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  • copy array from one class to another class array

    - by shishir.bobby
    hi all, i hv an array ("array A", which contains 3 objects, fox ex, to,from,message) in class "A". and in class "B",i m having another array ("array B"),which fills tableview,of class "B" only. i need to fill tableview,with the values of class A's array (i.e array A,with the object values, to,from,message). how can i do it?? how to copy array from another class ? i hope i m clear with my question regards shishir

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  • How to sort an array by some specific key?

    - by vinothkumar
    I have an array look like below. $array[0]['keyword'] = 'cricket '; $array[0]['noofhits'] = '26'; $array[0]['keyword'] = 'food '; $array[0]['noofhits'] = '17'; $array[0]['keyword'] = 'mypax'; $array[0]['noofhits'] = '22'; $array[0]['keyword'] = 'next'; $array[0]['noofhits'] = '22'; $array[0]['keyword'] = 'nextbutton'; $array[0]['noofhits'] = '22'; $array[0]['keyword'] = 'picture'; $array[0]['noofhits'] = '18'; I want to sort the array using the noofhits. How can I do? Advance Thanks for your advice.

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  • php array_unique

    - by dotty
    HAy, i have an array Array( [0] => Array ( [0] => 33 [user_id] => 33 [1] => 3 [frame_id] => 3 ) [1] => Array ( [0] => 33 [user_id] => 33 [1] => 3 [frame_id] => 3 ) [2] => Array ( [0] => 33 [user_id] => 33 [1] => 8 [frame_id] => 8 ) [3] => Array ( [0] => 33 [user_id] => 33 [1] => 3 [frame_id] => 3 ) [4] => Array ( [0] => 33 [user_id] => 33 [1] => 3 [frame_id] => 3 ) ) As you can see key 0 is the same as 1,3 and 4. And key 2 is different from them all. When running the array_unique function on them, the only left is Array ( [0] => Array ( [0] => 33 [user_id] => 33 [1] => 3 [frame_id] => 3 ) ) any ideas why array_unique isn't working as expected?

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  • Scala Array constructor?

    - by Lukasz Lew
    scala> val a = Array [Double] (10) a: Array[Double] = Array(10.0) scala> val a = new Array [Double] (10) a: Array[Double] = Array(0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0) Why these two expressions have different semantics?

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  • PHP 2D Array output all combinations

    - by stukerr
    Hi there, I've had this problem bending my mind for a while now (head cold doesn't help either!), basically I have a PHP array which looks like this example: $array[0][0] = 'apples'; $array[0][1] = 'pears'; $array[0][2] = 'oranges'; $array[1][0] = 'steve'; $array[1][1] = 'bob'; And I would like to be able to produce from this a table with every possible combination of these, but without repeating any combinations (regardless of their position), so for example this would output Array 0 Array 1 apples steve apples bob pears steve pears bob But I would like for this to be able to work with as many different arrays as possible. Many thanks!

