The Big Programming Thread - Page 216

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Thread Rules
1. This is not a "do my homework for me" thread. If you have specific questions, ask, but don't post an assignment or homework problem and expect an exact solution.
2. No recruiting for your cockamamie projects (you won't replace facebook with 3 dudes you found on the internet and $20)
3. If you can't articulate why a language is bad, don't start slinging shit about it. Just remember that nothing is worse than making CSS IE6 compatible.
4. Use [code] tags to format code blocks.

Recognizable

Netherlands1552 Posts

December 12 2012 16:47 GMT

#4301

Hey, I have a question. I understand why the following prints: MichaelLieberman. But I don't understand if I change line 4 from, x[0] + x[1] to i[0] + i[1] it prints: Mi.
Python btw.

n = ["Michael","Lieberman"]
def myFun(x):
	for i in x:
		return x[0] + x[1] # What happens when you change x to i?
print myFun(n)

ghrur

United States3786 Posts

December 12 2012 16:52 GMT

#4302

On December 13 2012 01:47 Recognizable wrote:
Hey, I have a question. I understand why the following prints: MichaelLieberman. But I don't understand if I change line 4 from, x[0] + x[1] to i[0] + i[1] it prints: Mi.

n = ["Michael","Lieberman"]
def myFun(x):
	for i in x:
		return x[0] + x[1] # What happens when you change x to i?
print myFun(n)

So when you change x to i, you're saying that for each element in the array, you return the first and the second element of the string i. So you return "Michael"[0] and "Michael"[1] which turns out to be "Mi". The reason it doesn't do "Li" too is because you're using return which stops the loop right there. So I think anyway.

Recognizable

Netherlands1552 Posts

December 12 2012 17:03 GMT

#4303

Ok, I understand it for strings. However I don't completely understand it for integers. Let's say we change it to n = [1,2]. I guess I understand why you can't go i[0] + i[1] because i has become a variable which loops(don't know if right word) through n=[1,2] if I am not mistaken?

LocusCoeruleus

Norway32 Posts

December 12 2012 17:12 GMT

#4304

i will assume the value of all the elements in n. So in the first iteration it will be 1 and second it will be 2, so you are not mistaken.

For your first question, in the first iteration i = "Michael" and in the second iteration i = "Lieberman".
For a list A, A[2] will return the third element. For the string S, how ever, S[2] gives you the the characters from 0 to (but not including) 2.
(Reading everything above me again, I see that everything has been answered quite nicely, but whatever)

Recognizable

Netherlands1552 Posts

December 12 2012 17:17 GMT

#4305

Ok thanks

adwodon

United Kingdom592 Posts

December 12 2012 21:46 GMT

#4306

Hey guys,
Two questions, both completely separate so answer either if you can

1)
I was just thinking coming home from work, I come from a physics background not a comp sci one so I'm not aware of some of the basics. Anyway, I remembered a question I had at an interview a while back about finding powers of 2. I mentioned several ways including looking at the binary, if it has a single 1, its a power of two.

Just thinking about that again, it sounds straight forward but then I realized I actually don't know how I'd implement that, could anybody give me an example? C preferably but C++ / Java / Python / whatever non-obscure language is fine if it suits.

2)
It's been a while since I did anything website related (5+ years), I'm considering setting up a personal site with a blog element, essentially to act as an both an extension for my CV and a kind of notebook / progress measure for personal projects.

Is there any kind of IDE / package I should consider using? All I remember is Dreamweaver and I have no idea if thats still useful or if there are superior / solid alternatives or as its just a small site, if I'm better off just using Notepad++ and cranking it out by hand.

I'm guessing HTML / CSS are still the main things with a splash of Javascript, but for a blog section is it reasonable to write something yourself, are there libraries / things to make this straight forward? I don't want a template site, but at the same time I don't really want to spend the next 6 months writing something with barely any functionality and full of annoying bugs, as I'd be learning every step of the way. What sort of languages would I expect to learn? I know a bit of Ruby but that's all as far as what I would consider web languages.

