How can I find the first element returned from a  type of list returned by SQLite? I have this code I am reviewing:             var phrases = db2                 .Query&lt;Phrase&gt;("SELECT C.Id As CategoryId" +                               " FROM Category AS C" +                               " JOIN Phrase AS P ON C.Id =          P.CategoryId");   phrases has a type of List&lt;Phrase&gt;  Now I would like to find the first element of the list. But with my IDE when I hover over "phrases."  on another line all I see is the option to get a .count .add .addRange .find and similar.   Can someone explain how I can get the first element?
How do I get the first element from an IEnumerable in .net? I often want to grab the first element of an IEnumerable&lt;T&gt; in .net, and I haven't found a nice way to do it.  The best I've come up with is:  foreach(Elem e in enumerable) {   // do something with e   break; }   Yuck!  So, is there a nice way to do this?
Electric potential is related to Electric Field by negative integral of Field with distance.If charges move away from the positive terminal (Assuming conventional flow) why is there no drop in electric potential even though the charges are moving in the direction of electric field?  Some of the answers I read for the similar question said that the electrons in wires rearrange themselves in a way to make the electric field uniform in the wire.However,this doesn’t explain the absence of potential drop in the wires (due to electric field) since the field is still there. Resolve build errors due to circular dependency amongst classes I often find myself in a situation where I am facing multiple compilation/linker errors in a C++ project due to some bad design decisions (made by someone else :) ) which lead to circular dependencies between C++ classes in different header files (can happen also in the same file). But fortunately(?) this doesn't happen often enough for me to remember the solution to this problem for the next time it happens again.  So for the purposes of easy recall in the future I am going to post a representative problem and a solution along with it. Better solutions are of-course welcome.     A.h  class B; class A {     int _val;     B *_b; public:      A(int val)         :_val(val)     {     }      void SetB(B *b)     {         _b = b;         _b-&gt;Print(); // COMPILER ERROR: C2027: use of undefined type 'B'     }      void Print()     {         cout&lt;&lt;"Type:A val="&lt;&lt;_val&lt;&lt;endl;     } };       B.h  #include "A.h" class B {     double _val;     A* _a; public:      B(double val)         :_val(val)     {     }      void SetA(A *a)     {         _a = a;         _a-&gt;Print();     }      void Print()     {         cout&lt;&lt;"Type:B val="&lt;&lt;_val&lt;&lt;endl;     } };       main.cpp  #include "B.h" #include &lt;iostream&gt;  int main(int argc, char* argv[]) {     A a(10);     B b(3.14);     a.Print();     a.SetB(&amp;b);     b.Print();     b.SetA(&amp;a);     return 0; } Does it make sense to combine PCA and LDA? Assume I have a dataset for a supervised statistical classification task, e.g., via a Bayes' classifier. This dataset consists of 20 features and I want to boil it down to 2 features via dimensionality reduction techniques such as Principal Component Analysis (PCA) and/or Linear Discriminant Analysis (LDA).  Both techniques are projecting the data onto a smaller feature subspace: with PCA, I would find the directions (components) that maximize the variance in the dataset (without considering the class labels), and with LDA I would have the components that maximize the between-class separation.  Now, I am wondering if, how, and why these techniques can be combined and if it makes sense.  For example:   transforming the dataset via PCA and projecting it onto a new 2D subspace transforming (the already PCA-transformed) dataset via LDA for max. in-class separation   or    skipping the PCA step and using the top 2 components from a LDA.   or any other combination that makes sense. How to execute a JavaScript function when I have its name as a string Why do clocks measure arc-length? Apologies in advance for the long question.  