How does one find which tags they created?
Can I find out which tags I have created?
What's the best way to invite a user to answer a question on Stack Overflow? On several occasions, I was reading a question thinking "I know a guy that knows the answer to this" and that guy (or gal) also happens to be a registered user. How can I bring this to the attention of this user? AWK: Nested Conditional Subset of Rows I have a pretty specific question, and have been able to find lots on conditional subsetting using awk but none that lends enough explicit code for me to generalize to my situation. I have a file 'keys' and a file 'features'. The 'keys' table contains two variables, KEY and GROUP, toy example below.      KEY        GROUP     ---        -----     1          GROUP0     2          GROUP0     3          GROUP1     4          GROUP1     5          GROUP2     6          GROUP2     The file 'features' contains a list of features of widgets like so      ID          FEATURE      VALUE     --          -------      -----     A           num_user     10     A           KEY          4     B           num_user     2     B           KEY          2     B           battery      Large     C           num_user     10     C           KEY          15     D           num_user     2     D           KEY          2     D           battery      Small     E           num_user     2     E           KEY          7     E           battery      Small   I am trying to select all rows for an ID which has a value of 'KEY' that is in the 'KEY' column of 'keys' for a hardcoded list of 'GROUP' values. The desired result is      ID          FEATURE      VALUE     --          -------      -----     A           num_user     10     A           KEY          4     B           num_user     2     B           KEY          2     B           battery      Large     D           num_user     2     D           KEY          2     D           battery      Small   Any ideas? A challenge on the parabola Good Night ... Here I am editing the question and explaining the purpose of the same ... Mathematics course, and I love this science, I am very curious, and these days, studying a book of calculus found in (book from the library of the University) a sheet full of challenges, to share with a colleague, he gave me this challenge, which he also failed to respond ... I tried, I tried, I used the formulas of quadratic equation, and I've been trying since Friday (here in Brazil is Sunday at 22:33), and could not, and I ask your help, and please believe me, I struggled, tried to make substitutions, tried both, and I'm confused ... I must find the values of $ a, b $ and $ c $?  Consider a parabola whose roots, other are $x'$ and $x''$, and such that $x'=-x''$ where $x'&gt; 0$. Let $$A = (x', 0)$$ $$B = (x'', 0)$$ and $V$ the vertex of the parabola. Suppose that the equation of this parabola is $$y = ax^2 +bx+ c$$. Calculate the value of the discriminant $$\Delta=b^2-4ac$$, so that the triangle is equilateral AVB. Formal Proof that area of a rectangle is $ab$ I tried to prove that the area of a rectangle is $ab$ given side lengths $a$ and $b$.  The best I can do is the assume the area of a $1\times1$ square is $1$. Then not the number of $1\times1$ squares that fit in an $a\times b$ rectangle is $ab$. Therefore area is $a\cdot b$. This does not seem rigorous however. Find $\lim_{x \to 0} (\frac{\tan(x)}{x})^{\frac{1}{x}}$ Find $$\lim_{x \to 0} (\frac{\tan(x)}{x})^{\frac{1}{x}}$$   My first idea to solve this was to try to evaluate it and then apply the L'Hospital's rule. This is what I managed to achieve: $$\left(\frac{\tan(x)}{x}\right)^\frac{1}{x}=e^{\frac{1}{x}\ln\left(\frac {\tan(x)}x \right)} $$ However, this is problematic, because the L'Hospital's rule is not applicable in the exponent, therefore my transformation was not a good shot. What transformation should I apply to get the desired form? How to calculate 95% CI for cumulative incidence rate using R? I would like to show that  $$\forall n\in\mathbb{N}^*, \quad \sqrt{\frac{n}{n+1}}\notin  \mathbb{Q}$$  I'm interested in more ways of proofing this.  My method :  suppose that $\sqrt{\frac{n}{n+1}}\in  \mathbb{Q}$ then there exist $(p,q)\in\mathbb{Z}\times \mathbb{N}^*$ such that $\sqrt{\frac{n}{n+1}}=\frac{p}{q}$ thus   $$\dfrac{n}{n+1}=\dfrac{p^2}{q^2} \implies nq^2=(n+1)p^2 \implies n(q^2-p^2)=p^2$$ since $p\neq q\implies p^2\neq q^2$ then  $$n=\dfrac{p^2}{(q^2-p^2)}$$  since $n\in \mathbb{N}^*$ then $n\in  \mathbb{Q}$   I'm stuck here and I would like to see different ways to prove $\sqrt{\frac{n}{n+1}}\notin  \mathbb{Q}$ Understanding dummy (manual or automated) variable creation in GLM If a factor variable (e.g. gender with levels M and F) is used in the glm formula, dummy variable(s) are created, and can be found in the glm model summary along with their associated coefficients (e.g. genderM)  If, instead of relying on R to split up the factor in this way, the factor is encoded in a series of numeric 0/1 variables (e.g. genderM (1 for M, 0 for F), genderF (1 for F, 0 for M) and these variables are then used as numeric variables in the glm formula, would the coefficient result be any different?  Basically the question is: does R use a different coefficient calculation when working with factor variables versus numeric variables?  Follow-up question (possibly answered by the above): besides just the efficiency of letting R create dummy variables, is there any problem with re-coding factors as a series of numeric 0,1 variables and using those in the model instead? Learning development with Force platform. Any good books? I'm a bit familiar with Java and SQL, any good books to get on coding in Apex for salesforce? Integral $\int^{ \pi /2}_{0} \ln (\sin x)\ dx$ $$\int^{ \pi /2}_{0} \ln (\sin x)\  dx$$  The answer is $- \frac{\pi}{2} \ln 2$.  