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  • adding array values

    - by christian
    Array ( [0] => Array ( [datas] => Array ( [name] => lorem [id] => 1 [type] => t1 [due_type] => Q1 [t1] => 1 [t2] => 1 [t3] => 1 ) ) [1] => Array ( [datas] => Array ( [name] => lorem [id] => 1 [type] => t2 [due_type] => Q1 [t1] => 0 [t2] => 1 [t3] => 0 ) ) [2] => Array ( [datas] => Array ( [name] => name [id] => 2 [type] => t1 [due_type] => Q1 [t1] => 1 [t2] => 0 [t3] => 1 ) ) [3] => Array ( [datas] => Array ( [name] => name [id] => 2 [type] => t2 [due_type] => Q1 [t1] => 1 [t2] => 0 [t3] => 0 ) ) ) I want to add the values of each array according to its id, but I am having problem getting the values using these code: I want to compute the sum of all type according to each due_type and combining them into one array. $totals = array(); $i = -1; foreach($datas as $key => $row){ $i += 1; $items[$i] = $row; if (isset($totals[$items[$i]['datas']['id']])){ if($totals[$items[$i]['datas']['id']]['due_type'] == 'Q1'){ if($totals[$items[$i]['datas']['id']]['type'] == 't1'){ $t1+=$totals[$items[$i]['datas']['id']]['t1']; }elseif($totals[$items[$i]['datas']['id']]['type'] == 't2'){ $t2+=$totals[$items[$i]['datas']['id']]['t2']; }elseif($totals[$items[$i]['datas']['id']]['type'] == 't3'){ $t3+=$totals[$items[$i]['datas']['id']]['t3']; } $totals[$items[$i]['datas']['id']]['t1_total'] = $t1; $totals[$items[$i]['datas']['id']]['t2_total'] = $t2; } } else { $totals[$items[$i]['datas']['id']] = $row['datas']; $totals[$items[$i]['datas']['id']]['t1_total'] = $items[$i]['datas']['t1']; $totals[$items[$i]['datas']['id']]['t2_total'] = $items[$i]['datas']['t2']; } }

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  • PHP compare two dimension array

    - by Jerry
    Hello guys I would like to know how to compare two two-dimension arrays value. First array Array 1 ( [0] => Array ( [0] => a ) [1] => Array ( [0] => b ) [2] => Array ( [0] => c ) } Second one Array 2 ( [0] => Array ( [0] => a ) [1] => Array ( [0] => d ) [2] => Array ( [0] => e ) } I need to know if my loop could compare the arrays to check the matched value. In my case, array1[0][0]=a matches array2[0][0]=a. If it matches, php will output some html. My foreach loop foreach ($array1 as $arrays){ foreach($arrays as $array){ //need to compare array2 here not sure how to do it. } } I would appreciate any helps. Thanks!

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  • php - arrange array to display in score order

    - by Phil Jackson
    hi, i have wrote a script to produce an array of data but now want to display in order of score. The array outputs as follows; [display_name] => Array ( [0] => ACT_Web_Designs [1] => user1_design [2] => user2_design ) [proffesion] => Array ( [0] => Web Developer [1] => web developer [2] => Web Developer ) [score] => Array ( [0] => 15 [1] => 6 [2] => 15 ) [img] => Array ( [0] => ./?0000=gif&0001=3fadb8c362ff39f3322909899ff14760&0002=prof_pic [1] => [2] => ) so in a nutshell I am wanting it to be converted as follows; [display_name] => Array ( [0] => ACT_Web_Designs [1] => user2_design [2] => user1_design ) [proffesion] => Array ( [0] => Web Developer [1] => web developer [2] => Web Developer ) [score] => Array ( [0] => 15 [1] => 15 [2] => 6 ) [img] => Array ( [0] => ./?0000=gif&0001=3fadb8c362ff39f3322909899ff14760&0002=prof_pic [1] => [2] => ) I have been looking at asort() but cant get anything to work. any help would be much appreciated.

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  • Get index values for an array to print in value attribute for radio buttons

    - by kexxcream
    Problem: To get the index values of an array to print accordingly in value attribute of radio buttons. The array $_SESSION['items']: Array ( [2] => Array ( [category] => 2 [question] => Array ( [6] => Källorna refereras separat [7] => Vissa försök till sammanbindning [8] => En del sammanfattningar [9] => Olika forskningslinjer jämförs och sammanfattas [10] => Kontraster, jämförelser, sammanfattningar; centrala likheter och skillnader framhävs ) [title] => Integration av källorna ) ) I have a PHP function that looks like this: function itemsLayout ($array) { for ($i = 1; $i <= count($array['question']); $i++) { $form .= '<input type="radio" name="'.$array['category'].'" id="'.$array['category'].'" value="INDEX VALUE FOR QUESTION ARRAY HERE">'; } return $form; } PHP code: I get the index by using the following: $key = key($_SESSION['items']); $current = $_SESSION['items'][$key]; And I print the first index by using: echo itemsLayout($current); Question: How do I get the index values 6, 7, 8, 9, 10 to print in the value attribute for each radio button?

