In: graphics.h:   class window {      friend class mouse;  private:      static int width;         // in pixels      static int height;        // in pixels      static vector&lt;object&gt; objects;      static size_t selected_obj;      static mouse mus;      static int move;  public:      static void setmove (int move_) { window::move = move_; } //&lt;--- problem      static void push_back (const object&amp; obj)          { objects.push_back (obj); }      static void setwidth (int width_) { width = width_; }      static void setheight (int height_) { height = height_; }      static void main();  };   In one of my functions in a class called interp.cpp: I'm trying to do this:  window::setmove(4);   But sadly, I get this error:   interp.o: In function `window::setmove(int)': /afs/cats.ucsc.edu/users/m/graphics.h:72: undefined reference to `window::move'   It's weird because in another function inside the interp.cpp, I'm able to use window::push_back(new_shape);  Any idea what could be wrong? Thank you.  EDIT: Because it's been marked as duplicate: I don't see how it could be a compiler issue because I'm able to use the other functions inside window class.
Can anyone explain why following code won't compile? At least on g++ 4.2.4.  And more interesting, why it will compile when I cast MEMBER to int?  #include &lt;vector&gt;  class Foo {   public:       static const int MEMBER = 1;   };  int main(){       vector&lt;int&gt; v;       v.push_back( Foo::MEMBER );       // undefined reference to `Foo::MEMBER'     v.push_back( (int) Foo::MEMBER ); // OK       return 0; }
What is the Java string pool and how is "s" different from new String("s")? I am using TeXworks 0.4.5 r1280 with MikTeX 2.9 on Windows 7 64-bit. When I add  \usepackage{datetime}   and try to typeset the document, I get a dialog window like this:    I select "Install", confirm UAC request and then get the following error on the console:  ! LaTeX Error: File `datetime.sty' not found.   If I try to typeset the document again - the dialog and the error appear again.  Same thing happens when I've tried installing wrapfig and a few other packages at random: cases, comma, dowith, microtype.  Other packages have installed fine before. What is wrong with my setup? Please let me know if you need more info about my setup in comments.  I've tried this solution: . Didn't work. Is the following proof correct, or are certain steps illegal?  Suppose $f:G_1\to G_2$ is a surjective group isomorphism, $N_1\lhd G_1, N_2\lhd G_2$ normal subgroups, with $N_1=f^{-1}(N_2)$. Prove that $(G_1/N_1)\cong (G_2/N_2)$.  Proof:  As $f$ is surjective, we an use the first isomorphism theorem to get $G_1/\ker(f)\cong G_2$, which implies that $$(G_1/\ker f)/(N_1/\ker f)\cong G_2/(N_1/\ker f)$$  Using the first isomorphism theorem we also know that $$N_1/\ker (f)\cong f(N_1)=f(f^{-1}(N_2))=N_2$$  Combining this we can see that $(G_1/\ker f)/(N_1/\ker f)\cong G_2/(N_1/\ker f)\cong G_2/N_2$.  Using the third isomorphism theorem we have that: $$ (G_1/\ker f)/(N_1/\ker f)\cong G_1/N_1$$  And from the last two lines we have $G_1/N_1\cong G_2/N_2$ $\tag*{$\Box$}$ How do I install gfortran? I am trying to install gfortran in my computer with Ubuntu 12.04 by using these commands,  sudo -i apt-get update apt-get install gfortran   but I found the program saying "unable to locate package gfortran". Then on a Ubuntu page I found some question regarding the same problem and I went through the solution from them but again I couldn't install the gfortran package.  Can anybody tell me how to install it? Problem: Let $X$ and $Y$ be independent and suppose that each has a $\text{Uniform}(0,1)$ distribution. Let $Z = \min\{X, Y\}$. Find the density $f_Z(z)$ for $Z$. Hint: It might be easier to first find $\mathbb{P}(Z &gt; z)$.  Attempted Solution:  Given that $X, Y \sim \text{Uniform}(0,1)$, how do we not just have the following?  $$ f_Z(t) = f_X(t) = f_Y(t) = \begin{cases} 1 &amp; \text{if } 0 \le t \le 1 \\                                           0 &amp; \text{otherwise}                             \end{cases} $$  Of course I'm highly suspicious of this answer because it's not making use of the fact that $X$ and $Y$ are independent, nor is it making use of the provided hint. Why should I put a ~ before \ref or \cite? My question is in the title of this post, but extends to other referencing commands like \eqref, \citep, \citet (and the like) as well. On the existence of a continuous bijection $f\colon [0,1]\to [0,1]\times [0,1]$ Let $f$ be a continuous function on $[0,1]$ such that $f([0,1])=[0,1]\times[0,1].$ Then show that $f$ is not one-one.  Hints will be appreciated. JSF returns blank/unparsed page with plain/raw XHTML/XML/EL source instead of rendered HTML output Help finding the limit of $\lim_{n \to \infty}\prod_{k=1}^{n}\left(1+\frac{1}{n}f\left(\frac{k}{n}\right)\right)$. Ubuntu GNOME 17.04: wi-fi not working -- mac address keeps changing? My Panda USB wi-fi adapter works just fine on 16.10, but when I try to connect to my wi-fi router in 17.04, GNOME network manager reports "Connection failed." I did some tinkering, and noticed that my MAC address for my wifi adapter, according to GNOME, is DIFFERENT every time I make it forget my wifi settings and try to reconnect. Any leads on a possible fix or work-around?   I'm running Ubuntu GNOME 17.04, kernel 4.10.0-19-generic, GNOME 3.24.0. What is the "correct" way of embedding text into math mode? Lets say I have something like:   \[ Let x = number of cats \]   (This is a contrived example, but pretend we are actually inside a long align environment block...) "Let" and "number of cats" need to be written like normal text, while x= needs to be math type text.  I've seen this:  \[ \mbox{Let } x = \mbox{ number of cats} \]   and this:  \[ \textrm{Let } x = \textrm{ number of cats} \]   They seem to produce equivalent output, but I'm not sure which is the "correct" one. Also, they are not ideal in that the extra space needs to be placed inside the text mode block, otherwise it is ignored as math mode and butts the text right up against the math.  