How to store custom objects in an ArrayList
What is a NullPointerException, and how do I fix it?
Proof that a sequence converges. Space ship manned by only AI returns home to a different earth, back in time Linear transformations map lines to lines I need help with this question:  Let $T$ be a linear map from $\mathbb{R}^n \to \mathbb{R}^m$, and let $L$ be a line in $\mathbb{R}^n$. Show that $T(L)$ is also a line (is it not enough to just consider lines in the plane). Move iTunes folder outside Music folder iTunes by default stores its settings, the iTunes Library.itl file, and the music library audio tracks, inside the \Users\username\Music\iTunes folder.  I don't want my Music folder polluted with program settings, especially because I'm synchronizing that folder on the cloud. Via the settings panel I can only change the path of the audio tracks, but the settings remain in the default position.  Is there a way to completely move the iTunes folder to another path?  PS: I've looked at the answers to , but the focus there is on moving the media library only, not the whole iTunes directory. I need a quick algorithm to select 5 random elements from a generic list. For example, I'd like to get 5 random elements from a List&lt;string&gt;. Is GTA Online cross-platform? I'm attempting to install Ubuntu 14.04.1 (the only version I have handy) onto my old Compaq Presario R3000 (this Ubuntu was recommended for this computer). The installation has stalled at about 60% and hasn't moved in an hour.  The message at the bottom says: Configuring bcmwl-kernal-source (i386).  I have no experience with Ubuntu (or with Linux, for that matter), coming from a strictly-Windows background). Can anyone offer some advice? How to validate an email address using a regular expression? Over the years I have slowly developed a  that validates MOST email addresses correctly, assuming they don't use an IP address as the server part.  I use it in several PHP programs, and it works most of the time.  However, from time to time I get contacted by someone that is having trouble with a site that uses it, and I end up having to make some adjustment (most recently I realized that I wasn't allowing 4-character TLDs).  What is the best regular expression you have or have seen for validating emails?  I've seen several solutions that use functions that use several shorter expressions, but I'd rather have one long complex expression in a simple function instead of several short expression in a more complex function. Mixing and Distributing Qualifiers ($\forall x$, $\exists x$) ls: cannot access .gvfs: Permission denied Whenever I run an application in terminal as root (such as sudo gedit  /etc/default/varnish) , subsequently when I open another terminal I get "ls: cannot access .gvfs: Permission denied" error at top line of terminal.   I found a solution on the net     umount /path/to/.gvfs   rm -rf .gvfs   but it only fixes problem temporarily.  It appears that I have two mounted instances of gvfs in my system    $ sudo mount |grep gvfs     gvfsd-fuse on /run/user/1000/gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,relatime,user_id=1000,group_id=33)     gvfsd-fuse on /home/****/.gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,relatime,user_id=0,group_id=0)   I don't know if it's related to, a few months ago I had to change permission in my home folder like     sudo chown -R $USER:www-data   Would you please help me fix it?  EDIT: After I unmount /run/user/1000/gvfs I don't get that error. Can I use my device on a different carrier? Sometimes it's desirable to use a device on a carrier that does not officially sell it. Often this is due to regional differences and travel (for example, using a US phone in Europe) but also because some carriers simply do not offer phones that other carriers do.  How can I determine if it's possible to use my phone on a different carrier, and which carriers it is compatible with?    This community wiki is intended to be a canonical answer to a common question. If you feel that improvements could be made then please edit the post(s). Solve the recurrence relation:$ T(n) = \sqrt{n} T \left(\sqrt n \right) + n$ Edit source differs from question view? Or are we each other's friends now? conflict between hyperref and enumitem packages (and \refstepcounter) The solution outlined for each of the following two posts appears to conflict with the hyperref package.         Here is a MWE.  The desired output for the example is  1. Outer 2.1. This is line 2.1, in block 2.  2.2. Inner again 3. Outer again 4. Let’s end here   This output is indeed produced by the LaTeX source below, but if I uncomment the \usepackage{hyperref} line in the header, LaTeX gives me an error just after the \skipitem command, as described in the inline comment below:  \documentclass{article}  % \usepackage{hyperref} % if we uncomment the above line, we get an error as described below  \usepackage{enumitem}  \newlist{steps}{enumerate}{9} \setlist[steps]{itemindent=0pt} \setlist[steps,1]{label=\arabic*.,ref=\arabic*} \setlist[steps,2]{label=\arabic{stepsi}.