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  • What pitfalls (if any) are there to learning C/C++ as a first language?

    - by thyrgle
    Hi, I first learned C++ then moved to C (I know kind of backwards) and then I found a thing called iTunes U and began watching programming classes to extend my knowledge and noticed that Stanford and MIT both did not teach C or C++ as an introduction to computer programming. Then I began to wonder why they didn't begin teaching C/C++ because I thought C/C++ was more efficient than Java (which Stanford taught) and Python (which MIT taught). Or maybe there is no particular reason?

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  • How to prevent regex from stopping at the first match of alternatives ?

    - by miket2e
    If I have the string hello world , how can I modify the regex world|wo|w so that it will match all of "world", "wo" and "w" rather than just the single first match of "world" that it comes to ? If this is not possible directly, is there a good workaround ? I'm using C# if it makes a difference: Regex testRegex = new Regex("world|wo|w"); MatchCollection theMatches = testRegex.Matches("hello world"); foreach (Match thisMatch in theMatches) { ... }

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  • PHP - How do i display the first 20 li then have a link to display more (gallery)

    - by Matt Rogers
    I want to display about 20 li on a page with a link at the bottom that says something like 'display more'. This link will then clear the first 20 and display the next 20. How should i go about doing this. (I am creating a gallery) I was thinking about using PHP and MySQL. However for what I am doing I do not really need to store it in a database so is there an easier way of doing it only using html, php or javascript? thanks

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  • Heaps of Trouble?