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  • Retrieve values from multimdimensional array

    - by vincentlerry
    I have a great difficulty. I need to retrieve [title], [url] and [abstract] values ??from this multidimensional array. Also, I have to store those values in mysql database. thanks in advance!!! Array ( [bossresponse] = Array ( [responsecode] = 200 [limitedweb] = Array ( [start] = 0 [count] = 20 [totalresults] = 972000 [results] = Array ( [0] = Array ( [date] = [clickurl] = http://www.torchlake.com/ [url] = http://www.torchlake.com/ [dispurl] = www.torchlake.com [title] = Torch Lake, COLI Inc, Highspeed, Dial-up, Wireless ... [abstract] = Welcome to COLI Inc. Chain O' Lake Internet. Local Northern Michigan ISP, offering Dialup Internet access, Wireless access, Web design, and T1 services in Northern ... ) [1] = Array ( [date] = [clickurl] = http://en.wikipedia.org/wiki/Torch_Lake_(Antrim_County,_Michigan) [url] = http://en.wikipedia.org/wiki/Torch_Lake_(Antrim_County,_Michigan) [dispurl] = en.wikipedia.org/wiki/Torch_Lake_(Antrim_County,_Michigan) [title] = Torch Lake (Antrim County, Michigan) - Wikipedia, the free ... [abstract] = Torch Lake at 19 miles (31 km) long is Michigan's longest lake and at approximately 18,770 acres (76 km²) is Michigan's second largest lake. Within it are several ... ) this is the entire code that generates this array: require("OAuth.php"); $cc_key = ""; $cc_secret = ""; $url = "http://yboss.yahooapis.com/ysearch/limitedweb"; $args = array(); $args["q"] = "car"; $args["format"] = "json"; $args["count"] = 20; $consumer = new OAuthConsumer($cc_key, $cc_secret); $request = OAuthRequest::from_consumer_and_token($consumer, NULL,"GET", $url, $args); $request-sign_request(new OAuthSignatureMethod_HMAC_SHA1(), $consumer, NULL); $url = sprintf("%s?%s", $url, OAuthUtil::build_http_query($args)); $ch = curl_init(); $headers = array($request-to_header()); curl_setopt($ch, CURLOPT_HTTPHEADER, $headers); curl_setopt($ch, CURLOPT_URL, $url); curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE); $rsp = curl_exec($ch); $results = json_decode($rsp, true);

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  • In C, as free() knows an array size, why isn't there a function that gets the array size? [closed]

    - by user354959
    Possible Duplicate: If free() knows the length of my array, why can’t I ask for it in my own code? Searching around (including here at stackoverflow), I got that malloc() allocates an array and also creates a header to control the array info. In this header, there's also the array size. free() use such information to know how to deallocate that array. So, if the array size info is "there" (somewhere in the memory), why there isn't a function that returns an array size, looking for this at the array header? Or am I missing something?

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  • PHP Flatten Array with multiple leaf nodes

    - by tafaju
    What is the best way to flatten an array with multiple leaf nodes so that each full path to leaf is a distinct return? array("Object"=>array("Properties"=>array(1, 2))); to yield Object.Properties.1 Object.Properties.2 I'm able to flatten to Object.Properties.1 but 2 does not get processed with recursive function: function flattenArray($prefix, $array) { $result = array(); foreach ($array as $key => $value) { if (is_array($value)) $result = array_merge($result, flattenArray($prefix . $key . '.', $value)); else $result[$prefix . $key] = $value; } return $result; } I presume top down will not work when anticipating multiple leaf nodes, so either need some type of bottom up processing or a way to copy array for each leaf and process (althought that seems completely inefficient)

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  • Comment associative array in PHP Documentor

