Nullpointerexception when I perform depth first search
What is a NullPointerException, and how do I fix it?
Why can't I set a default output audio device in Ubuntu 19.10? Requested value 'CheckBoxes' was not found There's a weird thing happening with my Windows application. I have created a Sharepoint utility to automate the creation of content-types and site-columns. At a point where I try to get my available fields, I get this error:     An unhandled exception of type 'Microsoft.SharePoint.Client.ServerException' occurred in Microsoft.SharePoint.Client.Runtime.dll   Additional information: Requested value 'CheckBoxes' was not found.   It is throwing an exception at:  ClientContext clientContxt = new ClientContext(clientContext.Url);         clientContext.Credentials = clientContext.Credentials;         Web web = clientContext.Web;          clientContext.Load(web, w =&gt; w.AvailableFields);         clientContext.ExecuteQuery(); //**exception thrown here**   Kindly please suggest what is wrong here? Pagination go to first page if i'm on last post I have a custom post type and used get_next_post() and get_previous_post() to create a simple pagination. However i need to create a pagination loop (if that is the correct term). If i'm on the first post i need to browse back to the last post and if i'm on the last post i need to browse to the first post. Is there a simple way to do this that i haven't seen but Google, this site and the codex has not been very helpfull :/  The current code i have is. I've tried to use wp_list_pages and other functions but with little luck.  $next_post = get_next_post(); $prev_post = get_previous_post(); $nav = array(     "next_post" =&gt; array(         "url"       =&gt; get_permalink($next_post-&gt;ID),         "id"        =&gt; $next_post-&gt;ID,         "titill"    =&gt; get_field("vinstri_titill", $next_post-&gt;ID)." - ".get_field("vinstri_undirtitill", $next_post-&gt;ID)     ),     "prev_post" =&gt; array(         "url"       =&gt; get_permalink($prev_post-&gt;ID),         "id"        =&gt; $prev_post-&gt;ID,         "titill"    =&gt; get_field("vinstri_titill", $prev_post-&gt;ID)." - ".get_field("vinstri_undirtitill", $prev_post-&gt;ID)     ), ); understanding Debian dpkg package version numbers Command to switch to tty1? I want to make a list of commands to install everything so it's easier the next time I do it, however, I wanted one of the commands to do CTRL+ALT+F1 for me. If $|G/H|=4$ then $G$ is union of three proper subgroups Let $G$ be a finite group and $N$ a normal subgroup of $G$ of index $4$. Prove that $G$ has a subgroup of index $2$ (hence normal) and if $G/H$ is not cyclic then $G$ is equal to the union of three proper subgroups.   I understand that $G/H$ is isomorphic to $\mathbb{Z}_4$ or $\mathbb{Z}_2\times \mathbb{Z}_2$. Let $K^*$ be a subgroup of $G/H$ of order $2$. Then $K^*\cong K/H$ for some $K$ such that $H$ is normal in $K$ and hence $[G:K]=[G/H:K/H]=4/2=2$.  If $G/H$ is not cyclic then $G/H\cong \mathbb{Z}_2\times \mathbb{Z}_2$. I don't know how to prove $G$ is the union of three subgroups. Could it be true that as $G/H$ is the union of three proper subgroups $K_1^*,K_2^*,K_3^*$ then the corresponding groups $K_1,K_2,K_3$ have union equal to $G$? Would anyone give me a hint to prove this?  Thank you. I've a set of regions within a city. I'd like to split each region into five subregions and then get the centroid of each one of them.  I know how to make a vector grid, but the problem I have is that the generated points are not taking into account the regions, they're only related to the general map.  I have this:    And I want to split each region in five equal size areas. Something like this:    I'd like to build a visualization similar to this one: . In order to accomplish that, I need to split each region, because the original ones are pretty big. How to encrypt external devices using a cross-platform solution? Reading through my textbook I came across the following problem and I am looking for some help solving it.  Prove that,  $$\binom{3n}{r}=\binom{3n-1}{r} + \binom{3n-1}{r-1}$$  In a previous problem I solved the following but I wasn't sure how to use it to prove the question above. Prove,  $$\binom{n}{r}=\binom{n}{n-r}$$  Solution:  left Side,  $$\binom{n}{r}=\frac{n!}{(n-r)!r!}$$  right side,  $$\binom{n}{n-r}=\frac{n!}{(n-(n-r))!(n-r)!} = \frac{n!}{(n-r)!r!