    - by Paul White NZ
    If you’re not already a regular reader of Brad Schulz’s blog, you’re missing out on some great material.  In his latest entry, he is tasked with optimizing a query run against tables that have no indexes at all.  The problem is, predictably, that performance is not very good.  The catch is that we are not allowed to create any indexes (or even new statistics) as part of our optimization efforts. In this post, I’m going to look at the problem from a slightly different angle, and present an alternative solution to the one Brad found.  Inevitably, there’s going to be some overlap between our entries, and while you don’t necessarily need to read Brad’s post before this one, I do strongly recommend that you read it at some stage; he covers some important points that I won’t cover again here. The Example We’ll use data from the AdventureWorks database, copied to temporary unindexed tables.  A script to create these structures is shown below: CREATE TABLE #Custs ( CustomerID INTEGER NOT NULL, TerritoryID INTEGER NULL, CustomerType NCHAR(1) COLLATE SQL_Latin1_General_CP1_CI_AI NOT NULL, ); GO CREATE TABLE #Prods ( ProductMainID INTEGER NOT NULL, ProductSubID INTEGER NOT NULL, ProductSubSubID INTEGER NOT NULL, Name NVARCHAR(50) COLLATE SQL_Latin1_General_CP1_CI_AI NOT NULL, ); GO CREATE TABLE #OrdHeader ( SalesOrderID INTEGER NOT NULL, OrderDate DATETIME NOT NULL, SalesOrderNumber NVARCHAR(25) COLLATE SQL_Latin1_General_CP1_CI_AI NOT NULL, CustomerID INTEGER NOT NULL, ); GO CREATE TABLE #OrdDetail ( SalesOrderID INTEGER NOT NULL, OrderQty SMALLINT NOT NULL, LineTotal NUMERIC(38,6) NOT NULL, ProductMainID INTEGER NOT NULL, ProductSubID INTEGER NOT NULL, ProductSubSubID INTEGER NOT NULL, ); GO INSERT #Custs ( CustomerID, TerritoryID, CustomerType ) SELECT C.CustomerID, C.TerritoryID, C.CustomerType FROM AdventureWorks.Sales.Customer C WITH (TABLOCK); GO INSERT #Prods ( ProductMainID, ProductSubID, ProductSubSubID, Name ) SELECT P.ProductID, P.ProductID, P.ProductID, P.Name FROM AdventureWorks.Production.Product P WITH (TABLOCK); GO INSERT #OrdHeader ( SalesOrderID, OrderDate, SalesOrderNumber, CustomerID ) SELECT H.SalesOrderID, H.OrderDate, H.SalesOrderNumber, H.CustomerID FROM AdventureWorks.Sales.SalesOrderHeader H WITH (TABLOCK); GO INSERT #OrdDetail ( SalesOrderID, OrderQty, LineTotal, ProductMainID, ProductSubID, ProductSubSubID ) SELECT D.SalesOrderID, D.OrderQty, D.LineTotal, D.ProductID, D.ProductID, D.ProductID FROM AdventureWorks.Sales.SalesOrderDetail D WITH (TABLOCK); The query itself is a simple join of the four tables: SELECT P.ProductMainID AS PID, P.Name, D.OrderQty, H.SalesOrderNumber, H.OrderDate, C.TerritoryID FROM #Prods P JOIN #OrdDetail D ON P.ProductMainID = D.ProductMainID AND P.ProductSubID = D.ProductSubID AND P.ProductSubSubID = D.ProductSubSubID JOIN #OrdHeader H ON D.SalesOrderID = H.SalesOrderID JOIN #Custs C ON H.CustomerID = C.CustomerID ORDER BY P.ProductMainID ASC OPTION (RECOMPILE, MAXDOP 1); Remember that these tables have no indexes at all, and only the single-column sampled statistics SQL Server automatically creates (assuming default settings).  The estimated query plan produced for the test query looks like this (click to enlarge): The Problem The problem here is one of cardinality estimation – the number of rows SQL Server expects to find at each step of the plan.  The lack of indexes and useful statistical information means that SQL Server does not have the information it needs to make a good estimate.  Every join in the plan shown above estimates that it will produce just a single row as output.  Brad covers the factors that lead to the low estimates in his post. In reality, the join between the #Prods and #OrdDetail tables will produce 121,317 rows.  It should not surprise you that this has rather dire consequences for the remainder of the query plan.  In particular, it makes a nonsense of the optimizer’s decision to use Nested Loops to join to the two remaining tables.  Instead of scanning the #OrdHeader and #Custs tables once (as it expected), it has to perform 121,317 full scans of each.  The query takes somewhere in the region of twenty minutes to run to completion on my development machine. A Solution At this point, you may be thinking the same thing I was: if we really are stuck with no indexes, the best we can do is to use hash joins everywhere. We can force the exclusive use of hash joins in several ways, the two most common being join and query hints.  A join hint means writing the query using the INNER HASH JOIN syntax; using a query hint involves adding OPTION (HASH JOIN) at the bottom of the query.  The difference is that using join hints also forces the order of the join, whereas the query hint gives the optimizer freedom to reorder the joins at its discretion. Adding the OPTION (HASH JOIN) hint results in this estimated plan: That produces the correct output in around seven seconds, which is quite an improvement!  As a purely practical matter, and given the rigid rules of the environment we find ourselves in, we might leave things there.  (We can improve the hashing solution a bit – I’ll come back to that later on). Faster Nested Loops It might surprise you to hear that we can beat the performance of the hash join solution shown above using nested loops joins exclusively, and without breaking the rules we have been set. The key to this part is to realize that a condition like (A = B) can be expressed as (A <= B) AND (A >= B).  Armed with this tremendous new insight, we can rewrite the join predicates like so: SELECT P.ProductMainID AS PID, P.Name, D.OrderQty, H.SalesOrderNumber, H.OrderDate, C.TerritoryID FROM #OrdDetail D JOIN #OrdHeader H ON D.SalesOrderID >= H.SalesOrderID AND D.SalesOrderID <= H.SalesOrderID JOIN #Custs C ON H.CustomerID >= C.CustomerID AND H.CustomerID <= C.CustomerID JOIN #Prods P ON P.ProductMainID >= D.ProductMainID AND P.ProductMainID <= D.ProductMainID AND P.ProductSubID = D.ProductSubID AND P.ProductSubSubID = D.ProductSubSubID ORDER BY D.ProductMainID OPTION (RECOMPILE, LOOP JOIN, MAXDOP 1, FORCE ORDER); I’ve also added LOOP JOIN and FORCE ORDER query hints to ensure that only nested loops joins are used, and that the tables are joined in the order they appear.  