    - by Abenil
    Hey Guys, i hope someone can help me out on this one here. I use several associative arrays in my php application and i'm using to php documentor to comment my sources. I never really did specified comments for the arrays in an array, but now i need to do that and dont know how. $array = array('id' => 'test', 'class' => 'tester', 'options' => array('option1' => 1, 'option2' => 2)) So how do i comment this array in the correct way for @var and @param comments? I could do this like this, but dunno, if this is correct: @param string $array['id'] @param string $array['class'] @param int $array['options']['option1'] But how to this for the var part? I hope someone can lead me to the right direction. Thanks in advance for any help. Regards

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  • Finding a list of indices from master array using secondary array with non-unique entries

    - by fideli
    I have a master array of length n of id numbers that apply to other analogous arrays with corresponding data for elements in my simulation that belong to those id numbers (e.g. data[id]). Were I to generate a list of id numbers of length m separately and need the information in the data array for those ids, what is the best method of getting a list of indices idx of the original array of ids in order to extract data[idx]? That is, given: a=numpy.array([1,3,4,5,6]) # master array b=numpy.array([3,4,3,6,4,1,5]) # secondary array I would like to generate idx=numpy.array([1,2,1,4,2,0,3]) The array a is typically in sequential order but it's not a requirement. Also, array b will most definitely have repeats and will not be in any order. My current method of doing this is: idx=numpy.array([numpy.where(a==bi)[0][0] for bi in b]) I timed it using the following test: a=(numpy.random.uniform(100,size=100)).astype('int') b=numpy.repeat(a,100) timeit method1(a,b) 10 loops, best of 3: 53.1 ms per loop Is there a better way of doing this?

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  • How to handle array element between int and Integer

    - by masato-san
    First, it is long post so if you need clarification please let me know. I'm new to Java and having difficulty deciding whether I should use int[] or Integer[]. I wrote a function that find odd_number from int[] array. public int[] find_odd(int[] arr) { int[] result = new int[arr.length]; for(int i=0; i<arr.length; i++) { if(arr[i] % 2 != 0) { //System.out.println(arr[i]); result[i] = arr[i]; } } return result; } Then, when I pass the int[] array consisting of some integer like below: int[] myArray = {-1, 0, 1, 2, 3}; int[] result = find_odd(myArray); The array "result" contains: 0, -1, 0, 1, 0, 3 Because in Java you have to define the size of array first, and empty int[] array element is 0 not null. So when I want to test the find_odd() function and expect the array to have odd numbers (which it does) only, it throws the error because the array also includes 0s representing "empty cell" as shown above. My test code: public void testFindOddPassValidIntArray() { int[] arr = {-2, -1, 0, 1, 3}; int[] result = findOddObj.find_odd(arr); //check if return array only contain odd number for(int i=0; i<result.length; i++) { if(result[i] != null) { assert is_odd(result[i]) : result[i]; } } } So, my question is should I use int[] array and add check for 0 element in my test like: if(result[i] != 0) { assert is_odd(result[i] : result[i] } But in this case, if find_odd() function is broken and returning 0 by miscalculation, I can't catch it because my test code would only assume that 0 is empty cell. OR should I use Integer[] array whose default is null? How do people deal with this kind of situation?

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  • Change index order in array (php)

    - by Trikks
    Hi Been kind of stuck on this one for a while now, so any help would be appreciated. I have one array (left) that contains a list of elements, the goal is to sort another arrays (right) keys with the values from the left array. The left array Array ( [0] => ID [1] => FirstName [2] => LastName [3] => Address ) The right array Array ( [0] => Array ( [FirstName] => Pim [Address] => Finland [LastName] => Svensson [ID] => 3 ) [1] => Array ( [FirstName] => Emil [Address] => Sweden [LastName] => Malm [ID] => 5 ) ) What I'm trying to accomplish would be similar to this Array ( [0] => Array ( [ID] => 3 [FirstName] => Pim [LastName] => Svensson [Address] => Finland ) Anyone? :) Oh, I'm running php 5.3, if it helps!

