Python interface for R Programming Language
How do Rpy2, pyrserve and PypeR compare?
What is wrong with this fake proof $e^i = 1$? Like when you want to say that you'll tell a short version of something which would alter the accuracy of the original version.  I was thinking of "In a nutshell" and "To make a long story short", but I felt like those phrases did not place emphasis on the "there might be an insignificant loss of accuracy" aspect. There is an image in the Wikipedia about the :    I have a rough understanding of the stress tensor: you imagine cutting out a tiny cube from the fluid and form a matrix out of the forces on each side of it: it's not hard to see that the forces that push the faces outwards so the x direction of the force on the x side, the y direction of the force on the y side, etc. is the pressure, while other forces directions are the shear stress (these can be non-zero is some jelly-like substance).  Now the first question: Why is the lower half of the spatial part is the momentum flux, while only the upper one is the shear stress on the image? Is the image wrong? Isn't the spatial part describe the classical stress tensor?   The second question is: what's the intuition behind the temporal part?  So if I have a Minkowski spacetime and cut out a cube of it, then it's quite hard to see why "pressure" on the temporal face means density, also why "shear stresses" on the temporal face translate to momentum density, not speaking of the "temporal force components" the spatial faces...  Also stresses are measured in Pascals, mass density is measured in $kg/m^3$ while momentum density is in the units of $Ns/m^3$. These units doesn't seem to be compatible, but still they are in the same matrix, why?  EDIT: It has been pointed out that momentum flux refer to the entire blue part, it indeed have a lighter frame I didn't notice. Then yet another subquestion: what's the difference between the momentum density and momentum flux? Clusering based on categorical variables? I am working on a project and currently experimenting cluster analysis. The dataset is mainly categorical variables and discrete numbers. Please pardon my poor programming skills as I am not very familiar with MathJax, but I will try to summarize the data set in words in the following three examples.   imagine column 1 is participants names of course, from column2 - 5, each column's value range from 1 (least important) - 5 (most important). so in this case, column 2-8 only have discrete data.  column 6 for example, since this is a multiple choice question. row 1 chose "nice" as an answer, however, row 2 chose "poor". In this case, we have one column that contains multiple categorical answers. for column 7-9, this is another type of multiple choice questions. However this time, each column represents only one answer. column 7 only allows string values "true", column 8 only allows string "somewhat". So in this case, we have multiple columns that represent multiple answers of the same question.    Any ideas how to solve this? Thankful to any input!   A2       A3       A4       A5       A6       A7         A8         A9 1        4        5        4        nice     true       somewhate  false 2        4        3        1        poor     true                  false 1        5        2        1        nice                somewhate     3        2        1        5        nice     true                  false Does Thunderbolt support chaining multiple monitors to a MacBook Pro? This has always been a big want for me. I love using multi-monitor setups but only want a single laptop. Does the 2011 MBP Thunderbolt support chaining multiple monitors together? I know that you can add a monitor and say, a few hard drives, but can you connect it to 2 27" monitors and your flat screen TV (for example)? Thanks! Make rigid body collide with hair particles How do I handle elastic contacts in a simulation with friction I'm trying to simulate a wheel as it hits the ground.  Problem 1  Suppose a disc is dropped from a height. It has initial velocity of $-x,-y$ caused by throwing and gravity. It has no initial angular velocity. When it hits the ground it should have some rotation resulting from the collision.   How to calculate force that caused torque for that rotation?   Problem 2  The same disc is dropped from height. It doesn't have velocity on the side direction. It already spinning fast. When it hits the ground the spinning should cause some translation.   How to calculate force that caused this linear acceleration? Some rotation must be lost in the collision, how to calculate that?   To simplify assume static $friction = 1$ and $g = -10$. How to insert the content of a file into another file before a pattern (marker)? File1 contents:  line1-file1      "1"  line2-file1      "2" line3-file1      "3"  line4-file1      "4"    File2 contents:  line1-file2     "25"   line2-file2     "24"   Pointer-file2   "23"   line4-file2     "22"  line5-file2     "21"   After the execution of perl/shell script, File2 content should become:  line1-file2     "25"   line2-file2     "24"  line1-file1      "1"  line2-file1      "2" line3-file1      "3"  line4-file1      "4"  Pointer-file2   "23"  line4-file2     "22"  line5-file2     "21"   i.e  paste the contents of File1 in File2 before the line that contains "Pointer". I have a VB background and I'm converting to C# for my new job.  I'm also trying to get better at .NET in general.  I've seen the keyword "T" used a lot in samples people post.  What does the "T" mean in C#?  For example:  public class SomeBase&lt;T&gt; where T : SomeBase&lt;T&gt;, new()   What does T do? Why would I want to use it? Dual boot Ubuntu (14.04) with Windows 10 I have ran into some issues when it went down to install Steam on Debian. The console as well as Apper comes up with missing dependencies  and the package isn't going to install. How to write the word 'smart phone' in a research article? Smart phone Smartphone Smart-phone   I am really looking for some help regarding hyphen (-) issue in text found in research article. Is there any standard method how to use it? When to use (and not use) the rule of three How Do I Opt Out of Privileges? According to  , gold badge holders can now unilaterally close questions as duplicates. I hold seven gold badges for tags on SO, which means I can, on my own, close questions as duplicates for those tagged questions.  Simply put: DO NOT WANT.  However, I do not see any ability in SO (the only SE site I spend much time in) to opt out of any privileges, let alone this one.  Is there a spot that I'm just not seeing that allows me to turn off this new privilege? If a Matrix Has Only Zero as an Eigen-Value Then It Is Nilpotent Is there a way I can share the 3.42 GB download of Windows 8.1 from Windows Store? Python idiom to return first item or None I'm sure there's a simpler way of doing this that's just not occurring to me.  I'm calling a bunch of methods that return a list.  The list may be empty.  If the list is non-empty, I want to return the first item; otherwise, I want to return None.  This code works:  my_list = get_list() if len(my_list) &gt; 0: return my_list[0] return None   It seems to me that there should be a simple one-line idiom for doing this, but for the life of me I can't think of it.  Is there?  Edit:  The reason that I'm looking for a one-line expression here is not that I like incredibly terse code, but because I'm having to write a lot of code like this:  x = get_first_list() if x:     # do something with x[0]     # inevitably forget the [0] part, and have a bug to fix y = get_second_list() if y:     # do something with y[0]     # inevitably forget the [0] part AGAIN, and have another bug to fix   What I'd like to be doing can certainly be accomplished with a function (and probably will be):  def first_item(list_or_none):     if list_or_none: return list_or_none[0]  x = first_item(get_first_list()) if x:     # do something with x y = first_item(get_second_list()) if y:     # do something with y   I posted the question because I'm frequently surprised by what simple expressions in Python can do, and I thought that writing a function was a silly thing to do if there was a simple expression could do the trick.  But seeing these answers, it seems like a function is the simple solution. raw_input in python without pressing enter Changed permissions of / to user Script to raise a single window to the front