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• Google Datastore low-level api query by key property

  - by Keyur

  I'm using the low-level google datastore api and I want to query by the key property and another property (let's call it category). I need to query based on a list of keys for which I'll use the IN operator. I know that the max. number of values you can provide for the IN clause is 30. I have 2 questions: Does the limit of 30 IN values apply to the key property as well? Do I need to create a composite index on {_key_ + category} or just on {category} for this query? Thanks, Keyur

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• Pointing to vectors

  - by Matt Munson

  #include <iostream> #include <vector> using namespace std; int main () { vector <int> qwerty; qwerty.push_back(5); vector <int>* p = &qwerty; cout << p[0]; //error: no match for 'operator<<' in 'std::cout << * p' } I'm generally unclear on how to use pointers with vectors, so I'm pretty mystified as to why this is not working. To my mind, this should print 5 to screen.

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• SQL vs MySQL: Rules about aggregate operations and GROUP BY

  - by Phazyck

  In this book I'm currently reading while following a course on databases, the following example of an illegal query using an aggregate operator is given: Find the name and age of the oldest sailor. Consider the following attempt to answer this query: SELECT S.name, S.age FROM Sailors.S The intent is for this query to return not only the maximum age but also the name of the sailors having that age. However, this query is illegal in SQL--if the SELECT clause uses an aggregate operation, then it must use only aggregate operations unless the query contains a GROUP BY clause! Some time later while doing an exercise using MySQL, I faced a similar problem, and made a mistake similar to the one mentioned. However, MySQL didn't complain and just spit out some tables which later turned out not be what I needed. Is the query above really illegal in SQL, but legal in MySQL, and if so, why is that? In what situation would one need to make such a query?

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• Database EAV model, record listing as per search

  - by Shyam Sunder Verma

  I am building a dynamic application. I have three tables : ( EAV model style) 1: Items ( ItemId, ItemName) 2: Fields (FieldId, FieldName) 3: Field Values ( ItemID, FieldId, Value) Can you tell me how to write SINGLE query to get starting 20 records from ALL items where FieldId=4 is equal to TRUE. Expected Result : Columns = ItemID | Name | Field1 | Field2 | Field3 Each Row= ItemId | ItemName| Value1 | Value2 | Value3 Important concerns : 1: Number of fields per item are not known 2: I need one to write ONE query. 3: Query will be running on 100K records, so performance is concern. 4: I am using MySQL 5.0, so need solution for MYSQL Should I denormalize the tables if above query is not possible at all ? Any advice ?

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• matlab noninteger step indexing

  - by rlbond

  So, I have a vector: k = 1:100; And I want to take 19 elements from it, which are roughly equally-spaced. So I write this: m = k(1:(99/18):end); This works great, except for a tiny problem: Warning: Integer operands are required for colon operator when used as index m = 1 7 12 18 23 29 34 40 45 51 56 62 67 73 78 84 89 95 100 Now, I understand why this comes up, but I'd like to get rid of that warning. Is there a "right" way to do this without a warning?

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• C# implicit conversions

  - by Chris Boden

  Hello, I'm currently working on an application where I need to load data from an SQL database and then assign the retrieved values into the properties of an object. I'm doing this by using reflection since the property names and column names are the same. However, many of the properties are using a custom struct type that is basically a currency wrapper for the decimal type. I have defined an implicit conversion in my struct: public static implicit operator Currency(decimal d) { return new Currency(d); } This works fine when I use it in code. However, when I have this: foreach (PropertyInfo p in props) { p.SetValue(this, table.Rows[0][p.Name], null); } It throws an ArgumentException stating that it cannot convert from System.Decimal to Currency. I'm confused since it works fine in any other circumstance.