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  • PHP modifying and combining array

    - by Industrial
    Hi everyone, I have a bit of an array headache going on. The function does what I want, but since I am not yet to well acquainted with PHP:s array/looping functions, so thereby my question is if there's any part of this function that could be improved from a performance-wise perspective? I tried to be as complete as possible in my descriptions in each stage of the functions which shortly described prefixes all keys in an array, fill up eventual empty/non-valid keys with '' and removes the prefixes before returning the array: $var = myFunction ( array('key1', 'key2', 'key3', '111') ); function myFunction ($keys) { $prefix = 'prefix_'; $keyCount = count($keys); // Prefix each key and remove old keys for($i=0;$i<$keyCount; $i++){ $keys[] = $prefix.$keys[$i]; unset($keys[$i]); } // output: array('prefix_key1', 'prefix_key2', 'prefix_key3', '111) // Get all keys from memcached. Only returns valid keys $items = $this->memcache->get($keys); // output: array('prefix_key1' => 'value1', 'prefix_key2' => 'value2', 'prefix_key3'=>'value3) // note: key 111 was not found in memcache. // Fill upp eventual keys that are not valid/empty from memcache $return = $items + array_fill_keys($keys, ''); // output: array('prefix_key1' => 'value1', 'prefix_key2' => 'value2', 'prefix_key3'=>'value3, 'prefix_111' => '') // Remove the prefixes for each result before returning array to application foreach ($return as $k => $v) { $expl = explode($prefix, $k); $return[$expl[1]] = $v; unset($return[$k]); } // output: array('key1' => 'value1', 'key2' => 'value2', 'key3'=>'value3, '111' => '') return $return; } Thanks a lot!

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  • Nested foreach loops for associative array combinations

    - by JohnL
    I have an associative array as follows: $myarray = array('a'=>array(), 'b'=>array(), 'c'=>array(), 'd'=>array()); I want to be able to get all pairs of elements in the array. If it wasn't an associative array, I would use nested for loops, like: for($i=0; $i<count($myarray); $i++) { for($j=$i+1; $j<count($myarray); $j++) { do_something($myarray[$i], $myarray[$j]); } } I have looked at using foreach loops, but as the inner loop goes through ALL elements, some pairs are repeated. Is there a way to do this? Thanks!

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  • PHP find array keys

    - by Jens Törnell
    In PHP I have an array that looks like this: $array[0]['width'] = '100'; $array[0]['height'] = '200'; $array[2]['width'] = '150'; $array[2]['height'] = '250'; I don't know how many items there are in the array. Some items can be deleted which explains the missing [1] key. I want to add a new item after this, like this: $array[]['width'] = '300'; $array[]['height'] = '500'; However the code above don't work, because it adds a new key for each row. It should be the same for the two rows above. A clever way to solve it? An alternative solution would be to find the last key. I failed trying the 'end' function.

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  • PHP Sort Array By SubArray Value

    - by Sjwdavies
    I've got the following structue of array: Array ( [0] => Array ( [configuration_id] => 10 [id] => 1 [optionNumber] => 3 [optionActive] => 1 [lastUpdated] => 2010-03-17 15:44:12 ) [1] => Array ( [configuration_id] => 9 [id] => 1 [optionNumber] => 2 [optionActive] => 1 [lastUpdated] => 2010-03-17 15:44:12 ) [2] => Array ( [configuration_id] => 8 [id] => 1 [optionNumber] => 1 [optionActive] => 1 [lastUpdated] => 2010-03-17 15:44:12 ) ) What's the best way for order the array, in an incremental way based on the optionNumber? So the results look like: Array ( [0] => Array ( [configuration_id] => 8 [id] => 1 [optionNumber] => 1 [optionActive] => 1 [lastUpdated] => 2010-03-17 15:44:12 ) [1] => Array ( [configuration_id] => 9 [id] => 1 [optionNumber] => 2 [optionActive] => 1 [lastUpdated] => 2010-03-17 15:44:12 ) [2] => Array ( [configuration_id] => 10 [id] => 1 [optionNumber] => 3 [optionActive] => 1 [lastUpdated] => 2010-03-17 15:44:12 ) )