\arabic*.,ref=\arabic{stepsi}.\arabic*}  \makeatletter \newcommand{\skipitem}{\refstepcounter{\@enumctr}} \makeatother  \begin{document} \begin{steps} \item Outer   \skipitem   % If we uncomment the \usepackage{hyperref} line in the header,   % we get the following error after the \skipitem above   %    % ./tmp.tex:20: Missing number, treated as zero.   % &lt;to be read again&gt;    %   \c@0.0ptxcii    % l.20   \skipitem   % \item[] \label{blockA}   \label{blockA}\item[]   \begin{steps}   \item\label{lineB} This is line~\ref{lineB}, in block~\ref{blockA}.   \item Inner again   \end{steps} \item Outer again \item Let's end here \end{steps}  \end{document}   Can someone help me work around this conflict, so I can use hyperref with this use of enumitems and refstepcounter?  Thanks.  (Note: smaller working examples are possible, but they give different error messages.  I'm hoping to get the current example working.) How to open pdf file in Windows Phone 8? Enumerate within TikZ node What is the technical term for when people thinks the actions of the few represent the many.  Like when people thinks everyone in a specific group behaves a certain way cause of how a few people in the group behave, including non stereotypical behaviours. I thought it was "generalisation" but I couldn't find any articles using that term. Can you make a non-polar electrolytic capacitor out of two regular electrolytic capacitors? There was some discussion on this question        which I don't see as being conclusively resolved:   "turns out that what might LOOK like two ordinary electrolytics are not, in fact, two ordinary electrolytics." "No, do not do this. It will act as a capacitor also, but once you pass a few volts it will blow out the insulator." 'Kind of like "you can't make a BJT from two diodes"' "it is a process that a tinkerer cannot do"   So is a non-polar (NP) electrolytic cap electrically identical to two electrolytic caps in reverse series, or not?  Does it not survive the same voltages?  What happens to the reverse-biased cap when a large voltage is placed across the combination?  Are there practical limitations other than physical size?  Does it matter which polarity is on the outside?  I don't see what the difference is, but a lot of people seem to think there is one.  Summary:   As posted in one of the comments, there's a sort of electrochemical diode going on:     The film is permeable to free electrons but substantially impermeable to ions, provided the temperature of the cell is not high. When the metal underlying the film is at a negative potential, free electrons are available in this electrode and the current flows through the film of the cell. With the polarity reversed, the electrolyte is subjected to the negative potential, but as there are only ions and no free electrons in the electrolyte the current is blocked. —    Normally a capacitor cannot be reverse-biased for long, or large currents will flow and "destroy the center layer of dielectric material via electrochemical reduction":     An electrolytic can withstand a reverse bias for a short period, but will conduct significant current and not act as a very good capacitor. —    However, when you have two back-to-back, the forward-biased capacitor prevents a prolonged DC current from flowing.  :     For circuit positions when reverse voltage excursions are unavoidable,   two similar capacitors in series connected “back to back” ... will create a   non-polar capacitor function ... This works because almost all the circuit voltage   is dropped across the forward biased capacitor, so that the reverse   biased device sees only a negligible voltage.   :     The oxide dielectric construction that is used in tantalum capacitors   has a basic rectified property which blocks current flow in one   direction and at the same time offers a low resistance path in the   opposite direction. Is there a modal command for MakeLowercase? Analog to  question I've been wondering WHAT is to \lowercase as \bfseries is to \textbf? (... and for uppercase, or \MakeLowercase and MakeUppercase, etc., which also seem to be text-block commands as explained .)  Is there a "switch"/"declaration"/"modal command" (what is the correct term here?, is there a difference between them?) that would allow one to turn something into lowercase/uppercase in the same way that one would use \scshape for example, i.e., not as text-block command with the curly braces?  Or how to create such a command?  I couldn't find any clue on the forum. input() error - NameError: name '...' is not defined