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• Object comparison in JavaScript

  - by spankmaster79

  What is the best way to compare Objects in JavaScript? Example: var user1 = {name : "nerd", org: "dev"}; var user2 = {name : "nerd", org: "dev"}; var eq = user1 == user2; alert(eq); // gives false I know that "Two objects are equal if they refer to the exact same Object", but is there a way to check it another way?? Using this way works for me.....but is it the only possibility? var eq = Object.toJSON(user1) == Object.toJSON(user2); alert(eq); // gives true

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• How to file split at a line number

  - by superspace

  I want to split a 400k line long log file from a particular line number. For this question, lets make this an arbitrary number 300k. Is there a linux function that allows me to do this? I know split lets me split the file in equal parts either by size or line numbers but that's not what I want. I want to the first 300k in one file and the last 100k in the second file. Any help would be appreciated. Thanks!

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• Get a list of documents from Mongo DB

  - by Andrei Neagu

  Hello, I want to do something like this: List<int> fff = new List<int>(); fff.Add(1); fff.Add(2); fff.Add(5); Mongo m = new Mongo(); m.Connect(); var dataBase = m.GetDatabase("database"); var collection = dataBase.GetCollection("coll"); IMongoQuery queryable = collection.AsQueryable(); MongoQueryProvider prov = new MongoQueryProvider(collection); var query = new MongoQuery(prov); var ffppp = from p221 in query where fff.Contains((int)p221["oid"]) select p221; This throws this error : The method 'Contains' could not be converted into a constant. I saw that mongo has an operator $in. Does any one know how can I use it from c#? (http://www.mongodb.org/display/DOCS/Advanced+Queries) Thanks

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• Selecting an element based on class and id in jquery - similar to an SQL query

  - by Ankur

  I want to select an element based on it's class and id, I have seen many similar questions on SO. Most of the users ask why would you ever want to do this - which is not very helpful, trust me, this is the simplest way of solving my UI problem, and it's a very unique and intereting UI related to making generic triple stores easily browsable. At the moment the jQUery looks like this: $(document).ready( function() { $(".predicates").hide(); $(".objects").hide(); $("#subject").click( function() { $("#predId, .predicates").toggle(); // this line } ); $("#predId").click( function() { $("#objId, .objects").toggle(); } ); }); The line I am really interested in is this $("#predId, .predicates").toggle(); how do I get the select to ensure that both #predId and .predicates are true, at the moment it seems that only one or the other has to be true. Is there something like an && operator?

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• Deriving streambuf or basic_ostringstream?

  - by Helltone

  Hi, I want to derive a stringstream so that I can use the operator<< to construct a message which will then be thrown. The API would look like: error("some text") << " more text " << 42 << std::endl; This should do a throw "some text more text 42" So what I did is make an errorbuf (inheriting from streambuf) which overloads the 'overflow' method and then create an ostream(&errorbuf). I wonder if I shouldn't instead inherit from basic_ostringstream or something...

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• C# Socket Server

  - by Snoopy

  In .NET 3.5 a new socket classes was released: http://msdn.microsoft.com/en-us/library/bb968780.aspx i found a sample but some questions regarding best practicses are remaining: http://code.msdn.microsoft.com/nclsamples/Wiki/View.aspx?title=Socket%20Performance m_numConnections (the maximum number of connections the sample is designed to handle simultaneously) is probably equal to the amount of cpu cores i have. m_receiveBufferSize is the size for one packet? like 8kb? how should i handle a length byte? opsToPreAlloc i dont understand. is this if i code a transparent proxy? Regarding the multithreading, what should be used? The reactive extension seem to be a good choice. Has anyone tried this in a real world project? Are there better options? I had bad experiences with the .NET thread pool in the past.

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• Question regarding built-in type variable is returned from a function

  - by Chan

  Hello, If a local object is returned in a function call, it has to do at least three steps: 1. Copy constructor is called to hold a copy. 2. Destroy local object. 3. A copy is return. For example: x = y + z If x is an integer object. A copy of y + z should be returned, then a new object is created, then assignment operator of x will take this object as parameter. So my question is: Is the same process used for built-in type such as int, double...? If they're not the same, how's it done? Thanks, Chan

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• Expression.OrElse, dynamically creating a condition.