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  • Elegant way to aggregate multi-dimensional array by index key

    - by Stephen J. Fuhry
    How can I recursively find the total value of all children of an array that looks something like this? [0] => Array ( [value] => ? // 8590.25 + 200.5 + 22.4 [children] => Array ( [0] => Array ( [value] => ? // 8590.25 + 200.5 [children] => Array ( [0] => Array ( [value] => 8590.25 // leaf node ) [1] => Array ( [value] => 200.05 // leaf node ) ) ) [1] => Array ( [value] => 22.4 // leaf node ) ) )

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  • Sorting multidimensional array on inner value php [duplicate]

    - by Silver89
    This question already has an answer here: Reference: all basic ways to sort arrays and data in PHP 4 answers Say I have the following array, how can I sort it on sort_by? Array ( [10] => Array ( [Masthead_slide] => Array ( [id] => 1456464564 [sort_by] => 1 ) ) [6] => Array ( [Masthead_slide] => Array ( [id] => 645454 [sort_by] => 10 ) ) [7] => Array ( [Masthead_slide] => Array ( [id] => 4547 [sort_by] => 5 ) ) )

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  • Elegant way to aggregate multi-demensional array by index key

    - by Stephen J. Fuhry
    How can I recursively find the total value of all children of an array that looks something like this? [0] => Array ( [value] => ? // 8590.25 + 200.5 + 22.4 [children] => Array ( [0] => Array ( [value] => ? // 8590.25 + 200.5 [children] => Array ( [0] => Array ( [value] => 8590.25 // leaf node ) [1] => Array ( [value] => 200.05 // leaf node ) ) ) [1] => Array ( [value] => 22.4 // leaf node ) ) )

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  • Oracle parameter array binding from c# executed parallel and serial on different servers

    - by redir_dev_nut
    I have two Oracle 9i 64 bit servers, dev and prod. Calling a procedure from a c# app with parameter array binding, prod executes the procedure simultaneously for each value in the parameter array, but dev executes for each value serially. So, if the sproc does: select count(*) into cnt from mytable where id = 123; if cnt = 0 then insert into mytable (id) values (123); end if; Assuming the table initially does not have an id = 123 row. Dev gets cnt = 0 for the first array parameter value, then 1 for each of the subsequent. Prod gets cnt = 0 for all array parameter values and inserts id 123 for each. Is this a configuration difference, an illusion due to speed difference, something else?

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  • Subterranean IL: Generics and array covariance

