Why does float(3.1) + float(18.8) give such weird decimals?
Is floating point math broken?
Is the complement of a prime ideal closed under both addition and multiplication? I just read a description of CVE-2020-1971, which indicates that a patch was released today for openSSL, the patched version is 1.1.1i -- my version of Ubuntu 20.04.1 LTS is running openSSL version 1.1.1f.  From last March. Software Updater says that my system is up to date.  I am hoping that this openSSL update is released by Canonical by tomorrow... can anyone confirm they are working on this? I want to simplify the following:  $\sum_{n=0}^x 1 - (1 - I[a \ne 0] * I[a \ge b]) * (1 - I[b \ne 0] * I[b \ge c]) * (1 - I[c \ne 0] * I[c \ge d]) * (1 - I[d = 0])$ Learning Regular Expressions This question attempts to collect a community-maintained list of quality books on the  programming language, targeted at various skill levels.  C is a complex programming language that is difficult to pick up on-the-go by reading online tutorials. A comprehensive book is often the best way to learn the language, and finding a good book is the first step. It is important to avoid badly-written books, and even more importantly, books that contain serious technical errors.  Please suggest edits to the accepted answer to add quality books, with an approximate skill level and a short blurb/description about each book. (Note that the question is locked, so no new answers will be accepted. A single answer is being maintained with the list)  Feel free to debate book choices, quality, headings, summaries, skill levels, and anything else you see that is wrong. Books that are deemed satisfactory by the C community here will stick around on the list; the rest will be regularly removed.  For books that have reviews by the Association of C and C++ Users (ACCU), a link to those reviews should be added along with the book.  See also:   Other C-related resources in the   A similar list for :      This question was discussed on  as part of the .The consensus was to keep it undeleted and actively maintained. I'm brand new to jQuery and have some experience using Prototype. In Prototype, there is a method to "flash" an element &mdash; ie. briefly highlight it in another color and have it fade back to normal so that the user's eye is drawn to it. Is there such a method in jQuery? I see fadeIn, fadeOut, and animate, but I don't see anything like "flash". Perhaps one of these three can be used with appropriate inputs? Securely erase hard drive using the Disk Utility A real solution to ambiguous tags Introduction Even within a site's relatively focused range of topics (it's unlikely anybody on SO will think  refers to the solar system's star), some tags are inherently ambiguous. There are currently two unsatisfying options:  Declare that the tag is only to be used for one of the possible meanings, write a tag wiki explaining it, and link from there to the newly created tags for the other meanings, and keep cleaning up the tag whenever someone ignores the tag wiki popup explaining the tag. Use alternative tags for all meanings (e.g. cd-command and compact-disk, instead of cd), and keep removing the undesired tag whenever it pops up again. And it will. The  is not deterring users from creating unnecessary or redundant tags.  Blacklisting tags is a complicated ritual occurring once in a blue moon, at most, and involves a dragon, a team member, and sacrifices to be burnt. It also makes those other, unambiguous tags not easier to discover, if it were applied to ambiguous tags. I don't consider this to be a viable option in this case. Proposal Here's the proposed : Just like a tag can be declared a synonym of another tag, basically changing all references to the synonym to references to that other, canonical tag, it should be possible to make it a disambiguation tag (or ambiguous tag) for multiple other tags. It's a logical progression from &quot;You entered a term with the same meaning as this other term&quot; to &quot;You entered a term with the same meaning as one of these other terms, please choose the correct meaning&quot;. Use of the tag should force the user to choose one of the alternatives presented:  &quot;, did you mean  or ?&quot; &quot;, did you mean , , or ?&quot; &quot;You entered 'vm', did you mean  or ?&quot; &quot;You entered 'sas', did you mean  or ?&quot; &quot;You entered 'kvm', did you mean  or ?&quot; &quot;You entered 'mono', did you mean  or ?&quot; &quot;You entered 'dsl', did you mean  or ?&quot;  Not all of these examples might be good (I'm not sure e.g. about the first two SO examples), but that's not really the point here anyway — they should just convey how this could be used. Note how some of these tags would probably never be used if users had to type them, and yet everything shorter is ambiguous. This could probably be mostly solved by extending the synonym/tag wiki excerpt dropdowns shown when a user enters the tags, proposing the alternatives for an ambiguous tag entered by the user (like a synonym suggestion, but with multiple options). If ignored, the user could still be shown an error message like with blacklisted tags, but by designing the selection dialog well, it should be possible to prevent that in most cases.  