My understanding is that in GR, massive observers move along timelike curves $x^\mu(\lambda)$, and if an observer moves from point $x^\mu(\lambda_a)$ to $x^\mu(\lambda_b)$, then his clock will measure that an amount of time $t_{ba}$ given by the curve's arc length; $$   t_{ba} = \int_{\lambda_a}^{\lambda_b}d\lambda \sqrt{-g_{\mu\nu}(x(\lambda))\dot x^\mu(\lambda)\dot x^\nu(\lambda)} $$ will have elapsed where $g_{\mu\nu}$ is a metric on spacetime with signature $(-,+,+,+)$.    Why is this so?     Here is how I would attempt to justify this fact in special relativity with $g_{\mu\nu} = \eta_{\mu\nu}$.  Consider an inertial observer $O$ in $\mathbb R^{3,1}$, and suppose that this observer sees a clock, which I'll call observer $O'$ moving around on a curve $x^\mu(\lambda)$.  If $O'$ were also an inertial observer, then given any event with coordinates $x^\mu$ as measured by $O$, observer $O'$ would measure the coordinates of the event to be $x'^\mu = \Lambda^\mu_{\phantom\mu\nu} x^\nu + x_0^\mu$ for some Lorentz transformation $\Lambda$.  If $O'$ is not inertial, then this is no longer true, and there is some more complicated family of transformations, say $T_\lambda$ between events as seen by both observers.  I would argue, however, that if we were to partition the interval $[\lambda_a, \lambda_b]$ into a large number $N$ of intervals $I_1=[\lambda_a, \lambda_i], \dots, I_N=[\lambda_{N-1}, \lambda_b]$ with $\lambda_n = \lambda_a+n\epsilon_N$ and $\epsilon_N=(\lambda_b-\lambda_a)/N$, then on each interval $I_n$, $O'$ is approximately an inertial observer in the sense that $$   T_{\lambda_n} = P_n + \mathcal O(\epsilon_N), \qquad (\star) $$ for some Poincare transformation $P_n$.  Then we would note that since $O'$ is stationary in his own reference frame, he measures his worldline to have the property $\dot x'^\mu(\lambda) = (\dot t(\lambda), \mathbf 0)$ so that $$   I_{ba}=\int_{\lambda_a}^{\lambda_b}d\lambda \,\sqrt{-\eta_{\mu\nu}\dot x'^\mu\dot x'^\mu} = \int_{\lambda_a}^{\lambda_b} d\lambda \, \sqrt{\dot t^2} = t(\lambda_b) - t(\lambda_a) = t_{ba} $$ On the other hand the integral on the left can be written as a Riemann sum using the partition above, and we can invoke ($\star$) above to get \begin{align}   I_{ba}  &amp;= \lim_{N\to\infty}\left[\sum_{n=1}^N \epsilon_N\sqrt{-\eta_{\mu\nu}\dot x'^\mu(\lambda_n)\dot x'^\nu(\lambda_n)}\right] \notag\\ &amp;= \lim_{N\to\infty}\left[\sum_{n=1}^N \epsilon_N\sqrt{-\eta_{\mu\nu}\dot x^\mu(\lambda_n)\dot x^\nu(\lambda_n)} + \mathcal{O}(\epsilon_N^2)\right] \notag\\ &amp;= \int_{\lambda_a}^{\lambda_b}d\lambda \,\sqrt{-\eta_{\mu\nu}\dot x^\mu\dot x^\mu} \end{align} Combining these two computations gives the desired result.  How do others feel about this argument?  I'm not completely comfortable with it because of the assumption $(\star)$ I made on $T_\lambda$.  I imagine that in GR a similar argument could be made by invoking local flatness of the metric. List AppDomains in Process Story about a sentient star system, in a 50s or early 60s anthology Story about a sentient star system, possibly in f&amp;sf anthology from 50s or early 60s. I know this is a long shot, but I hope someone will know of this story.  I believe it was called "Darkness or into the darkness". End of the story had it spinning off a galaxy, (possibly ours?). I'm trying to create globally-unique identifiers in JavaScript.  I'm not sure what routines are available on all browsers, how &quot;random&quot; and seeded the built-in random number generator is, etc. The GUID / UUID should be at least 32 characters and should stay in the ASCII range to avoid trouble when passing them around. So from what I understand transistors are one of the most important electronic elements for computers because of TTL (Transistors-Transistors Logic).  Is this true? If so, how exactly does this work? I have heard the terms "logic gate" and "logic block" tossed around. Proving $\sum\limits_{i=1}^k | \langle x,v_i \rangle \langle y,v_i\rangle| \leq \|x\|\cdot \|y\|$ Let $V$ be a real inner product space, and let $v_1,v_2, \dots ,v_k$ be an orthonormal set of vectors. How do you prove that  $$\sum_{i=1}^k | \langle x,v_i \rangle \langle y,v_i\rangle| \leq \|x\|\cdot\|y\|?