  - by Jim

  Hi, I am attempting to create a dynamic where clause using the standard expression API. var query = ( from p in Parties orderby p.PartyId orderby p.FullName select p ).AsQueryable(); Expression<Func<Party, bool>> @fn = (p) => SqlMethods.Like(p.FullName, "%smith%") || SqlMethods.Like(p.Person.FirstName, "%smith%"); Expression<Func<Party, bool>> @sn = (p) => SqlMethods.Like(p.Person.FirstName, words[0]); ParameterExpression pe = Expression.Parameter(typeof(Party), "p"); Expression orelse = Expression.OrElse( Expression.Lambda(@fn, pe), Expression.Lambda(@sn, pe) ); The expressions above will ultimately be added to a where clause. I need to add a bunch of 'likes'. How do I do this? I get InvalidOperationException on the operator OrElse I have also tried Expression.Or Thanks Regards Craig.

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• Placement new in gcc

  - by Roman Prikhodchenko

  I need to find a workaround for a bug with placement new in g++. I now it was fixed in gcc-4.3 but I have to support versions 4.2 and 4.1. For example, following code compiles with an error "error: no matching function for call to 'operator new(long unsigned int, void*&)" template<class T, template<typename> class Alloc> inline void* type_ctor() { Alloc<T> a; void* p = a.allocate(1); new(p) T; return p; } ..... type_ctor<A, NewAllocator >();

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• Postgres column casting...

  - by Simon

  I have a query SELECT assetid, type_code, version, name, short_name, status, languages, charset, force_secure, created, created_userid, updated, updated_userid, published, published_userid, status_changed, status_changed_userid FROM sq_ast WHERE assetid = 7 which doesn't work and throws ERROR: operator does not exist: character varying = integer LINE 4: FROM sq_ast WHERE assetid = 7 I can get it to work by doing SELECT assetid, type_code, version, name, short_name, status, languages, charset, force_secure, created, created_userid, updated, updated_userid, published, published_userid, status_changed, status_changed_userid FROM sq_ast WHERE assetid = '7' Please note the quoting of the 7 in the WHERE clause... I am deploying an huge application and I cannot rewrite the core... similarly I don't want to risk changing the type of the column... I'm no Postgres expert... please help... Is there an option for strict casting of columns???

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• performance of parameterized queries for different db's

  - by tuinstoel

  A lot of people know that it is important to use parameterized queries to prevent sql injection attacks. Parameterized queries are also much faster in sqlite and oracle when doing online transaction processing because the query optimizer doesn't have to reparse every parameterized sql statement before executing. I've seen sqlite becoming 3 times faster when you use parameterized queries, oracle can become 10 times faster when you use parameterized queries in some extreme cases with a lot of concurrency. How about other db's like mysql, ms sql, db2 and postgresql? Is there an equal difference in performance between parameterized queries and literal queries?

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• Why is this HTTP request continually looping?

  - by alex

  I'm probably overlooking something really obvious here. Comments are in to help explain any library specific code. public function areCookiesEnabled() { $random = 'cx67ds'; // set cookie cookie::set('test_cookie', $random); // try and get cookie, if not set to false $testCookie = cookie::get('test_cookie', false); $cookiesAppend = '?cookies=false'; // were we able to get the cookie equal ? $cookiesEnabled = ($testCookie === $random); // if $_GET['cookies'] === false , etc try and remove $_GET portion if ($this->input->get('cookies', false) === 'false' AND $cookiesEnabled) { url::redirect(str_replace($cookiesAppend, '', url::current())); // redirect return false; } // all else fails, add a $_GET[] if ( ! $cookiesEnabled) { url::redirect(url::current().$cookiesAppend); } return $cookiesEnabled; }

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• match at the beginning of any line, including the first

  - by JoelFan

  According the the Perl documentation on regexes: By default, the "^" character is guaranteed to match only the beginning of the string ... Embedded newlines will not be matched by "^" ... You may, however, wish to treat a string as a multi-line buffer, such that the "^" will match after any newline within the string ... you can do this by using the /m modifier on the pattern match operator. The "after any newline" part means that it will only match at the beginning of the 2nd and subsequent lines. What if I want to match at the beginning of any line (1st, 2nd, etc.)?