If there are any good resources / websites then please do share, I could probably find out a lot of this by the powers of google but I find its often best to ask in a forum as you tend to get a more well rounded answer

Thanks in advance!

heishe

Germany2284 Posts

December 12 2012 22:03 GMT

#4307

On December 13 2012 06:46 adwodon wrote:
Hey guys,
Two questions, both completely separate so answer either if you can

Thanks in advance!

On [1]. :


bool isPowerOfTwo(int value)
{
    return value && !(value & (value - 1)); //hint: value && [...] is there just because otherwise it would consider 0 to be a power of zero
}

I'll leave it to you to figure out why this works. Should be a nice exercise in bitwise-operations.

On [2]:

As a project page, a simple plain text HTML site with some colored links that link to textual descriptions and downloads of your projects will suffice. Any person you'd want to be hired by will look for function, not form in your online CV. And for that, the only thing you really need is Notepad++

RoyGBiv_13

United States1275 Posts

December 12 2012 22:18 GMT

#4308

On December 13 2012 06:46 adwodon wrote:
Hey guys,
Two questions, both completely separate so answer either if you can

Thanks in advance!

1)
Theres a few ways I can think to do this, but I like your way, so I'm going to use it.
in C:


int ispow2(int num)
{
while(! (num%2)) num=num>>1; //slide the number over to the first bit
return !(num>>1); //if its the only bit in the number, then the number is a power of 2
}

Or using a mask


int ispow2(int num)
{
int mask = 1;
while(mask) //this works because bitshifting using << will bitshift all the way to 0 when overflowed.
if(!(mask ^ num)) return true;
else mask = mask << 1;

return false;
}

EDIT: oooo I like heishe's implementation way better.
2) Wordpress has become the defacto standard for blogs. You can also check out Svbtle, which looks pretty cool. It's way easier than it ever has been to set up a personal blog.

tofucake

Hyrule19257 Posts

December 12 2012 22:18 GMT

#4309

web design IDEs are the devil

RoyGBiv_13

United States1275 Posts

December 12 2012 22:20 GMT

#4310

On December 13 2012 07:18 tofucake wrote:
web design IDEs are the devil

oh tofucake, its always 'X' is the devil with you. Tell us how you really feel about web design IDE's ^_^

tofucake

Hyrule19257 Posts

December 12 2012 22:25 GMT

#4311

They spit out barely manageable code based on tables and frames. Trying to tweak the layout is basically impossible. Forget about adding functionality and different screens (ie: doing anything a modern website would want to do) with PHP or ASP or even javascript. AJAX confuses every single one of them as well.

IDEs are responsible for sites like this

adwodon

United Kingdom592 Posts

December 12 2012 22:36 GMT

#4312

On December 13 2012 07:03 heishe wrote:
On [1]. :


bool isPowerOfTwo(int value)
{
    return value && !(value & (value - 1)); //hint: value && [...] is there just because otherwise it would consider 0 to be a power of zero
}

I'll leave it to you to figure out why this works. Should be a nice exercise in bitwise-operations.

Looks shiney, I'll have a better look at it tomorrow (its about my bed time right now!).

On December 13 2012 07:18 RoyGBiv_13 wrote:
2) Wordpress has become the defacto standard for blogs. You can also check out Svbtle, which looks pretty cool. It's way easier than it ever has been to set up a personal blog.

Appreciate the info, I'll probably start with something simple and expand from there, Svbtle looks interesting o.O

On December 13 2012 07:18 tofucake wrote:
web design IDEs are the devil

I do remember being told many years ago that things like Dreamweaver, whilst having their uses were generally for people with no interest or talent for actual code and also had a tendency to spew out rather unpleasant messes, I'm guessing things haven't changed in that sense much then!

jjwhg

United States155 Posts

December 12 2012 23:28 GMT

#4313

On December 13 2012 06:46 adwodon wrote:
I was just thinking coming home from work, I come from a physics background not a comp sci one so I'm not aware of some of the basics. Anyway, I remembered a question I had at an interview a while back about finding powers of 2. I mentioned several ways including looking at the binary, if it has a single 1, its a power of two.