}$$  Therefore completing the proof,  but how do I solve my first question? thanks! How to create a Java application which can be run by a click? Facebook changed how their API serves profile pictures that are requested with a non-application scope UID. Now because before Graph API 2 all applications shared the original UID, if users like me has joined Stack Exchange websites before  2, rolling out their profile pictures are not shown properly. Facebook suggests to use an access token to query those profile pictures, either an application access token or a user access token. A user access token is not a reliable option at all, because it would only work for users who joined website with Facebook and are currently logged in. Application access tokens, by the way, can be used by creating sort of a proxy that would retrieve a profile picture from Facebook and display it. Yet still it's not perfect. I think the best solution is to communicate with Facebook and ask them to offer an opportunity to convert old Graph API 1 UID's to application scope user IDs. I'd like to find the files in the current directory that contain the text "chrome".  $ find . -exec grep chrome find: missing argument to `-exec'   What am I doing wrong? What is an example of pairwise independent random variables which are not independent? I've just read in a stochastics textbook:     Let $(\Omega, P)$ be a discrete probability space.      (a) The events $A_i \subseteq \Omega, i=1,2, \dots$ are called independent, if   $$P(A_{i_1} \cap A_{i_2} \cap \dots \cap A_{i_k}) = P(A_{i_1}) \cdot \dots \cdot A_{i_k}$$   for all finite sets $\{i_1, \dots, i_k\} \subsetneq \{1,2, \dots\}$ and all $k \geq 2$.      (b) The events $A_i \subseteq \Omega, i=1,2,\dots$ are called pairwise independent, if   $$P(A_{i_1} \cap A_{i_2}) = P(A_{i_1}) \cdot P(A_{i_2})$$   for all pairs $\{i_1, i_2\} \subsetneq \{1,2,\dots\}$.   After these two definitions it states that pairwise independent events are not always independent.  Do you have an example? Transit visa requirements at Narita Airport Consider the group $\mathbb{Q}$ under addition of rational numbers. If $H$ is a subgroup of $\mathbb{Q}$ with finite index, then $H = \mathbb{Q}$.  I just saw this on our exam earlier and was stumped on how to show this. Any ideas? Meaning of the chemical potential for a boson gas My lecturer told me that $\mu$, the , is zero or negative, and in the following example, mathematically it acts as a normalization constant. But is there any physical insight about why boson gas can be zero or negative? I think it is due to the fact the photon gas can pop up from nowhere (i.e. vacuum fluctuation). $$ f_{BE}(\varepsilon)=\dfrac{1}{e^{(\varepsilon-\mu)/(k_B T)}-1}$$ When and where was “portfolio career“ first used? The expression  meaning:  the fact of having several part-time jobs at once, rather than one full-time job:    A portfolio career is suitable for people who want to have a variety in their work life.   (Cambridge Dictionary) Etymonline has no information about the origin of the above expression, can anyone tell me when and where it was first used? Can I install items on Ubuntu, without using the command line? Everything I've googled so far has used the command line? My intent is to show that a composition of bijections is also a bijection by showing the existence of an inverse.  But my approach requires the associativity of function composition.  Let $f:  X \rightarrow Y, g:  Y \rightarrow Z, h:  Z \rightarrow W$ be functions.  $((f \circ g) \circ h)(x) = h((f \circ g)(x)) = h(g(f(x)))$, and $(f \circ (g \circ h))(x) = (g \circ h)(f(x)) = h(g(f(x)))$.  However, I am having problems in justifying that the two compositions, $(f \circ g) \circ h$ and $f \circ (g \circ h)$, have the same domain and range.  When I consulted ProofWiki, whose link is at the bottom, I got even more confused.  Specifically, for $(f \circ g) \circ h = f \circ (g \circ h)$ to be defined, ProofWiki requires that dom$g =$ codom$f$ and dom$h =$ codom$g$.  First of all, I think that it should be dom$g =$ range$f$ ....  Moreover, as you can see in the example below, you actually have to adjust domains and ranges of $f, g, h$ for the requirement to hold true.  Let $f: \mathbb R \rightarrow \mathbb R$ be $f(x) = 2x$, $g: \mathbb R^+ \rightarrow \mathbb R$ be $g(y) = ln(y)$, $h: \mathbb R \rightarrow \mathbb R$ be $h(z) = z - 10$.   Then $((f \circ g) \circ h)(x) = ln(2x) - 10 = (f \circ (g \circ h))(x)$, with dom$((f \circ g) \circ h) = \mathbb R^+$ = dom$(f \circ (g \circ h))$.  As a result, we need to set dom$f = \mathbb R^+$, range$f = \mathbb R^+$; dom$g$, range$g$, dom$h$, and range$h$ remain the same.  Am I allowed to do that?  This adjustment implies that when we say dom$f = X$, $f$ must be defined for all elements in $X$, but $X$ may not be the entire set of elements for which $f$ is defined. Compiz Problems (Plugin Missing, Performance Issues, Development) I've recently set up the compiz cube again, and I was very happy to find that that flickering window glitch is gone! However, the cube reflection and deformation plugins are also gone.  I am running a fresh 12.10 install, and I have every compiz plugin package installed (including compiz-plugins-extra, which supposedly contains everything that compiz-fusion-plugins-extra package used to have), but those two plugins are still missing. I haven't been able to find anyone with the same problem.   Any ideas? (thanks!)