The new estimated execution plan is: This new query runs in under 2 seconds. Why Is It Faster? The main reason for the improvement is the appearance of the eager Index Spools, which are also known as index-on-the-fly spools.  If you read my Inside The Optimiser series you might be interested to know that the rule responsible is called JoinToIndexOnTheFly. An eager index spool consumes all rows from the table it sits above, and builds a index suitable for the join to seek on.  Taking the index spool above the #Custs table as an example, it reads all the CustomerID and TerritoryID values with a single scan of the table, and builds an index keyed on CustomerID.  The term ‘eager’ means that the spool consumes all of its input rows when it starts up.  The index is built in a work table in tempdb, has no associated statistics, and only exists until the query finishes executing. The result is that each unindexed table is only scanned once, and just for the columns necessary to build the temporary index.  From that point on, every execution of the inner side of the join is answered by a seek on the temporary index – not the base table. A second optimization is that the sort on ProductMainID (required by the ORDER BY clause) is performed early, on just the rows coming from the #OrdDetail table.  The optimizer has a good estimate for the number of rows it needs to sort at that stage – it is just the cardinality of the table itself.  The accuracy of the estimate there is important because it helps determine the memory grant given to the sort operation.  Nested loops join preserves the order of rows on its outer input, so sorting early is safe.  (Hash joins do not preserve order in this way, of course). The extra lazy spool on the #Prods branch is a further optimization that avoids executing the seek on the temporary index if the value being joined (the ‘outer reference’) hasn’t changed from the last row received on the outer input.  It takes advantage of the fact that rows are still sorted on ProductMainID, so if duplicates exist, they will arrive at the join operator one after the other. The optimizer is quite conservative about introducing index spools into a plan, because creating and dropping a temporary index is a relatively expensive operation.  It’s presence in a plan is often an indication that a useful index is missing. I want to stress that I rewrote the query in this way primarily as an educational exercise – I can’t imagine having to do something so horrible to a production system. Improving the Hash Join I promised I would return to the solution that uses hash joins.  You might be puzzled that SQL Server can create three new indexes (and perform all those nested loops iterations) faster than it can perform three hash joins.  The answer, again, is down to the poor information available to the optimizer.  Let’s look at the hash join plan again: Two of the hash joins have single-row estimates on their build inputs.  SQL Server fixes the amount of memory available for the hash table based on this cardinality estimate, so at run time the hash join very quickly runs out of memory. This results in the join spilling hash buckets to disk, and any rows from the probe input that hash to the spilled buckets also get written to disk.  The join process then continues, and may again run out of memory.  This is a recursive process, which may eventually result in SQL Server resorting to a bailout join algorithm, which is guaranteed to complete eventually, but may be very slow.  The data sizes in the example tables are not large enough to force a hash bailout, but it does result in multiple levels of hash recursion.  You can see this for yourself by tracing the Hash Warning event using the Profiler tool. The final sort in the plan also suffers from a similar problem: it receives very little memory and has to perform multiple sort passes, saving intermediate runs to disk (the Sort Warnings Profiler event can be used to confirm this).  Notice also that because hash joins don’t preserve sort order, the sort cannot be pushed down the plan toward the #OrdDetail table, as in the nested loops plan. Ok, so now we understand the problems, what can we do to fix it?  We can address the hash spilling by forcing a different order for the joins: SELECT P.ProductMainID AS PID, P.Name, D.OrderQty, H.SalesOrderNumber, H.OrderDate, C.TerritoryID FROM #Prods P JOIN #Custs C JOIN #OrdHeader H ON H.CustomerID = C.CustomerID JOIN #OrdDetail D ON D.SalesOrderID = H.SalesOrderID ON P.ProductMainID = D.ProductMainID AND P.ProductSubID = D.ProductSubID AND P.ProductSubSubID = D.ProductSubSubID ORDER BY D.ProductMainID OPTION (MAXDOP 1, HASH JOIN, FORCE ORDER); With this plan, each of the inputs to the hash joins has a good estimate, and no hash recursion occurs.  The final sort still suffers from the one-row estimate problem, and we get a single-pass sort warning as it writes rows to disk.  Even so, the query runs to completion in three or four seconds.  That’s around half the time of the previous hashing solution, but still not as fast as the nested loops trickery. Final Thoughts SQL Server’s optimizer makes cost-based decisions, so it is vital to provide it with accurate information.  We can’t really blame the performance problems highlighted here on anything other than the decision to use completely unindexed tables, and not to allow the creation of additional statistics. I should probably stress that the nested loops solution shown above is not one I would normally contemplate in the real world.  It’s there primarily for its educational and entertainment value.  I might perhaps use it to demonstrate to the sceptical that SQL Server itself is crying out for an index. Be sure to read Brad’s original post for more details.  My grateful thanks to him for granting permission to reuse some of his material. Paul White Email: [email protected] Twitter: @PaulWhiteNZ