Just thinking about that again, it sounds straight forward but then I realized I actually don't know how I'd implement that, could anybody give me an example? C preferably but C++ / Java / Python / whatever non-obscure language is fine if it suits.

While this is probably overkill: In modern x86 CPUs there's actually an instruction that counts the number of bits set in a word, also known as the population count. The following GCC-specific C code

int power_of_two(unsigned int x)
{
    return __builtin_popcount(x) == 1;
}

compiles to 5 branch-free instructions on my machine (be sure to build with "-msse4.2")

power_of_two:
	popcntl	%edi, %edi
	xorl	%eax, %eax
	cmpl	$1, %edi
	sete	%al
	ret

which I think is just about as efficient as you're going to get.

Deleted User 101379

4849 Posts

December 13 2012 01:23 GMT

#4314

On December 13 2012 07:18 tofucake wrote:
web design IDEs are the devil

I wholeheartedly agree.

For a blog, just use wordpress. Doing it yourself is a waste of time.
There is nothing wrong without using it and some existing theme, what matters in the end is the content anyways.

heishe

Germany2284 Posts

December 13 2012 14:16 GMT

#4315

Question: Given an ordinary 16-bit number, what's the fastest way to count the number of all consecutive 1s from the left? I've looked at the graphics.stanford site, but I can't apply anything.

E.g. 1110101011000101 should return 3; 1111100100100111... should return 5, etc.

Checking single bits is trivial, but it's also incredibly slow.

So far the only thing I've tried are lookup tables, which are even slower than looping through all bits manually (probably because a lookup table doesn't fit into the cache at all, resulting in tons of cache misses)

AmericanUmlaut

Germany2597 Posts

December 13 2012 14:21 GMT

#4316

Why is checking single bits incredibly slow? Can't you just do sixteen bitwise shifts and use two variables to keep track of your current and max count of consecutive 1s?

Edit: Clarify what I mean with a bit of pseudocode


val = 0111100111010111;
max = 0;
cur = 0;
for(i = 0; i < 16; i++) {
  if(val & 1) {
    cur++;
  }
  else {
    if(cur > max) max = cur;
    cur = 0;
  }

  val >> 1; //Bitwise shift to the right by one
}

Oh, I see that you asked for counting from the left. The result is the same regardless of which direction you count, but if you want to start from the left, just use "val & 0b1000000000000000" and "val << 1" instead.

heishe

Germany2284 Posts

December 13 2012 15:13 GMT

#4317

On December 13 2012 23:21 AmericanUmlaut wrote:
Why is checking single bits incredibly slow? Can't you just do sixteen bitwise shifts and use two variables to keep track of your current and max count of consecutive 1s?

I already have a version that you're suggesting, but that piece of code is executed roughly 155 million times per second, which is why potentially 16 more loop iterations are incredibly slow.

AmericanUmlaut

Germany2597 Posts

December 13 2012 15:19 GMT

#4318

Hmm... Is there a specific threshhold you're looking for, or do you really need the maximum number?

gyth

657 Posts

December 13 2012 17:23 GMT

#4319

There is probably an assembly instruction to do what you're looking for, or its complement.
http://en.wikipedia.org/wiki/Find_first_set

heishe

Germany2284 Posts

December 13 2012 17:35 GMT

#4320

On December 14 2012 02:23 gyth wrote:
There is probably an assembly instruction to do what you're looking for, or its complement.
http://en.wikipedia.org/wiki/Find_first_set

Well, I'm already using bsf/bsr in another part of the program, problem is as far as I know there's only the instruction that starts from the LSB and counts the first found set bit "starting from the right". If there was a similar instruction that would do the same thing but started "from the left", I could just invert the 16 bit and execute that instruction on it. But so far, I haven't found anything.

(bsf and bsr produce the same results, they just start out differently.).

Essentially bsf and bsr count trailing zeroes. I need something that counts leading zeroes.

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