BufferedReader causes NPE
What is a NullPointerException, and how do I fix it?
Writing function definition in header files in C++ I have a class which has many small functions. By small functions, I mean functions that doesn't do any processing but just return a literal value. Something like:  string Foo::method() const{     return "A"; }   I have created a header file "Foo.h" and source file "Foo.cpp". But since the function is very small, I am thinking about putting it in the header file itself. I have the following questions:   Is there any performance or other issues if I put these function definition in header file? I will have many functions like this. My understanding is when the compilation is done, compiler will expand the header file and place it where it is included. Is that correct? Dark Energy / Accelerating universe: naive question QGIS: ESRI shapefile .shp no longer appears in list of file types under Add Vector Layer I recently installed QGIS 2.18.4 after using ArcMap for many years. I want to add several existing ESRI shapefiles (.shp) created in Arc, to a QGIS project. To add a new layer in QGIS, I select the "Add Vector Layer" button and then click "Browse" to go find the layer. Then I navigate to the proper folder. However, the drop down menu at the lower right of the "Open an OGR Supported Vector Layer" window does not include ESRI Shapefile on the list. The first one on the list is GeoJSON. Everything after "G" appears to be okay (all the way down to "X-Plane/Flightgear") but any file formats from A through F have been cut from the list. Since "ESRI" starts with "E" it does not show up on the list. Strange thing is when I first installed QGIS, ESRI Shapefile appeared on this list. So I must have accidently done something to change how that list is populated and I want to know how to change it back.   If I type in the name of the ESRI shapefile that I know is in the folder, it and all other files starting with those letters appears in the prompt window that appears. And QGIS will correctly load the Shapefile. However, I want to be able to see all those ESRI Shapefiles without having to know and type in the first letter so it will appear in the prompt. I want it to appear in the file type drop down (or drop up?) menu. Modified Hamilton's Principle overconstraining a system by imposing too many boundary conditions In Hamiltonian Mechanics, a version of Hamilton's principle is shown to evolve a system according to the same equations of motion as the Lagrangian, and therefore Newtonian formalism. In particular, letting $\delta$ indicate a variation of the path through phase space, $$ \delta \int_{t_1}^{t_2} \big(p_i \dot{q}_i - H(p_i,q_i,t)\big)dt = 0 $$ is shown to generate the same motion equations as those found by the Legendre transform of the Lagrangian equations of motion. Essentially, when we calculate the Euler-Lagrange equations for the above integrand, we find $$ \dot{p}_i + \frac{\partial H}{\partial q_i} = 0\text{, } \dot{q}_i - \frac{\partial H}{\partial p_i} = 0. $$ Now, since these are both first-order equations of motion, we require $2n$ boundary conditions to obtain a solution, where $n$ is the number of particles. This is perfectly consistent with the Lagrangian formalism which had $2n$ boundary conditions due to its $n$ distinct second-order motion equations.   An important thing to note is that since $p_i\dot{q}_i - H(p_i,q_i,t)$ contains no $\dot{p}_i$ dependence, the variation in $p_i(t)$ need not be zero at the endpoints of the path. This is not the case with $q_i(t)$ because the $\dot{q}_i$ dependance results in the appearance of boundary terms that must be set to zero in order to obtain the motion equations.  Now, as far as the boundary conditions are concerned, this makes sense. For the path to be uniquely specified given Newtonian mechanics, we require $2n$ boundary conditions. These can either be the usual Newtonian choice of initial positions and velocities/momenta, or they can be the initial and final positions. Both are perfectly acceptable mathematically speaking. If we were required, in the modified Hamilton's principle variation, to enforce that the variations in the momenta also be zero at the initial and final times, this would mean $2n$ additional boundary conditions, which would generally over constrain a Newtonian system.   However, my confusion is this: when textbooks (Goldstein in particular) consider generating functions, they require that the integrand be invariant to the addition of a total time derivative of a function $F(q_i,p_i,t)$ of the phase space coordinates. However, adding such a function will in general add some $\dot{p}_i$ dependence to the integrand, which will then add boundary terms to the equations of motion unless we require that the $p_i(t)$ variations be zero at the boundaries. This is fine as far as it goes. We can always define the variation however we like. The point is just that it spits out the right motion equations at the end of the day. But my confusion lies in how this doesn't generally over constrain the system. If we specify all the positions and momenta at both the initial and the final times, might not the required path to connect those points in phase space be non-Newtonian? Does this only matter if you're actually trying to use the stationary action principle to find the paths, rather than just using it to find the motion equations? Why does a balloon spiral in the air instead of moving in a straight line? I need a regular expression to select all the text between two outer brackets.  Example: some text(text here(possible text)text(possible text(more text)))end text  Result: (text here(possible text)text(possible text(more text))) What kind of lubrication should I use for a lock? I recently made the Yale lock on my front door much easier to open by spraying some WD-40 in there.  