Have array like   arr = [1,2,3,4,5,6,7,8,9,10].   How I can get array like this:  [1,2,5,6,9,10]   take 2 elements with step 2(::2)  I try something like arr[:2::2].it's not work
I need a good explanation (references are a plus) on Python's slice notation.   To me, this notation needs a bit of picking up.   It looks extremely powerful, but I haven't quite got my head around it.
Placing nodes midway on paths using |- or -| in tikz I make frequent use of |- and -| with computed points in diagrams, like so:  \coordinate (a) at (0,0); \foreach \x in (1,...,10) {     \coordinate (b) at ({whateverFunction(\x)},{whateverOtherFunction(\x)});     \draw (a) |- (b); %Straight line segment     \draw (b) -| (a); %Another straight line segment     \coordinate (a) at (b); }   To get plots like:   I would like to be able to label the midpoint of each line segment, but doing something like node[midway,above] {a} does not act like expected:    I want something like:    Why does midway not work, and what is a good workaround here? Meaning of "over in" I read this sentence in a book:     I just took part in a study over in the Psychology Department.   Why do we add over in front of in the here? Proof of Frullani's theorem Why is Harry Potter a half-blood? Being born to James and Lily, how can Harry be called a half-blood?  It is said that admission in Hogwarts is given to children of Muggles when they show magical talents as a child, and Lily was no exception to this. This makes Lily a witch, so why is Harry half-blooded? Is there a simple way to explain how Kepler's third law follows from the inverse square law that of gravity  (and laws of motion)  For example for Kepler's second law we can say it's because Gravity is a central force  What is it about the law of gravity which ultimately gives rise to Kepler's third law? How to do a secure erase on a Macbook Pro? I have a Macbook Pro with SSD and I would like to completely erase every single bits of data on it. How would I do that? On a PC, I would just run Darik's Boot And Nuke, but can I run it on a Mac too, or is there a better alternative? How do you clear a stringstream variable? I've tried several things already,  std::stringstream m; m.empty(); m.clear();   both of which don't work. Uncountable chain of subsets of $\mathbb N$ A PCI Express 3.0 Video Card into a PCI-E 1.0 x16 slot Perhaps I'm trying to push my mobo too hard, but I think I have read that PCI-e 3.0 is back compatible to previous PCI-e specifications. But 3.0 into a 1.0, is it possibly asking too much?  I'm thinking about buying an AMD . My mobo is an Asus p5g41-m le How are the file metadata stored in Windows? (I'm using Windows XP but I guess it's similar in all the recent versions of Windows.)  When you create for example a new empty text document, you'll find in its properties that it has size of 0 bytes. Zero bytes means no information. No data. But still, the file has some name, it can still carry the dates of last access, modification and creation. It carries the information whether it's hidden file or not, whether it's read-only or not...  So where are all the metadata stored? Converting an array of objects to an array of their primitive types If you have an array of Java objects which have a primitive type (for example Byte, Integer, Char, etc).  Is there a neat way I can convert it into an array of the primitive type?  In particular can this be done without having to create a new array and loop through the contents.  So for example, given  Integer[] array   what is the neatest way to convert this into  int[] intArray   Unfortunately, this is something we have to do quite frequently when interfacing between Hibernate and some third party libraries over which we have no control.  It seems this would be a quite common operation so I would be surprised if there's no shortcut.  Thanks for your help! How to write permutations as product of disjoint cycles and transpositions $$\sigma=\begin{pmatrix} 1 &amp; 2 &amp;3 &amp; 4&amp; 5&amp; 6&amp;7 &amp;8 &amp;9 &amp;10 &amp; 11 \\ 4&amp;2&amp;9&amp;10&amp;6&amp;5&amp;11&amp;7&amp;8&amp;1&amp;3    \end{pmatrix}$$  (1) I am asked to write this permutation in $S_{11}$ as a product of disjoint cycles and also as a product of transpositions.   (2) Also find the order of the element. Is this permutation even or odd?  I think these are the disjoint cycles  $E_{1}=(1,4,10)$, $\;\operatorname{order}E_1 =3$  $E_{2}= (3,9,8,7,11),\;$ $\operatorname{order}E_{2} =5$  $E_{3}=(5,6),\;$ $\operatorname{order}E_{3} =2$  $S_{11}$=  $E_{1} \cdot E_{2}\cdot  E_{3}$  The order of $\operatorname{order}E_{3}$ is even so the order of the permutation is even. Why are they asking this? And what is the significance of it being even or odd?  Transpositions – I have read this a couple times but the one example in my textbook is rather unclear, I am not sure what this means?  I think the transposition for $E_{1}$ is $(1,4)(1,10)$ but I'm not sure what this means. Do small title edits constitute trivial edits? What is the difference between "What are you?" and "Who are you?"? I am often asked "Who are you?" and "What are you?", but I don't know the difference between these two questions.  Please explain it to me. How can I set up password-less SSH login? I want to be able to login to a remote via ssh without having to enter the password all the time.   How do I set it up? Is a different command required to execute a password-less session? Positive operator has a positive spectrum? Let $T : \operatorname{dom}(T) \rightarrow H $ be a positive self-adjoint operator, is it then true that $\sigma(T) \subset [0,\infty)$? This is something that sounds natural and I guess that it is true. For bounded operator, this can be easily shown, but the only proof I know cannot be adapted to unbounded operator, so I was wondering if anybody here knows how to show this? Annuity notation in LaTeX How do you write this in LaTeX: What pins on a nRF51822 do I connect to a STM32 programmer? I want to be able to program a  with  but I'm not sure what pins on the actual chip I should use, and all the guides seemed to be using a different chip Why do diamonds shine? C++ copy-construct construct-and-assign question