Ideally there would be some command which allowed embedding text into an equation, which correctly inserted leading and trailing space around the block as required without adding space inside the text block.  How does one do that "correctly"? How does the PPA fit into the scenario of publishing an application to the Ubuntu Software Center? I've been going through docs for the past couple of hours but I haven't understood what the PPA is? I have a cross-platform Java application that I'd like to publish to the Ubuntu Software Center. My application is open-source and I'm using Github.   Apparently, publishing applications to the store isn't as simple as uploading a deb package - am I right? I need to create an account on Launchpad and put all my code there.  I don't intend to move from Git to Bzr merely for the sake of publishing to the app store but luckily, one is able to set up source-code mirroring from Github to Launchpad.  Since my application is still very premature, it'll have updates fairly often. When I build my application on my machine, do I simply go my Ubuntu App Developer page and upload the new DEB package or do they build my application from source?  What exactly is the PPA for? I don't think I'll need too many of the Launchpad features so I'd like to stick to Github if possible. I came upon an interesting way to relatively quickly compute modular exponentiation with large numbers. However, I do not fully understand it and was hoping for a better explanation.  The method basically states if you have $x^y \bmod N$, it can be computed by repeatedly multiplying by $x$ modulo $N$:  $$x \bmod N \rightarrow x^2 \bmod N \rightarrow x^4 \bmod N \rightarrow x^8 \bmod N \rightarrow \ldots \rightarrow x^{2[\log y]} \bmod N$$  This is supposed to calculate in $\mathcal{O}(\log_2 N)$ time.  An example is given:  $$x^{25} = x^{11001_2} = x^{10000_2} * x^{1000_2} * x^{1_2} = x^{16} * x^8 * x^1$$  I don't really understand how this is faster. Aren't you just doing the same thing? You're just breaking down the exponents into representations of base $2$. Also where does $x^{2[\log y]}$ come from? If you're breaking it down into exponents of two where does a logarithm come from? (Aren't logs representations of numbers in exponents of $10$?)  I'm sorry if this appears to be a really stupid question - I have minimal knowledge in math.  Also, I wasn't sure if this belongs here or on stack exchange, so give me the word and I'll move it. How do I speed up my design work? I've been a designer for 6 months, having previously had a 1st class honours degree in Graphic Communication. My issue is that I am a Very good designer, but I work too slow. It's beginning to drive a rift between myself and the creative director who desperately wants me to work quicker. I've always been a workaholic even in university, I put in 110% all the time, really work my ass off, but I'm still not fast enough to complete projects quickly, my work is good, but it's not efficient.  Do you guys have any tips to help me speed up? USB flash drive not working or is appearing as an empty disk drive, Disk Management reports "No Media" with 0 bytes size I have a USB flash drive which is no longer recognized by my computer. Windows Disk Management and DiskPart report No Media with no storage space (0 bytes) on the drive and I cannot partition or format the drive:     If the drive appears in Windows Explorer, trying to access it returns an error message indicating that there is no disk inserted, such as the following:     Please insert a disk into drive X:.   Various disk partitioning and data recovery utilities don't recognize the drive or only give a generic name for the drive and cannot access the contents of the drive.  What can I do? How do I recover data from the drive?     This question comes up often and the answers are usually the same. This post is meant to provide a definitive, canonical answer for this problem. Feel free to edit the answer to add additional details. How to have full bleed between two facing pages Indesign I'd like to do it in Python. What I'd like to do in this example in C: #include &lt;stdio.h&gt;  int main() {     int i;     for (i=0; i&lt;10; i++) printf(&quot;.&quot;);     return 0; }  Output: ..........  In Python: &gt;&gt;&gt; for i in range(10): print('.') . . . . . . . . . . &gt;&gt;&gt; print('.', '.', '.', '.', '.', '.', '.', '.', '.', '.') . . . . . . . . . .  In Python print will add a \n or space, how can I avoid that? Now, it's just an example, don't tell me I can first build a string then print it. I'd like to know how to &quot;append&quot; strings to stdout. "Unable to locate package" while trying to install packages with APT I know that matrix multiplication in general is not commutative. So, in general:  $A, B \in \mathbb{R}^{n \times n}: A \cdot B \neq B \cdot A$  But for some matrices, this equations holds, e.g. A = Identity or A = Null-matrix $\forall B \in \mathbb{R}^{n \times n}$.  I think I remember that a group of special matrices (was it $O(n)$, the ?) exist, for which matrix multiplication is commutative.  For which matrices $A, B \in \mathbb{R}^{n \times n}$ is $A\cdot B = B \cdot A$? I am working on refactoring some old code and have found few structs containing zero length arrays (below). Warnings depressed by pragma, of course, but I've failed to create by "new" structures containing such structures (error 2233). Array 'byData' used as pointer, but why not to use pointer instead? or array of length 1? And of course, no comments were added to make me enjoy the process...  Any causes to use such thing? Any advice in refactoring those?  struct someData {    int nData;    BYTE byData[0]; }   NB It's C++, Windows XP, VS 2003