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  • Uninitialized constant Item::Types

    - by Rasmus
    Hi! First of, im a newbie ruby programmer so please bare with me if this is a very dumb question. I get this uninitialized constant error when i submit my nested forms. order.rb class Order < ActiveRecord::Base has_many :items, :dependent => :destroy has_many :types, :through => :items accepts_nested_attributes_for :items accepts_nested_attributes_for :types validates_associated :items validates_associated :types end item.rb class Item < ActiveRecord::Base has_one :types belongs_to :order accepts_nested_attributes_for :types validates_associated :types end type.rb class Type < ActiveRecord::Base belongs_to :items belongs_to :orders end new.erb.html <% form_for @order do |f| %> <%= f.error_messages %> <% f.fields_for :items do |builder| %> <table border="0"> <th>Type</th> <th>Amount</th> <th>Text</th> <th>Price</th> <tr> <% f.fields_for :type do |m| %> <td> <%= m.collection_select :type, Type.find(:all, :order => "created_at DESC"), :id, :name, {:prompt => "Select a Type" }, {:id => "selector", :onchange => "type_change(this)"} %> </td> <% end %> <td> <%= f.text_field :amount, :id => "amountField", :onchange => "change_total_price()" %> </td> <td> <%= f.text_field :text, :id => "textField" %> </td> <td> <%= f.text_field :price, :class => "priceField", :onChange => "change_total_price()" %> </td> <td> <%= link_to_remove_fields "Remove Item", f %> </td> </tr> </table> <% end %> <p><%= link_to_add_fields "Add Item", f, :items %></p> <p> <%= f.label :total_price %><br /> <%= f.text_field :total_price, :class => "priceField", :id => "totalPrice" %> </p> <p><%= f.submit "Create"%></p> <% end %> <%= link_to 'Back', orders_path %> create method in orders_controller.rb def create @order = Order.new(params[:order]) respond_to do |format| if @order.save flash[:notice] = 'Post was successfully created.' format.html { redirect_to(@order) } format.xml { render :xml => @order, :status => :created, :location => @order } else format.html { render :action => "new" } format.xml { render :xml => @order.errors, :status => :unprocessable_entity } end end end Hopefully you can see what i cant

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  • How to render the properties of the view model's base class first when using ViewData.ModelMetadata.

    - by Martin R-L
    When I use the ViewData.ModelMetadata.Properties in order to loop the properties (with an additional Where(modelMetadata => modelMetadata.ShowForEdit && !ViewData.TemplateInfo.Visited(modelMetadata))), and thereby create a generic edit view, the properties of the view model's base class are rendered last. Is it possible to use a clever OrderBy() or is there another way to first get the properties of the base class, and then the sub class'? Reverse won't do the trick since the ordering of each class' properties is perfectly fine. A workaround would of course be composition + delegation, but since we don't have mixins, it's too un-DRY IMHO, why I seek a better solution if possible.

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  • TextMate: Preview in Firefox without having to save document first?

    - by hced
    Using TextMate: Is it possible to assign a shortcut to preview/refresh the currently edited HTML document in, say, Firefox, without having to first hit Save? I'm looking for the same functionality as TextMate's built-in Web Preview window, but I'd prefer an external browser instead of TextMate's. (Mainly in order to use a JavaScript console such as Firebug for instance). Would it be possible to pipe the currently unsaved document through the shell and then preview in Firefox. And if so, is there anyone having a TextMate command for this, willing to share it?