    - by Simon Cooper
    Arrays in .NET are curious beasts. They are the only built-in collection types in the CLR, and SZ-arrays (single dimension, zero-indexed) have their own commands and IL syntax. One of their stranger properties is they have a kind of built-in covariance long before generic variance was added in .NET 4. However, this causes a subtle but important problem with generics. First of all, we need to briefly recap on array covariance. SZ-array covariance To demonstrate, I'll tweak the classes I introduced in my previous posts: public class IncrementableClass { public int Value; public virtual void Increment(int incrementBy) { Value += incrementBy; } } public class IncrementableClassx2 : IncrementableClass { public override void Increment(int incrementBy) { base.Increment(incrementBy); base.Increment(incrementBy); } } In the CLR, SZ-arrays of reference types are implicitly convertible to arrays of the element's supertypes, all the way up to object (note that this does not apply to value types). That is, an instance of IncrementableClassx2[] can be used wherever a IncrementableClass[] or object[] is required. When an SZ-array could be used in this fashion, a run-time type check is performed when you try to insert an object into the array to make sure you're not trying to insert an instance of IncrementableClass into an IncrementableClassx2[]. This check means that the following code will compile fine but will fail at run-time: IncrementableClass[] array = new IncrementableClassx2[1]; array[0] = new IncrementableClass(); // throws ArrayTypeMismatchException These checks are enforced by the various stelem* and ldelem* il instructions in such a way as to ensure you can't insert a IncrementableClass into a IncrementableClassx2[]. For the rest of this post, however, I'm going to concentrate on the ldelema instruction. ldelema This instruction pops the array index (int32) and array reference (O) off the stack, and pushes a pointer (&) to the corresponding array element. However, unlike the ldelem instruction, the instruction's type argument must match the run-time array type exactly. This is because, once you've got a managed pointer, you can use that pointer to both load and store values in that array element using the ldind* and stind* (load/store indirect) instructions. As the same pointer can be used for both input and output to the array, the type argument to ldelema must be invariant. At the time, this was a perfectly reasonable restriction, and maintained array type-safety within managed code. However, along came generics, and with it the constrained callvirt instruction. So, what happens when we combine array covariance and constrained callvirt? .method public static void CallIncrementArrayValue() { // IncrementableClassx2[] arr = new IncrementableClassx2[1] ldc.i4.1 newarr IncrementableClassx2 // arr[0] = new IncrementableClassx2(); dup newobj instance void IncrementableClassx2::.ctor() ldc.i4.0 stelem.ref // IncrementArrayValue<IncrementableClass>(arr, 0) // here, we're treating an IncrementableClassx2[] as IncrementableClass[] dup ldc.i4.0 call void IncrementArrayValue<class IncrementableClass>(!!0[],int32) // ... ret } .method public static void IncrementArrayValue<(IncrementableClass) T>( !!T[] arr, int32 index) { // arr[index].Increment(1) ldarg.0 ldarg.1 ldelema !!T ldc.i4.1 constrained. !!T callvirt instance void IIncrementable::Increment(int32) ret } And the result: Unhandled Exception: System.ArrayTypeMismatchException: Attempted to access an element as a type incompatible with the array. at IncrementArrayValue[T](T[] arr, Int32 index) at CallIncrementArrayValue() Hmm. We're instantiating the generic method as IncrementArrayValue<IncrementableClass>, but passing in an IncrementableClassx2[], hence the ldelema instruction is failing as it's expecting an IncrementableClass[]. On features and feature conflicts What we've got here is a conflict between existing behaviour (ldelema ensuring type safety on covariant arrays) and new behaviour (managed pointers to object references used for every constrained callvirt on generic type instances). And, although this is an edge case, there is no general workaround. The generic method could be hidden behind several layers of assemblies, wrappers and interfaces that make it a requirement to use array covariance when calling the generic method. Furthermore, this will only fail at runtime, whereas compile-time safety is what generics were designed for! The solution is the readonly. prefix instruction. This modifies the ldelema instruction to ignore the exact type check for arrays of reference types, and so it lets us take the address of array elements using a covariant type to the actual run-time type of the array: .method public static void IncrementArrayValue<(IncrementableClass) T>( !!T[] arr, int32 index) { // arr[index].Increment(1) ldarg.0 ldarg.1 readonly. ldelema !!T ldc.i4.1 constrained. !!T callvirt instance void IIncrementable::Increment(int32) ret } But what about type safety? In return for ignoring the type check, the resulting controlled mutability pointer can only be used in the following situations: As the object parameter to ldfld, ldflda, stfld, call and constrained callvirt instructions As the pointer parameter to ldobj or ldind* As the source parameter to cpobj In other words, the only operations allowed are those that read from the pointer; stind* and similar that alter the pointer itself are banned. This ensures that the array element we're pointing to won't be changed to anything untoward, and so type safety within the array is maintained. This is a typical example of the maxim that whenever you add a feature to a program, you have to consider how that feature interacts with every single one of the existing features. Although an edge case, the readonly. prefix instruction ensures that generics and array covariance work together and that compile-time type safety is maintained. Tune in next time for a look at the .ctor generic type constraint, and what it means.

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