$$  When does the equality hold?  I've been trying to do this with the Bessel and Cauchy-Schwarz inequalities, but I can't make it work yet. Any help would be greatly appreciated. Markdown code snippets don't allow trailing spaces Does a preprint include revisions made in response to peer review? This is related to but not answered in .  After an author has submitted a manuscript to a journal for peer-review, very often the journal asks the author to revise (and resubmit) the manuscript as a precondition for its publication. I am wondering whether the term preprint refers to the manuscript before or after its authors made these revisions.  If a preprint includes revisions made in response to peer review, then there is only a slight difference, mostly regarding layout and style, between the preprint and the article that is published in the journal. From a reader’s perspective, there would be no point in paying for access to the article, since they can simply download the preprint free of charge. A publisher would probably want to prevent their authors (by publication agreements etc.) from publishing preprints that differ from the respective journal article only in form.  As a side question, what kind of preprints do publishers tolerate, as a rule? Proving that an additive function $f$ is continuous if it is continuous at a single point Suppose that $f$ is continuous at $x_0$ and $f$ satisfies $f(x)+f(y)=f(x+y)$. Then how can we prove that $f$ is continuous at $x$ for all $x$? I seems to have problem doing anything with it. Thanks in advance. Prove that for all integers $n$, $n \geq 1$,   $$1 + 3 + 5 + \cdots + (2n - 1) = n\cdot n$$   How would I go about proving this? How to get Lookup ID from Search API with REST Stop making iPhone constantly change sound volume on its own? I regularly get interrupted in watching and listening to video by the volume randomly getting lowered without me asking for it. I get a system notification telling me &quot;the volume was lowered&quot;, telling me it's protecting my hearing based on the last 7 days of use. I've never been asked about enabling such a feature. I have already disabled what relevant items I can find under Sound &amp; Haptics, which is given as the source of the notification. It apparently doesn't matter though because it's not being respected. How can I make this stop? iPhone 8, iOS 14.2 What is this electronics symbol (four triangles?) The below image is from a service manual for a mobile radio. Note the small symbols that appear to be four triangles, point to point. I've searched far and wide and cannot identify that symbol. I have a nagging feeling that I've seen something like it in hydraulics, but never in an electronics diagram. I know that my radio doesn't have a bunch of stars of David printed inside it, so what could this symbol be?  Some of them have a line through the middle, others don't. Compare for example the ones near the labels "mic int" and "mic ext". Does this represent something, or is it a typo? Diffeomorphism & Weyl transformations in the 2D worldsheet of string theory and the existence of conformal gauge D. Tong's , chapter 5 (), feature the following in introducing the symmetries used in the Faddeev-Popov method:     We have two gauge symmetries: diffeomorphisms and Weyl transformations. We will schematically denote both of these by $\zeta$. The change of the metric under a general gauge transformation is $g \rightarrow g^{\zeta}$. This is shorthand for,   \begin{equation} g_{\alpha \beta} (\sigma) \rightarrow g_{\alpha \beta}^{\zeta} (\sigma)= e^{2\omega (\sigma)} \frac{\partial \sigma^{\gamma}}{\partial \sigma'^{\alpha}} \frac{\partial \sigma^{\delta}}{\partial \sigma'^{\beta}} g_{\gamma \delta} (\sigma) \end{equation}   In two dimensions these gauge symmetries allow us to put the metric into any form that we like — say, $\hat{g}$. This is called the fiducial metric and will represent our choice of gauge fixing.   Is there a proof available that shows that the combined symmetry allows us to "put the metric in any form (locally, of course)?" Is this property restricted to two dimensions? Prove that $(p+q)^m \leq p^m+q^m$ Can't do comparison on PageReference