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  • How to query one table and add rows to another using that first query? MySQL

    - by Nickelbids
    Hello, I have some users setup in a MySQL table with different variables. I am trying to figure out what would be the best way to do this. Basically I want to award all of my registered and active users with bids which are stored in another table. So for the Table "users" I have ran this query: SELECT * FROM users WHERE active = 1 AND admin = 0 ORDER BY users.id ASC Which will show all active users who are not administrators. Now I would like to give each one of these users which are identified by the "ID" field in another table. So in the "bids" table I would need to add a new row for each one of those users with all of the same values except for the "user_id" field which will basically match the "id" field of the table "users" What would be the best approach for this. There are approximately 6,000+ users coming up in the first query. Please be gentle as I am not a programmer. Just need some friendly advice.

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  • Why can I not get a WDS-originated PXE boot to progress past the first file download?

    - by Jeff Shattock
    I'm trying to work out an automated Windows install process, and thought I'd give WDS a look. After some promising initial progress, I seem to have hit a wall. I imported the boot and install WIMs, and created the capture WIM successfully. However, whenever I try to PXE boot the reference machine against the WDS server, it kinda craps out. It finds the server and downloads WDSNBP.COM successfully, and then gives the message "TFTP download failed." According to WireShark, the only communication between the WDS box and the client box is the successful TFTP request and download of boot\x86\WDSNBP.COM. No further requests are sent. The WDS log on the server shows the same thing, one successful download and no more activity. I've tried every combination of the following, with exactly zero change in behaviour: Win Server 2008R2 vs 2012 vs 2012R2 WDS virtualized on KVM, ESXi, VirtualBox, VMWare Workstation Client virtualized on KVM, ESXi, VirtualBox, VMWare Workstation Every network adaptor type offered by the virtualization platforms. "Actual" network vs isolated, virtual network. MS DHCP server vs Linux isc-dhcp-server Joined to a domain vs Stand-alone I tried changing the boot filename in DHCP to pxeboot.com instead, and it has no problem downloading that file instead, but it then crabs about Boot\BCD being corrupted. Also, with 2012, it doesnt appear that WDSNBP.com does the architecture detection, or at least does'nt report that it did. 2008 reports that it found x64, and then errors. I find myself out of things to check, and I dont see anything immediately wrong. Where do I go from here? WDS server is at 192.168.1.50, DHCP/DNS at 192.168.1.7. Console of the client computer after the boot: MAC: 52:54:00:28:94:0E UUID: blah blah Searching for server (DHCP)..... Me: 192.168.1.155, DHCP: 192.168.1.7, Gateway 192.168.1.1 Loading 192.168.1.50:boot\x86\wdsnbp.com ...(PXE).................done Downloaded WDSNCP... TFPT download failed Interesting parts of /etc/dhcp/dhcpd.conf on the Linux DHCP server: allow booting; allow bootp; option option-60 code 60 = string; option option-66 code 66 = string; option option-67 code 67 = string; subnet 192.168.1.0 netmask 255.255.255.0 { range 192.168.1.110 192.168.1.253; next-server 192.168.1.50; option tftp-server-name "192.168.1.50"; option option-60 "PXEClient"; filename "boot\\x86\\wdsnbp.com"; option bootfile-name "boot\\x86\\wdsnbp.com"; }

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  • How can I do an SELINUX filesystem relabel without rebooting first?

    - by Skaperen
    I can touch the file /.autorelabel and reboot and during the initialization coming back up it will do the SELINUX relabel for me. But I want to do this in a different situation where the system has just been copied to a hard drive image. I can chroot to the originating file tree, or chroot to the just populated device image and run it. I just can't find anything that says what to be run. This image is being made into an AMI on AWS EC2, and contains CentOS 6.3. But the time it takes to relabel is too long (6 minutes or more). I want to move the relabel to the image build where the extra time is not an issue (because it happens once instead of every time an AMI is launched). I can make this relabel be the very last thing just before the filesystem is unmounted for the last time until it becomes an AMI and will launch. I just need to know what to call to do it. I have searched man pages with no luck. I have searched system init scripts but where /.autorelabel is detected, it is unclear what is happening. Documents like http://www.centos.org/docs/5/html/5.2/Deployment_Guide/sec-sel-fsrelabel.html only tell how to do things that still really do the work after a reboot. I need to have the work doing BEFORE the "reboot" (unmount, build AMI, and launch ready to go). The big point is ... yes there will be a reboot ... but I want the relabel work to be done before that so it won't be done every time an AMI is launched (because it takes so long).

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  • What to do before connecting Ubuntu Server to the internet for the first time?