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• Track/Display Array Index As Part Of Cout (C++)

  - by John Smith

  Hi, I have a command line C++ program that lets you enter basic information about a person (ID number, name, age, etc.) and I want to output to a console in the following manner: ------------------------------------------------------------------- Index ID # First Name Last Name Age ------------------------------------------------------------------- 0 1234 John Smith 25 The person objects are stored in an array of Persons and I've overload the ostream (<<) operator to print out all of the fields like you see. The dashed lines and header come from a displayHdg() function. Anyhow, I have not been able to figure out how to get the proper index value for the array. Ideally, I'd like to generate the indices for each line, but all my attempts have failed. The array is looped through and each object printed in the main() function, and the ostream is overloaded in a person class, so I tried to use global variables as well as static variables, and all of those produce incorrect numbering (i.e. show 0, 1 the first time (for 2 objects), then change to 1, 2 on the next display). Any ideas?

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• Weird behaviour with vector::erase and std::remove_if with end range different from vector.end()

  - by Edison Gustavo Muenz

  Hi, I need to remove elements from the middle of a std::vector. So I tried: struct IsEven { bool operator()(int ele) { return ele % 2 == 0; } }; int elements[] = {1, 2, 3, 4, 5, 6}; std::vector<int> ints(elements, elements+6); std::vector<int>::iterator it = std::remove_if(ints.begin() + 2, ints.begin() + 4, IsEven()); ints.erase(it, ints.end()); After this I would expect that the ints vector have: [1, 2, 3, 5, 6]. In the debugger of Visual studio 2008, after the std::remove_if line, the elements of ints are modified, I'm guessing I'm into some sort of undefined behaviour here. So, how do I remove elements from a Range of a vector?

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• Why is window not identical to window.self in Internet Explorer?

  - by Jim Puls

  There's a convoluted backstory involving how I came across this, but why is the self property not exactly equal to the window itself? In Safari and Firefox and friends, the results are as I'd expect: > window == window.self true > window === window.self true The same isn't true in Internet Explorer, though: >> window == window.self true >> window === window.self false Can anybody account for the inconsistency? To exactly what is the self property of the window object pointing? It casts to something with equality, which is even more vexing.

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• How to convert this foreach loop into Linq code?

  - by a-galkin

  I am new one with Linq and I would like to modify my old c# code to use Linq. The idea of this code to select all tables where it's not set and reference’s field PrimaryTable equal "myTable" foreach (Table table in dbServer.Tables) { if (!table.IsSet) { foreach (Reference refer in table.References) { if (refer.PrimaryTable == "myTable") { tables.Add(table); } } } } After digging in internet I have got this code var q = from table in dbServer.Tables let refers = from refer in table.References where refer.PrimaryTable == "myTable" select refer.ForeignTable where refers.Contains(table.Name) select table; But it does not work at all and I need your help to make it works. Thanks in advance.

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• Problem understanding treesort in Haskell

  - by Jerry

  I am trying to figure out how exactly does treesort from here work (I understand flatten, insert and foldr). I suppose what's being done in treesort is applying insert for each element on the list thus generating a tree and then flattening it. The only problem I can't overcome here is where the list (that is the argument of the function) is hiding (because it is not written anywhere as an argument except for the function type declaration). One more thing: since dot operator is function composition, why is it an error when I change: treesort = flatten . foldr insert Leaf to treesort = flatten( foldr insert Leaf )?

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• Python - How to find a correlation between two vectors ?

  - by psihodelia

  Given two vectors X and Y I have to find their correlation, i.e. their linear dependence/independence. Both vectors have equal dimension. A resulted answer should be a floating point number from [-1.0 .. 1.0]. Example: X=[-1, 2, 0] Y=[ 4, 2, -0.3] Find y=cor(X,Y) such that y belongs to [-1.0 .. 1.0]. It should be a simple construction involving a list-comprehension. No external library is allowed. UPDATE: ok, if dot product is enough, then here is my solution: nX = 1/(sum([x*x for x in X]) ** 0.5) nY = 1/(sum([y*y for y in Y]) ** 0.5) cor = sum([(x*nX)*(y*nY) for x,y in zip(X,Y) ]) right?

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