Related topics   This topic is related, but the user seems to have accepted the inclusion of tag wiki excerpts in the tag suggestions drop-down as sufficient.   This requires extensive discussion and team involvement (i.e. the status quo)   This topic has no real solution, except manual retagging. UnboundLocalError: local variable 'url_request' referenced before assignment Removed Python 3 and now Ubuntu Software Center, terminal and other applications don't work I want to create this:  Note that wire lay chaotically, so spiral object not the same thing.  There is a cool way to do this task in 3ds max:  Basically he deform straight spline/curve and then bend it couple iterations (over 9000 degrees angle in max).   Blender can't bend that way with simple deform as I know, only up to 360 degree. Is there any other way to make wire wrapping similar to photo? Preventing other users from viewing my files Ref:   My Ubuntu user account name "user-3121" with type as "Administrator". There is one more account named "admin" with type as "Administrator". How can I be sure whether or not "admin" can login as me or otherwise view my files in "user-3121"?  I discussed this with the other user and we have tried to modify /etc/sudoers to protect my files:  Cmnd_Alias   SHELLS = /bin/sh,/bin/bash,/bin/ksh, /usr/bin/x11/passwd  Cmnd_Alias   SU = /usr/bin/su,/bin/su,/usr/bin/gksudo,/usr/bin/sudo,/usr/bin/su bash,/usr/bin/sudo /bin/bash,/usr/sbin/visudo  Cmnd_Alias   PASS = /usr/bin/passwd root,/bin/* * root,/bin/* * sysadmin,/bin/* * /home/sysadmin,/usr/bin/passwd  Cmnd_Alias      EDIT= /bin/* /etc/sudoers,/bin/* sudoers,/bin/* /etc/passwd,/bin/* passwd,/bin/* /etc/group,/bin/* group,/bin/* /etc/shadow,/bin/* shadow,/*/*/[a-z]* /etc/sudoers,/*/*/[a-z]* /etc/passwd,/*/*/[a-z]* /etc/group,/*/*/[a-z]* /etc/shadow,/*/*/[a-z]* sudoers,/*/*/[a-z]* passwd,/*/*/[a-z]* group,/*/*/[a-z]* shadow  Cmnd_Alias   CMDS = /usr/sbin/userdel * sysadmin,/usr/sbin/userdel sysadmin,/usr/sbin/deluser * sysadmin,/usr/sbin/deluser sysadmin  root    ALL=(ALL) ALL, !CMDS  %admin ALL=(ALL) ALL, !SHELLS, !SU, !CMDS, !PASS, !EDIT %sudo  ALL=(ALL) ALL,!SHELLS, !SU, !CMDS, !PASS, !EDIT  admin ALL=(ALL) ALL administrator ALL=(ALL) ALL   If "admin" can still read my data, how do I prevent that?  Also how does this configuration work, it allows "user-3121" to run some sudo commands, but it doesn't actually mention "user-3121" anywhere?  P.S. I'm the only person who knows the password for the "root" user, so that I can log in as root using the "su" command. What is the Total Reputation field on Area51? What does the total reputation field on Area51 represent?  It's obviously not the total of the user's reputation on the various sites, as that is the number listed as user reputation. How do I dynamically create the variable name in a PHP loop? If a microsite uses the same logo as the main site, which "home" should it link to? Which 'home' should the microsite's logo link to - the main site or the microsite?  Consider that...    The content and layout of the microsite is the only differentiator  Half the users reach the microsite from the main site.   The microsite doesn't have it's own URL (it's www.mainurl/microsite) The microsite has the same logo and header style (so you almost can't tell you've left the main site) PBKDF vs HKDF for pretty long key I'm developing a messenger application with encrypted chats.   In the first version of the app I've used PBKDF2 (10000 iterations, SHA1, random salt) to extend a short user password and generate keys to encrypt (AES256) and sign message (HMAC).  In the current version of the app I generate a random 130-bit key and use it instead of a user password. I think that using PBKDF2 with such a key has some overhead.   I see 2 possible ways to improve the current approach:   Keep using PBKDF2 and decrease iterations to 1 or 10. Don't use PBKDF2 and use HKDF instead.   What are the advantages and disadvantages of each approach? Are there any other options? Showing that $\int_0^\infty x^{-x} \mathrm{d}x \leq 2$. This integral is very closely related to the sophmores dream that states  $$ \int_0^1 x^{-x}\mathrm{d}x = \sum_{n=1}^\infty n^{-n} = 1.27\ldots $$  For example here   Now I want to bound the integral, and showing that is less that 2.  