I have changed it into $$\frac{1}{2} \int^{1}_{0} t d \ln t^{2}$$  But I didn't get the answer with it. Writing a compiler in its own language Intuitively, it would seems that a compiler for language Foo cannot itself be written in Foo. More specifically, the first compiler for language Foo cannot be written in Foo, but any subsequent compiler could be written for Foo.  But is this actually true? I have some very vague recollection of reading about a language whose first compiler was written in "itself". Is this possible, and if so how? "I will appreciate your help" I encounter the phrase "I will appreciate your help" from time to time and it doesn't sound too honest to me. To be more specific, I don't have a problem with this phrase coming from a friend, because he will appreciate my help by "owing me one". But a stranger from the Internet, that's different, methinks.  "How will you appreciate it?" pops in my mind every time I see it, because to me, appreciation equals reward and, well, you can't reward people over the internet much. It is quite possible that I am just wrong and this is a perfectly fine figure of speech. Suppose we have $n$ covariates $x_1, \dots, x_n$ and a binary outcome variable $y$. Some of these covariates are categorical with multiple levels. Others are continuous. How would you choose the "best" model? In other words, how do you choose which covariates to include in the model?  Would you model $y$ with each of the covariates individually using simple logistic regression and choose the ones with a significant association? Why do harmonics occur when you pluck a string? Why is it possible to answer a question twice? You normally answer a question and you have the opportunity to answer again. The second time, a message is shown suggesting the use of the edit tool, but I think it's kind of wrong. I mean, if you answer something and then you want to say anything else you know that you must use edit.  Why not disable the answer button if you already have an answer? valueOf() vs. toString() in Javascript Suggested edit review should show custom rejection reasons Somewhat similar to , but rather more limited: when Rejecting an edit, not only the count of custom rejection reasons but the actual text of the reasons should be shown, IMO. Just now I  on SO that tidied some code in a trivially correct and substantially superior way because it looked perfectly legitimate and I couldn't understand why anyone would bother with a custom rejection reason. As it turned out, it was code in a question, of course.  In this case, the rejection reason correctly identified the problem, and had I seen it, I would have been reminded of that aspect I had neglected to check. I am working to build a reinforcement agent with DQN. The agent would be able to place buy and sell orders for a day trading purpose. I am facing a little problem with that project. The question is "how to tell the agent to maximize the profit and avoid the transaction where the profit is less than 100$".  I want to maximize the profit inside a trading day and avoid to place the pair (limit buy order, limit sell order) if the profit on that transaction is less than 100$. The idea here is to avoid the little noisy movements. Instead, I prefer long beautiful profitable movements. Be aware that I thought using the "Profit &amp; Loss" as the reward.  "I want the minimal profit per transaction to be 100$" ==> It seems this is not something that is enforceable. I can train the agent to maximize profit per transaction, but how that profit is cannot be ensured.  At the beginning, I wanted to tell the agent, if the profit of a transaction is 50 dollars, I will remove 100 dollars, then It becomes a penalty of 50 dollars for the agent. I thought it was a great way to tell the agent to not place a limit buy order if you are not sure it will give us a minimal profit of 100$. It seems that all I would be doing there is simply shifting the value of the reward. The agent only cares about maximizing the sum of rewards and not taking care of individual transactions.   How to tell the agent to maximize the profit and avoid the transaction where the profit is less than 100$? With that strategy, what guarantee that the agent will never make a buy/sell decision that results in less than 100 dollars profit? Does the sum of reward - # transaction * 100 can be a solution? How to intuitively understand eigenvalue and eigenvector? I'm learning multivariate analysis and I have learnt linear algebra for two semester when I was a freshman.  Eigenvalue and eigenvector is easy to calculate and the concept is not difficult to understand.I found that there are many application of eigenvalue and eigenvector in multivariate analysis. For example     In principal components, proportion of total population variance due   to kth principal component equal    $$\frac{\lambda_k}{\lambda_1+\lambda_2+...\lambda_k}$$   I think eigenvalue product corresponding eigenvector has same effect as the matrix product eigenvector geometrically.   I think my former understanding may be too naive so that I cannot find the link between eigenvalue and its application in principal components and others.   I know how to induce almost every step form the assumption to the result mathematically. I'd like to know how to intuitively or geometrically understand eigenvalue and eigenvector in the context of multivariate analysis(in linear algebra is also good).  Thank you! Let $U$ be connected. A function $u \in H^1(U)$ is a weak solution   of Neumann's problem   \begin{equation} (*)\qquad\left\{ \begin{array}{rl} -\Delta = f &amp; \text{in } U \\ \frac{\partial u}{\partial \nu} = 0 &amp; \text{on } \partial U \end{array} \right. \end{equation}   if   $$ \int_U Du \cdot Dv \; dx = \int_Ufv \; dx $$   for all $v \in H^1(U)$. Let $f\in L^2(U)$. Prove $(*)$ has a weak solution   if and only if   $$ \int_U f \; dx =0. $$   For the only if part I set $v=1$. However I do not see where to start the if part. I was thinking of the Lax-Milgram theorem. That's where I'm now.