I have seen both in Cygwin and in Linux the [ command under /bin. Whenever I run it I get an error saying: missing ].   I never understood what this command does, I just one day found it wanted to find out what it does (and whether it is useful or not)  NOTE: I would have checked the man entry, for an answer, but sadly, there is no entry in it for [
I was looking at coreutils and found this as one of the files included as part of coreutils:  /usr/bin/[. What is [ and what does it do? It is an executable. I just don't know what it does or how to use it. $ file /usr/bin/[   /usr/bin/[: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared  libs), for GNU/Linux 2.6.15, stripped    When I try to run it, I think it is defaulting to the bash built in line expansion. Instead of actually running the file. $ &quot;/usr/bin/[&quot; /usr/bin/[: missing ‘]’ $ /usr/bin/\[ /usr/bin/[: missing ‘]’
Let's consider it case by case:  Case 1: Charged particle is at rest. It has an electric field around it. No problem. That is its property.  Case 2: Charged particle started moving (it's accelerating). We were told that it starts radiating EM radiation. Why? What happened to it? What made it do this?  Follow up question: Suppose a charged particle is placed in uniform electric field. It accelerates because of electric force it experiences. Then work done by the electric field should not be equal to change in its kinetic energy, right? It should be equal to change in K.E + energy it has radiated in the form of EM Waves. But then, why don't we take energy radiated into consideration when solving problems? (I'm tutoring grade 12 students. I never encountered a problem in which energy radiated is considered.) Infinite Series Convergence Verification I'm currently tackling the following infinite series in order to determine if it converges or not:  $$\sum_{n=1}^\infty \frac{\sqrt{n+1}-\sqrt{n}}{n}$$  What I've done is rearranged this to make use of the comparison test, shown below:  $$= \frac{\sqrt{n+1}-\sqrt{n}}{n} * (\frac{\sqrt{n+1}+\sqrt{n}}{\sqrt{n+1}+\sqrt{n}})$$  $$= \frac{(n+1)-(n)}{n(\sqrt{n+1}+\sqrt{n})}$$  $$= \frac{1}{n(\sqrt{n+1}+\sqrt{n})}$$  $$= \frac{1}{n^{3/2}+\sqrt{n+1}} &lt; \frac{1}{\sqrt{n+1}} &lt; \frac{1}{\sqrt{n}} $$  However, $\frac{1}{n}$ is smaller than $\frac{1}{\sqrt{n}}$, therefore:  $$  \frac{1}{n}   &lt; \frac{1}{\sqrt{n}}$$   This implies divergence by the comparison test due to the fact $\frac{1}{n}$ diverges, and if a smaller series diverges any larger series must as well.  Is this right? If I've made any mistakes in any part, please tell me! I have basic programming skills (I program simple vba python and lisp scripts and had gone through a java course a long time ago), but it seems to me (judging by ESRI publications) that there is no escaping the need to learn .NET for future relevance in the GIS job market (even as an analyst). I've gone through a basic c# course and can program (regrettably) quite poor programs, but I have no idea how to link the c# knowledge to actually programming for ArcGIS in order to progress.  I know the differences aren't big, but does ArcGIS use .NET or C#?  I know the best way to further my experience in .NET is to program more and more but are there any good .NET programming books or courses I can learn techniques from?  In your opinion, what is the best way to learn to program in .NET for ArcGIS? Are there any useful books or websites?   I seek your comments regarding ArcGIS 10 (and will be saving them for later) but also for ArcGIS 9.3.1. Requires installation of untrusted packages? How can I revert a config file back to the originally installed version after I have edited it? I want to revert a file in /etc/ back to its originally-installed form. I have edited the file since it was installed. How can I revert this config file? Apt is smart enough not to overwrite edited config files, so how do I tell it that I want it to do so?  For argument's sake, let's say that I want to revert the file /etc/foo.conf from the package foo. What IDEs are available for Ubuntu? This question exists because it has historical significance, but it is not considered a good, on-topic question for this site, so please do not use it as evidence that you can ask similar questions here. See  for more information.   This is a community wiki for IDEs available on Ubuntu. Please post one IDE per answer (including more than just a screenshot or a link, please at least put a short description).  In your answer, tell us what the IDE is for (which language(s) or if it is RAD capable). Multirow and Multicolumn with missing lines Let $\{X_n\}_{n=1}^\infty$ be a sequence of random variables converging to a random variable $X$. Does this fact implies that $\lim\limits_{n\to \infty} EX_n = EX$?  If not, is there any other sufficient condition? Help me identify this fantasy book about two near-omnipotent wizard run empires Zip all files in directory? Is there a way to zip all files in a given directory with the zip command? I've heard of using *.*, but I want it to work for extensionless files, too. Handling data in a PHP JSON Object Trends data from Twitter Search API in JSON.  Grabbing the file using:  $jsonurl = "http://search.twitter.com/trends.json"; $json = file_get_contents($jsonurl,0,null,null); $json_output = json_decode($json);   How do I work with data from this object. As an array? Only really need to extract data from the [name] values.  JSON object contains:  stdClass Object (     [trends] =&gt; Array         (             [0] =&gt; stdClass Object                 (                     [name] =&gt; Vote                     [url] =&gt; http://search.twitter.com/search?q=Vote                 )              [1] =&gt; stdClass Object                 (                     [name] =&gt; Halloween                     [url] =&gt; http://search.twitter.com/search?q=Halloween                 )              [2] =&gt; stdClass Object                 (                     [name] =&gt; Starbucks                     [url] =&gt; http://search.twitter.com/search?q=Starbucks                 )              [3] =&gt; stdClass Object                 (                     [name] =&gt; #flylady                     [url] =&gt; http://search.twitter.com/search?q=%23flylady                 )              [4] =&gt; stdClass Object                 (                     [name] =&gt; #votereport                     [url] =&gt; http://search.twitter.com/search?q=%23votereport                 )              [5] =&gt; stdClass Object                 (                     [name] =&gt; Election Day                     [url] =&gt; http://search.twitter.com/search?q=%22Election+Day%22                 )              [6] =&gt; stdClass Object                 (                     [name] =&gt; #PubCon                     [url] =&gt; http://search.twitter.com/search?q=%23PubCon                 )              [7] =&gt; stdClass Object                 (                     [name] =&gt; #defrag08                     [url] =&gt; http://search.twitter.com/search?q=%23defrag08                 )              [8] =&gt; stdClass Object                 (                     [name] =&gt; Melbourne Cup                     [url] =&gt; http://search.twitter.com/search?q=%22Melbourne+Cup%22                 )              [9] =&gt; stdClass Object                 (                     [name] =&gt; Cheney                     [url] =&gt; http://search.twitter.com/search?q=Cheney                 )          )      [as_of] =&gt; Mon, 03 Nov 2008 21:49:36 +0000 ) According to , $[0,1]$ cannot be written as union of countable disjoint closed sets, is the same true about (uncountable) family of disjoint closed intervals ? How to convert Bedrock texture pack to Java Edition Minecraft I'm trying to use a texture pack designed for the Bedrock Edition of Minecraft, on Java Edition. I looked at the texture names for a Java Edition texture pack and compared those to the names of the Bedrock Edition texture names, and they aren't the same. A lot of the texture names are flipped, such as:   Jungle Log (Java = jungle_log.png) (Bedrock = log_jungle.png)   Most of the texture names are flipped, like the Jungle Log (and all of the other log types) and there are some .json files that are different and/or not there.  Trying to open the Bedrock texture pack would result in some animated blocks not being animated, other blocks play the wrong sound (or no sound) when broken, if OptiFine is in the mix, the skybox could be missing, which I am pretty sure would result in an always black sky, along with no sun and moon textures.  Any help in finding a program or site that can do this would be helpful, because I tried doing it manually, but almost every texture is different.  Any help is appreciated! Add max value to a new column in R Inbox chops messages in the middle of an HTML character reference The messages in the inbox get chopped if they too long, and the chop can occur in the middle of an HTML character reference as can be seen in this screenshot:    The character being chopped is [ and it appears inside a code snippet, in case it makes a difference. Bayes' Theorem Intuition I've been trying to develop an intuition based understanding of Bayes' theorem in terms of the prior, posterior, likelihood and marginal probability. For that I use the following equation: $$P(B|A) = \frac{P(A|B)P(B)}{P(A)}$$ where $A$ represents a hypothesis or belief and $B$ represents data or evidence. I've understood the concept of the posterior - it's a unifying entity that combines the prior belief and the likelihood of an event. What I don't understand is what does the likelihood signify? And why is the marginal probability in the denominator? After reviewing a couple of resources I came across this quote:     The likelihood is the weight of event $B$ given by the occurrence of $A$ ... $P(B|A)$ is the posterior probability of event $B$ , given that event $A$ has occurred.   The above 2 statements seem identical to me, just written in different ways. Can anyone please explain the difference between the two? Create line heatmap with QGIS I have loaded my bike tours as .gpx files into QGIS.    It already looks quite nice but I was wondering if it might be possible to create something similar to the strava heatmap with tools of QGIS.     I have tried to generate points along the lines and use kernel density but it just does not look nice as a raster layer.  Any suggestions? Significance of $\sqrt[n]{a^n} $?! There is a formula given in my module:  $$ \sqrt[n]{a^n} = \begin{cases}  \, a  &amp;\text{ if $n$ is odd } \\  |a| &amp;\text{ if $n$ is even } \end{cases} $$  I don't really understand the differences between them, kindly explain with an example. Travel grant for undergraduate researcher for attending conference I am an undergraduate student doing independent research. My research work is selected for presentation at a very reputed conference in the US. I am from India. I cannot afford to attend the conference at all. How should I fund my trip? The travel grant for students available from the conference is very low. It can hardly buy me a one way ticket. What are my options? I'm having some difficulty understanding how to interpret variable importance output from the Random Forest package. Mean decrease in accuracy is usually described as "the decrease in model accuracy from permuting the values in each feature".   Is this a statement about the feature as a whole or about specific values within the feature? In either case, is the Mean Decrease in Accuracy the number or proportion of observations that are incorrectly classified by removing the feature (or values from the feature) in question from the model?   Say we have the following model:  require(randomForest) data(iris) set.seed(1) dat &lt;- iris dat$Species &lt;- factor(ifelse(dat$Species=='virginica','virginica','other')) model.rf &lt;- randomForest(Species~., dat, ntree=25, importance=TRUE, nodesize=5) model.rf varImpPlot(model.rf)     Call:  randomForest(formula = Species ~ ., data = dat, ntree = 25,  proximity = TRUE, importance = TRUE, nodesize = 5)  Type of random forest: classification Number of trees: 25 No. of variables tried at each split: 2          OOB estimate of  error rate: 3.33% Confusion matrix:           other virginica class.error other        97         3        0.03 virginica     2        48        0.04     In this model, the OOB rate is rather low (around 5%). Yet, the Mean Decrease in Accuracy for the predictor (Petal.Length) with the highest value in this measure is only around 8.   Does this mean that removing Petal.Length from the model would only result in an additional misclassification of 8 or so observations on average?   How could the Mean Decrease in Accuracy for Petal.Length be so low, given that it's the highest in this measure, and thus the other variables have even lower values on this measure?