For the interval $[0,1]$ a good bound is rewriting it to $\exp(x\log x)$ and using the expansion   $$ 1 - x \log(x) + \frac12 (-x \log(x))^2$$  but how does one handle $[1,\infty)$ ? In this answer here     gives bounds to the integral, but they are not tight enough.. So to taste my question again, how does one prove that   $$ \int_0^\infty \frac{\mathrm{d}x}{x^x} \leq 2 $$ Running a 32-bit application in Debian Wheezy 64 bits: Missing libraries I want to play the game Aquaria in a Debian Wheezy 64 bits. The installation went ok, but when trying to play the game I get these errors:  ALSA lib conf.c:3314:(snd_config_hooks_call) Cannot open shared library libasound_module_conf_pulse.so ALSA lib control.c:951:(snd_ctl_open_noupdate) Invalid CTL hw:0 AL lib: alsa.c:1000: control open (0): No such file or directory Message: SDL_GL_LoadLibrary Error: Failed loading libGL.so.1   I have added 32 bit compatibility with dpkg --add-architecture i386 and I think that the required libraries are present in the system since typing locate libasound_module_conf_pulse.so yields:  /usr/lib/x86_64-linux-gnu/alsa-lib/libasound_module_conf_pulse.so  and locate libGL.so.1:  /etc/alternatives/glx--libGL.so.1-x86_64-linux-gnu /usr/lib/mesa-diverted/i386-linux-gnu/libGL.so.1 /usr/lib/mesa-diverted/i386-linux-gnu/libGL.so.1.2 /usr/lib/mesa-diverted/x86_64-linux-gnu/libGL.so.1 /usr/lib/mesa-diverted/x86_64-linux-gnu/libGL.so.1.2 /usr/lib/x86_64-linux-gnu/libGL.so.1 /usr/lib/x86_64-linux-gnu/fglrx/fglrx-libGL.so.1.2 /usr/lib/x86_64-linux-gnu/fglrx/libGL.so.1   However, it seems that Debian is ignoring them. What can I do to play Aquaria?  EDIT 1: ldd aquaria  linux-gate.so.1 =&gt;  (0xf77e1000) libSDL-1.2.so.0 =&gt; /opt/Aquaria/./libSDL-1.2.so.0 (0xf7748000) libopenal.so.1 =&gt; /opt/Aquaria/./libopenal.so.1 (0xf76fa000) libstdc++.so.6 =&gt; /opt/Aquaria/./libstdc++.so.6 (0xf760d000) libm.so.6 =&gt; /lib/i386-linux-gnu/i686/cmov/libm.so.6 (0xf75c3000) libgcc_s.so.1 =&gt; /opt/Aquaria/./libgcc_s.so.1 (0xf75b8000) libc.so.6 =&gt; /lib/i386-linux-gnu/i686/cmov/libc.so.6 (0xf7455000) libdl.so.2 =&gt; /lib/i386-linux-gnu/i686/cmov/libdl.so.2 (0xf7451000) libpthread.so.0 =&gt; /lib/i386-linux-gnu/i686/cmov/libpthread.so.0 (0xf7437000) librt.so.1 =&gt; /lib/i386-linux-gnu/i686/cmov/librt.so.1 (0xf742e000) /lib/ld-linux.so.2 (0xf77e2000) I am trying to customize the legend for this graph. First of all, I would like the legend width to be equal to the x-axis width. At the moment, I am setting the width of the legend via      /tikz/every even column/.append style={column sep=1.0cm}   and I could manually adjust this value so that length equals the x-axis width, but I guess that there is a more elegant way to set the width of the legend.  Second, I don't really like the little color rectangles in the legend. I only want one rectangle in front of every entry and not two. This rectangle does not need to have a border and should be vertically centred with the text. The alignment seems to be okay for the rectangle closer to the text, but not for one the far left.   Update: Thanks to Christian Feuersänger's comment, there is possibility to change the rectangles via   legend image code/.code={%     \draw[#1, draw=none] (0cm,-0.1cm) rectangle (0.6cm,0.1cm);   or what Gonzalo Medina proposed  legend image code/.code={%     \draw[#1, draw=none, /tikz/.cd, bar width=3pt, yshift=-0.2em, bar shift=0pt] %     plot coordinates {(2*\pgfplotbarwidth,0.6em)};   Third, I would like to the legend entries to start next to the text "Legend:".  I don't really know if with these requirements it still makes sense to use legend or if it would be better to add everything manually via after end axis/.code and \ref{label}.  Graph    MWE  \documentclass{article} \usepackage{pgfplots} \pgfplotsset{compat=newest}  \begin{document} \begin{tikzpicture}     \begin{axis}[                 ybar,                 height=6cm,                 title={Some plot},                 width=12cm,                 axis on top,                 y axis line style={opacity=0},                 axis x line*=bottom,                 tick align=inside,                 tick style={draw=none},                 xtick=data,                 ymajorgrids,                 major grid style={draw=white},                 xtick={1,2,3,4},                 legend style={                     at={(0.0,-0.15)},                     anchor=north west,                     legend columns=-1,                     /tikz/every even column/.append style={column sep=1.0cm}                         },             ]                    \addlegendimage{empty legend}             \addlegendentry{\textbf{Legend:}}                                \addplot coordinates {                 (1, 0.2064)                 (2, 0.3961)                 (3, 0.5961)                 (4, 0.7961)}; \addlegendentry[text width=45pt, text depth=]{legend a}             \addplot coordinates {                 (1, 0.3064)                 (2, 0.4961)                 (3, 0.6961)                 (4, 0.8961)}; \addlegendentry[text width=65pt, text depth=]{lengend b}     \end{axis}  \end{tikzpicture} \end{document} I have two groups with equal variances and unequal sample size (28 and 8) and I want to run a Student t-test. Can I use Mann-Whitney considering that the variances are equal? Do you think if there is a better test to compare the samples?