and 'n'.

On the ZX81, the "FOR" command was accessed by pressing the F key in a special "command mode", so by typing the F key twice you got "FOR F". Similarly "NEXT N".

Edit: Yes, duplicate of what axod said here: <http://news.ycombinator.com/item?id=1971594>

</comment><comment><USER17> I rarely use i-j-k, I tend to use n-m-r for some reason(possibly because I've been programming with iterator syntax so long, that I always think first of grabbing "thing n" rather than incrementing "counter i.") But on reflection I think this is probably less legible.

</comment><comment><USER18> For me i has always been "iterator variable" or something similar, j being the logical next iterator variable. Then again I haven't done this (code) for very long.

</comment><comment><USER19> When I first started studying graduate math, professors would write how my usage of i as bizarre since I would use it for my index in sums.

sqrt(-1) = 1,2,3,... ????? WTF

</comment><comment><USER20> I always used and thought of i as "index", c as "count", and n as "number".

<comment><USER21> More specifically, I think the N stands for "Natural number".</comment></comment>


Nasa captures images of supersonic shockwaves - <AUTHOR> https://phys.org/news/2019-03-nasa-captures-unprecedented-images-supersonic.html <comment><USER2> As mentioned by others, the photography technique looks like (and apparently is a software enabled variant of) Schlieren photography. This(1) is a neat example of an old school Schlieren set up capturing shockwave formation during whip cracks. I highly recommend it.

(1) [https://www.youtube.com/watch?v=AnaASTBn_K4](https://www.youtube.com/watch?v=AnaASTBn_K4)

<comment><USER3> Very cool. They didn't mention this in the video, but I immediately see one groundbreaking new phenomenon that they uncovered: at the exact moment the end of the whip goes supersonic, it is being accelerated by something other than the tension of the rope. It looks as though it's accelerated by the shockwaves, of which there appear to be two -- one on each side of the whip end, with a vacuum in between. It's as clear as day in the video. The rope in front of it clearly goes lax, and the whip end even changes direction to align with the shock wave propagation.

<comment><USER3> I thought about this and watched the video again, and I've changed my mind just a bit. It looks like, instead of being accelerated, the whip end is experiencing reduced drag in the vacuum between the two shock waves. We can see that it travels right behind the first shockwave, not lagging at all, and this is exactly what you would expect if it is both the source of the shockwave and the beneficiary of the vacuum behind it.

</comment></comment></comment><comment><USER4> How are these images captured? They look like Schlieren photography, but as I understand it that requires a mirror on the other side of the target being photographed.

<comment><USER5> It is a Schlieren technique, but in this case a series of images taken over a textured background are then processed with software in a way that amplifies the differences in air density. This page goes into a little more detail:

[https://technology.nasa.gov/patent/TOP2-271](https://technology.nasa.gov/patent/TOP2-271)

<comment><USER4> Thank you, that's really interesting!

</comment></comment></comment><comment><USER6> More on NASA's attempt to solve "quiet" supersonic flight.

[https://www.nasa.gov/QSF18](https://www.nasa.gov/QSF18)

</comment><comment><USER7> Anyone found hi-res versions of these images?

<comment><USER8> NASA has somewhat larger versions linked on this article, though they aren't really sufficient for larger wallpapers or prints:

[https://www.nasa.gov/centers/armstrong/features/supersonic-s...](https://www.nasa.gov/centers/armstrong/features/supersonic- shockwave-interaction.html)

The photos aren't yet listed in their image database[0], unfortunately. They also publish[1] high-resolution versions on Flickr, so it's possible the photos might make their way there in the near future. Alternatively, they give a phone number (202-358-1900) to inquire about media use of specific images.[2] Maybe I'll give them a call later if someone doesn't beat me to it. Or the high-res photos will make their way onto Flickr, since they already posted to Instagram as of yesteray.

0\. [https://nasasearch.nasa.gov/search/images?affiliate=nasa&que...](https://nasasearch.nasa.gov/search/images?affiliate=nasa&query=)

1\. [https://www.flickr.com/photos/nasahqphoto/](https://www.flickr.com/photos/nasahqphoto/)

2\. [https://www.nasa.gov/multimedia/guidelines/index.html](https://www.nasa.gov/multimedia/guidelines/index.html)

</comment></comment><comment><USER9> I really wonder who makes the decision of starting work on this kind of research, and how it goes.

I hope it's not in the lines of "Alright team, we've got two idle supersonic jets, a high speed camera, and enough jet fuel to burn for a day, so how about some ideas?"

<comment><USER10> It's probably "we now have two idle supersonic jets". Everything else they already had. And good budget planning always helps.

<comment><USER11> > It's probably "we now have two idle supersonic jets".

It's likely not. This kind of thing takes a lot of planning and coordination.

</comment></comment></comment><comment><USER12> I suspect the idea of quiet supersonics are also very interesting to the Department of Defense as well (for relatively obvious reasons).

</comment><comment><USER13> I wish there were video :(

</comment><comment><USER14> NASA is a an acronym and not a word</comment>


Chess is Too Easy (1998) - <AUTHOR> https://www.technologyreview.com/s/400154/chess-is-too-easy/ <comment><AUTHOR> I've been researching lately technology news & commentary from 20 years ago and most of the headlines feel weirdly actual.</comment>