    - by CodeMonkey
    I just finished installing Ubuntu Server 12.10 on an Asus Eee PC 1000H (to be used as a home server/sandbox) from USB. I installed this software during installation: OpenSSH server LAMP server Samba file server Virtual Machine host I won't use 2, 3 or 4 for a while though. Can/should I turn these off somehow? I have turned home directory encryption on. Security updates are installed automatically. I have chosen a strong password for the single user. I have never plugged in the internet cable so far. Before doing so I'd like to ask: What can/should I do/install to increase security before connecting to the internet? Firewall? Fail2ban? Users/Passwords? Encryption? Enable/Disable functionality? etc. I'm sorry if you get this question a lot. I've searched around quite a while, but it still feels like I might overlook something important.

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  • How to egrep the first character in second column?

    - by Steve
    using egrep, how can i print all lastnames start with K or k ??? Jennifer Cowan:548-834-2348:583 Laurel Ave., Kingsville, TX 83745:10/1/35:58900 Lesley Kirstin:408-456-1234:4 Harvard Square, Boston, MA 02133:4/22/62:52600 Jennifer Cowan:548-834-2348:583 Laurel Ave., kingsville, TX 83745:10/1/35:58900 Lesley kirstin:408-456-1234:4 Harvard Square, Boston, MA 02133:4/22/62:52600 William Kopf:846-836-2837:6937 Ware Road, Milton, PA 93756:9/21/46:43500 Arthur Putie:923-835-8745:23 Wimp Lane, Kensington, DL 38758:8/31/69:126000

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  • For large files compress first then transfer or rsync -z? which would be fastest?

    I have a ton of relativity small data files but they take up about 50 GB and I need them transferred to a different machine. I was trying to think of the most efficient way to do this. Thoughts I had were to gzip the whole thing then rsync it and decompress it, rely on rsync -z for compression, gzip then use rsync -z. I am not sure which would be most efficient since I am not sure how exactly rsync -z is implemented. Any ideas on which option would be the fastest?

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  • How to set up RAID-0 first time on new PC?

    - by jasondavis
    I have built basic PC's in the past but have never used a RAID array at all. SO now I am buying parts to build my new PC, it will be an intel i7 processor. My motherboard will have RAID support which I will use instead of an aftermarket raid controller for now. Also I plan to use 2 SSD drives in RAID-0 for my windows 7 OS. (Please note that I am aware of the issues with doing this, including lack of TRIM support when using RAID with SSD drives. I am OK with it not working as I can just re[place the drives in a year or so or wheneer they become more sluggish). SO here is my question part. If I assemble the motherboard, PSU, processor, RAM, vidm card, etc and then go to turn the PC on, it will have the 2 SSD drives hooked up. so I assume I will then soon the BIOS screen before I install windows? How to I go about making the 2 drives work in RAID-0 at this point? I do the raid part before installing my OS right? Please help with the steps involved from assembling the parts of the PC and then turning it on, to the part of getting the RAID-0 set up between the 2 drives and then installing my windows 7 OS from a Optical drive? Please help, all advice, instructions, tips appreciated as long as on topic. I do not need to be told that this is a bad idea as far as if 1 drive fails I losse it all, I plan on having a disk IMAGE to be able to restore my OS and software to a new set of drives at anytime needed in the event of drive failure. Same goes for lack of TRIM support. Thanks for reading and help =)

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  • Using Remote Desktop, connect to a Windows 7 domain user account without first logging on locally?

    - by Robert S Ciaccio
    I have a dell laptop (henceforth we'll call this the server) running Windows 7 Enterprise. The server is part of my company's domain. My primary user account is a domain account. When I am at home and not connected to the domain, I prefer to connect to the server using Remote Desktop Connection from my MacBook Pro (we'll call this the client). The problem is, that if I do not physically login to the server, I am unable to connect to it using RDC from the client. I have a local administrator account on the server, and connecting to it via RDC works just fine. I had a feeling that the Mac RDC application was not giving me the full story, so I attempted the same procedure from a Windows 7 client. When trying to login, I get this message: So basically, If I logon to the server physically with my domain user and lock the computer, I can then successfully logon from the client. Otherwise, I am unable to connect.

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