I am trying to merge two linked list. I wrote the following code to pick out nodes from ListB and link it to listA at appropriate place. However i am getting null pointer exception and not able to figure out why.. pls help    Node sortedMerge(Node headA, Node headB) {       if(headB.data&lt;=headA.data){          Node temp = headB.next;          headB.next=headA;          headA=headB;         if (temp == null) return headA;          else return sortedMerge(headA, temp);      }      else{          Node headAtemp = headA;          while(headA.next.data &lt;headB.data &amp;&amp; headA.next!=null){              headA = headA.next;          }          Node next = headA.next;          Node temp = headB.next;          //changing links          headA.next= headB;          headB.next = next;          if (temp == null) return headAtemp;          else return sortedMerge(headAtemp,temp);      }    }   The function should return the head of the sorted list.
What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?  What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?
Question on how to normalize regression coefficient Not sure if normalize is the correct word to use here, but I will try my best to illustrate what I am trying to ask. The estimator used here is least squares.  Suppose you have $y=\beta_0+\beta_1x_1$, you can center it around the mean by $y=\beta_0'+\beta_1x_1'$ where $\beta_0'=\beta_0+\beta_1\bar x_1$ and $x_1'=x-\bar x$, so that $\beta_0'$ no longer has any influence on estimating $\beta_1$.  By this I mean $\hat\beta_1$ in $y=\beta_1x_1'$ is equivalent to $\hat\beta_1$ in $y=\beta_0+\beta_1x_1$. We have reduced equation for easier least square calculation.  How do you apply this method in general? Now I have the model $y=\beta_1e^{x_1t}+\beta_2e^{x_2t}$, I am trying to reduce it to $y=\beta_1x'$. Error: invalid environment block. Press any key to continue...   The error message appears immediately after the boot screen on purple background. Ubuntu 12.04.1 LTS. For example, if I revisit a memory of mine where I'm inside a castle and talking to someone, can the present-me explore different parts of the castle meanwhile the past-me continues her conversation? Is that possible? The Unix environment has some excellent tools for parsing text in various forms. However, of late, the data is not in the traditional (historical) formats (CSV, TSV, record-based or some other delimiter-based) it used to be before. Data these days is exchanged in structured formats like XML/JSON.  I know there are some good tools like sed, awk and Perl which can chew down nearly any form of data out there. However, to work with this sort of structured data, often one has to write a complete program, and, given the little time available to extract information, one has to sit down and figure out the whole logic of what one wants to query and put it down programmatically. Sometimes this is not OK – basically because the information extracted from those files acts as inputs for further work; also because of the time it takes to search for the appropriate solution and code it up. A command line tool is needed with sufficient switches to find, query and dump data.  I'm looking for tools that take a XML/JSON or other forms of structured data and dump it into other formats like csv, etc., so that from there one could use other commands to get any information out of it.  Are there any command line utilities you know of which do this kind of a job? Are there already awk/Perl scripts available to this? Using the same secret key for encryption and authentication in a Encrypt-then-MAC scheme Is it a weakness to use a single shared secret for protecting messages using a Encrypt-then-MAC scheme?    Assuming a system is using AES-256-CBC and a SHA1-HMAC and the same secret key for both operations.  Upon intercepting one of these messages and attacker could brute force this secret key by trying to solve the HMAC or by successfully decrypting the first block of the ciphertext.  Performing  a decryption on a single block of AES-256 should be a lighter operation than performing a SHA1-HMAC on a potentially large message.  Therefore AES-256 is ideal for a massively parallel brute-force attack and the use of the SHA1-HMAC doesn't hurt the system.  Is this approach sound, or is there an attack that has not been taken into consideration? How to terminate a script? How can I adjust the mouse scroll speed? Nautilus not generating thumbnails in 13.10 Like the title says, Nautilus is not generating thumbnails for any file types (pictures, documents, pdf, etc) after a clean install of 13.10.  I have checked the preferences and it is set to always generate thumbnails.  Help please! Suppose I have a complex figure made ​​with TikZ. How do I use it several times without having to repeat all the commands?  \documentclass{article} \usepackage[utf8]{inputenc} \usepackage{tikz}  \begin{document}  Base figure  \begin{tikzpicture}   \draw[blue] circle (2);   \foreach \i in {0,10,...,350}     \draw[blue] (\i:1.9) -- (\i:2.1); \end{tikzpicture}  Using base figure.  \begin{tikzpicture}   \draw[blue] circle (2);   \foreach \i in {0,10,...,350}     \draw[blue] (\i:1.9) -- (\i:2.1);   \foreach \i in {0,10,...,350}     \node at (\i:2.7) {\tiny $\i^\circ$}; \end{tikzpicture}  Using base figure.  \begin{tikzpicture}   \draw[blue] circle (2);   \foreach \i in {0,10,...,350}     \draw[blue] (\i:1.9) -- (\i:2.1);   \foreach \i in {0,10,...,350}     \draw (0,0) -- (\i:2); \end{tikzpicture}  \end{document} Notification that ‘someone might be watching what you browse’ I got a notification on my iPhone. The notification reads     Someone might be watching what you browse. Follow 3 steps to encrypt your web traffic and change your IP address.   What does this mean? Induction Proof: Fibonacci Numbers Identity with Sum of Two Squares Using induction, how can I show the following identity about the fibonacci numbers? I'm having trouble with simplification when doing the induction step.  