However, I just saw  on WD-40 not being a “true” lubricant, and wondered whether it was appropriate to use on Yale locks? Automatically solve dependencies of deb packages while using dpkg -i Installing Deb packages on the command line, using dpkg -i, only installs the package in question and displays errors of unresolved dependencies (manually solved by apt-geting the packages it complains about). How do I automatically resolve dependencies when installing standalone packages? Why use a prime number in hashCode? Factorial of $i$ With $i^2=-1$, what is $i!$ ?  I'm not sure if it is $i$ or if it is an infinite product $i(i-1)(i-2)(i-3)(i-4)(i-5)\dots$  It would make sense either way.  Is it not defined? I've recently updated to Ubuntu 20.04. When English is not the chosen language all shortcuts don't work (such as Ctrl + C, Ctrl + A, etc). It was working fine in the previous versions. What happened and What can I do to fix it? I have created 50 spreadsheets for 50 students. Each spreadsheet contains 33 sheets, numbered 1 to 33, along with a couple of other sheets.  Within each of these 33 sheets, there are many protected ranges.  I accidentally protected a range that ought not to have been protected. So, with 50 students each having 33 sheets, I have over 1800 sheets to unprotected.  I want to loop through each sheet, 1 to 33, and remove protection from cell F15. Right now, I (the owner) am the only editor.  Since I'm not an actual programmer, I've clearly not done this properly.  Can someone offer a suggestion of how I can change my Google Script to make this work?  var ss = SpreadsheetApp.getActiveSpreadsheet(); var sheets = ss.getSheets();  for (var i=2; i  &lt;sheets.length; i++) {   var sheet = ss.getSheets()[i];   sheet.activate();   SpreadsheetApp.flush();   var range = sheet.getRange('F15');        var protection = range.protect().setDescription('Protected range');        var me = Session.getEffectiveUser();        protection.addEditor(me);        protection.removeEditors(protection.getEditors());        if (protection.canDomainEdit()) {          protection.remove();          } //end of if statement  } How can I use quotas in samba? I have a simple set-up with just one public share on my configuration file. How can I restrict its size? I am working on the following problem asked in an examination: "What is the maximum number of cylindrical pencils of 0.5 cm diameter that can be stood in a square shaped stand of 5cm × 5cm inner cross section? 1. 99    2. 121   3. 100    4. 105." A simple calculation yielded me the answer 100.  But the answer in the key is 105. I don't how such an answer is possible. Any hint please.....Thanks in advance! Irreducibility of a polynomial if it has no root (Capelli) Let $F$ be a field of arbitrary characteristic, $a\in F$, and $p$ a prime number. Show that $$f(X)=X^p-a$$ is irreducible in $F[X]$ if it has no root in $F$.   This  to a related question mentions the result is due to Capelli.  I can prove the result if $F$ has characteristic $p$ as follows.  Suppose $f$ is reducible: $f(X)=g(X)h(X)$ with $g(X)$ an irreducible factor of degree $m$, $1\le m&lt;p$. Then if $\alpha$ is a root of $g$ in some extension field $K$ of $F$, we have $$f(X)=X^p-\alpha^p=(X-\alpha)^p$$ so its divisor $g(X)$ must be of the form $(X-\alpha)^m$. Since the coefficient of $X^{m-1}$ in $g$ is in $F$, we have $m\alpha\in F$.  So $\alpha\in F$ because $m$ is invertible modulo $p$.  How would you show the result in other characteristics? Show that G is a group, if G is finite, the operation is associative, and cancellation law holds Help with Spivak Calculus Ch3 Problem 6a Yet again I find myself stuck on a Spivak question. This time it is simply the question that isn't clear to me. It states:  If $x_1, ..., x_n$ are distinct numbers, find a polynomial function $f_i$, of degree $n-1$ which is 1 at $x_i$ and 0 at $x_j$ for $j\neq i$. Hint: the product of all $(x-x_j)$ for $j\neq i$, is 0 at $x_j$ if $j\neq i$. This product is usually denoted by: $$\prod_{\begin{smallmatrix}{j=1}\\ {j\neq i} \end{smallmatrix}}^n(x-x_j)$$  To be honest I don't know where to begin. I understand how to create a polynomial equation for a given set of results but this is quite strange. I'm not sure exactly what the author is expecting.  The answer book shows:  $$f_i(x)={\prod_{\begin{smallmatrix}{j=1}\\ {j\neq i} \end{smallmatrix}}^n(x-x_j)}/{\prod_{\begin{smallmatrix}{j=1}\\ {j\neq i} \end{smallmatrix}}^n(x_i-x_j)}$$  Which I have expanded out just fine but don't know what to do with. LWC URL reading parameters I am trying to authorize another sandbox/production to create connection. When I first time authorize the connected app, it is returning the Auhorization Code in the URL, however when it gets redirected to LWC app, code is disappeared because the parameter is 'code' not like 'c__code'. So, pagereference state is unable to read the param value. It was working till the previous release, with Spring release it started behaving differently.  Original URL: not working    I tested with adding 'c__' to the param manually and it worked fine.  Modified URL: Working    is there a way we can rename the redirected URL params dynamically?  'code' to 'c__code' When should I start an LLC for my side work? I am doing quite a bit of sidework this year and wondering at what point would it be a good idea to "start a business" and bill through that. Current sensing nA to mA I need to sense the current of a target  through various periods of its on/sleep/off state and then need to send this data back to another interface MCU to read out the value. They will be both on the same board, powered by 5&nbsp;V USB.  However these are the requirements I have:   5&nbsp;V USB PSU Must be able to sense currents in the range of nA to mA with high    accuracy/precision. (I figure 1&nbsp;nA to 500&nbsp;mA) Only measure the current of the target MCU and not the interface. Must output maximum 3.3&nbsp;V interface MCU   I have looked at available options and current sense amplifiers by Texas Instruments can't sense nA due to higher bias currents. So therefore I feel I would need a precision amplifier.  However I am stuck on how to proceed with this because, I need to somehow have a automatic dynamic current range and I am fairly new to current sensing and am not really sure on all the specifics.