Identity: $$f_n^2 + f_{n+1}^2 = f_{2n+1}$$  I get to:  $$f_{n+1}^2 + f_{n+2}^2$$  Should I replace $f_{n+2}$ using the recursion? When I do that, I end up with the product of terms, and that just doesn't seem right. Any guidance on how to get manipulate during the induction step?  Thanks! Proof by Induction: For any integer $n > 0$, $3|(5^{2n} − 1)$. Can the size of pointers vary between data and function pointers? Computing the Galois group of $x^4+ax^2+b \in \mathbb{Q}[x] $ I want to compute the Galois group of some polynomials, but I want to see some examples first. For example this proposition could be helpful. I don't know how to prove it &lt;.&lt;  Let's consider a polynomial $f(x)= x^4+ax^2+b \in \mathbb{Z} $. Let $ \pm\alpha , \pm\beta$ denote the roots of $f(x)$. Note that $f$ is irreducible iff $\alpha^2, \alpha\pm\beta \notin \mathbb{Q}$. So let's suppose that $f(x)$ is irreducible. Let's denote $G$ the galois group of $f$ (the Galois group of the splitting field $\mathbb{Q}(\alpha,\beta)/\mathbb{Q}$)  $i)$ $G\cong V$ the Klein 4-group iff $\alpha\beta \in \mathbb{Q}$  $ii)$ $G\cong \mathbb{Z}_4$ iff $\mathbb{Q}(\alpha\beta)=\mathbb{Q}(\alpha^2)$  $iii)$ $G\cong D_8$ the dihedral group of order 8, iff $\alpha\beta \notin \mathbb{Q}(\alpha^2)$  I need help with the proofs of this propositions... Well I'm very lost with the computation of Galois group, some care must be exercised, I don't have it. For example I know that Galois group acts transitively, in the sense that $u\in K/F$ a galois extension, the other roots of the minimal polynomial of $m_u(x)\in F[x]$ are precisely the elements $ \sigma(u)$ , where $\sigma \in Gal(K/F)$. And clearly any automorphism, is determined by only the value of a basis. In this case we are working on $\mathbb{Q}(\alpha,\beta)$, so I have to know the values of both. But I have to respect the algebraic relations involving $\alpha , \beta$ I proved that in $i),ii)$ $\mathbb{Q}(\alpha,\beta) = \mathbb{Q}(\alpha) $ So I have to know the values on $\alpha$.   My first question is: If in both cases $i),ii)$, I have an extension of degree 4 (so $|G|=4$) I have to map $\alpha$ to determine the automorphism, but I have also to map $\alpha$ to all of it's four roots. Why the galois group in this case are different?  My second question is about $iii)$ I have no idea How to attack it. Exploring $ \sum_{n=0}^\infty \frac{n^p}{n!} = B_pe$, particularly $p = 2$. I was exploring the fact that $$ \sum_{n=0}^\infty \frac{n^p}{n!} = B_pe,$$ where $B_n$ is the $n$th Bell number.   I found this result by exploring the series on wolframalpha and looking up the sequence of numbers generated. I have no experience with Bell numbers other than knowing that they represent the number of ways of partitioning a set.   What I tried  Looking at the base case we can verify $$\sum_{n=0}^\infty \frac{n}{n!} = \sum_{n=1}^\infty \frac{1}{(n-1)!} = e. $$  But then I realize that $n^2$ is going to be difficult.  Specifically what I want  I am doing this for fun, so I just want to glean some sort of lesson out of this. Resources that let me learn for myself are just as good!   An accepted answer will have any one of the following:   A proof that $\sum_{n = 0}^{\infty} \frac{n^2}{n!} = 2e$ (this interests me a lot) A description of why this series is related to Bell numbers and the number of partitions of a set A link to some resource that introduces Bell numbers and/or their connection to this series. What's the difference between adjusting highlights and shadows in the basic panel and in the tone curve in the Develop Module of Lightroom? I wanted to reduce some highlights in a photo and found that there were two possible ways of doing so: one from the basic panel and another from the tone curve. when I reduce the highlights in one pane the value on the other is unchanged. Are both of them the same thing? Which would be better? zsh I/O redirection: redirecting and then piping; what is going on? Can somebody explain what is going on in this command:  echo foo &gt;&amp;2 | grep foo   Zsh (5.0.2) prints foo twice. One seemingly to stdout and one to stderr, since running these commands:  { echo foo &gt;&amp;2 | grep foo } &gt;/dev/null   and:  { echo foo &gt;&amp;2 | grep foo } 2&gt;/dev/null   both just print foo once. How to verify $H\otimes K \cong \bigoplus\limits_{i\in I}H$ Let $H,~K$ be the Hilbert space. if $\{v_{j}\}_{j\in J}\subset H$ and $\{w_{i}\}_{i\in I}\subset K$ are the orthonormal bases, then how to construct the isomorphic mapping: $H\otimes K \rightarrow \bigoplus\limits_{i\in I}H$. Here, the $H \otimes K$ is the tensor product of two Hilbert space. When I type "[singular noun] and [singular noun]" in Word 2010, I find some interesting things.        Bread and butter is essential. (the auto-correction warns me of subject-verb disagreement.)   Bread and butter are essential. (OK)   A knife and fork is on the table. (OK)   A knife and fork are on the table. (OK)   The needle and thread is new. (OK)   The needle and thread are new. (OK)   Cost and revenue is directly linked. (OK)   Cost and revenue are directly linked. (OK)      I wonder how a native speaker would consider this phenomenon. What are recommended, general strategies to start block-cipher design and/or analysis? I (and many others for that matter) have always been fascinated by the inner workings of the modern building block of cryptography: block ciphers.  Now, the ressources on the "black art" of design and analysis of these ciphers are sparse; especially for the entry level. Available information like  seems to be somewhat outdated (due to its age).  What's the general consensus of the “best” (read: recommended) strategy to start designing and/or analyzing modern cryptographical block ciphers?    Nota bene: Think of this as a general, wiki-alike question – for learning purposes only. (Besides that, this Q&amp;A would represent a place we can point people to when alike algorithm design questions arise.)