See the great Article "Shuffling" from Jeff. Enumerable.Where Method (System.Linq) | Microsoft Learn You don't need to convert it. You may consider using a linked list for an algorithm requiring so many removals. Can I knock myself prone? If you are not suppose to add something to the Collection then you should use/return IEnumerable. For instance, why does Croatia feel so safe? Theoretically every query that contains more than a single where/select isn't simple. Developers use AI tools, they just dont trust them (Ep. I.e. Secondly, when the count of random items to be returned is more than half of the original group, then its easier to remove source.Count - count items from the group than adding count items. How do I clone a list so that it doesn't change unexpectedly after assignment? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. We can use the AsEnumerable () function of LINQ to convert a list to an IEnumerable in C#. rev2023.7.5.43524. When you call the method you can specify the type: The trick is to declare right, if you define generic , then you have to stick to it in your methods, so if you have IEnumerable then elsewhere in your method you must use and not or any other type. If it calls it whenever it wants to perform a comparison between two elements then the it will get a different value back each time, which will screw up the sort. So if you have { 1, 2, 3, 4 }, this can end up in { 1, 2, 3 }, { 3, 4, 1 } etc for 3 items. How Can I create objects of List dynamically? How to install game with dependencies on Linux? To completely randomly shuffle your list (in place) you do this: To shuffle an array a of n elements (indices 0..n-1): If you only need the first 5 elements, then instead of running i all the way from n-1 to 1, you only need to run it to n-5 (ie: n-5). 1) If you need true random values which means there is no restriction on which elements to choose from (ie, once chosen item can be reselected): If you set the exception flag off, then you can choose random items any number of times. And TakeWhile, a more complex form of Take, accepts the first elements while a predicate method returns true. You want to use Where which is a deferred filtering. Where is used to filter. For people wondering why Kirk's comment have so many "ups", it's because you can't just name your variable the way you want ! I try to make selection like this: But it returns a collection of 161 elements (it is initial count of collection) and each item has value false. Filters a sequence of values based on a predicate. First, permute the array randomly, then select the first K elements. You have. More like "wtf is going on here" :). Safe to drive back home with torn ball joint boot? This is the best I could come up with on a first cut: Using a list of randoms within a range of 1 - total list count and then simply pulling those items in the list seemed to be the best way, but using the Dictionary to ensure uniqueness is something I'm still mulling over. Why is this? Any recommendation? I use an extension method for this. Convert an IEnumerable of unknown to a List, Error "Cannot implicitly convert type "System.Collections.Generic.IEnumerable' to 'System.Collections.Generic.List', Linq IEnumerable> back to List, c# instantiate a List using AS beginner question. I recently did this on my project using an idea similar to Tyler's point 1. What is wrong? Linq select based on IEnumerable property, Use linq to return a selection of data from list. See Terms and Conditions for details. start Int32 The value of the first integer in the sequence. And, also interestingly, the StdDev of Kyle's solution was significantly higher comparatively. @t3chb0t, it is more about how good your query conveys your intent. How Did Old Testament Prophets "Earn Their Bread"? for (1) you can shuffle a list faster than sorting is, for (2) you will be biasing your distribution by using %, Given the objection you raised about the cycle length of a rng, is there any way we. +1 for an alternative approach. Using such a structure to build up an array called s, here is some pseudopython: I suggest running through a few sample cases to see how this efficiently implements the above English explanation. Therefore, depending of the requirements, the best compromise of efficiency and simplicity is quite likely the FisherYates variant. source IEnumerable<TSource> An IEnumerable<T> to return an element from. 2) If I can tell that it is an IList, use drzaus' approach, with a twist. Making statements based on opinion; back them up with references or personal experience. Do large language models know what they are talking about? If you wanted to be even more efficient, you would probably use a HashSet of the indices, not the actual list elements (in case you've got complex types or expensive comparisons); And to make sure we don't have any collisions, etc. IEnumerable<T> is meant for querying collections only. Difference between machine language and machine code, maybe in the C64 community? start + count -1 is larger than Int32.MaxValue. The type parameter needs to be specified by the caller somewhere. ToArray (); } } Output 0 1 Enumerable.Range. 2) You could use yield to get rid of the additional list. aAll questions were loaded in the a QuestionSorter list, which was then sorted using the List's Sort function and the first k elements where selected. Here's my approach (full text here http://krkadev.blogspot.com/2010/08/random-numbers-without-repetition.html ). My extension method first checks to see if the enumeration is null and if so creates an empty list. This blog post talks more about Microsoft's MultiDictionary type available via NuGet. As far as I know List implements IEnumerable. To learn more, see our tips on writing great answers. Is there a better way to return a list of all keys than just doing a foreach on the KeyValuePair List? For all the answers below, be aware that you need to have the System.Linq namespace imported. This however means, that now i can't return the ListOfStrings anymore. 586), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Testing native, sponsored banner ads on Stack Overflow (starting July 6), Temporary policy: Generative AI (e.g., ChatGPT) is banned. If the type of source implements ICollection, that implementation is used to obtain the count of elements. Which will actually filter the list based on the criterion given as the predicate, and the list shall get filtered upon the call of the ToList() method, as Where is deferred filtering. Removing exact instances of elements in one list from another, Why Enumerable.Except() Might Not Work the Way You Might Expect. That's why 2 got removed giving {6). I think the selected answer is correct and pretty sweet. 3) I guarantee that the items will be returned in the same order that they were encountered. Exceptions ArgumentOutOfRangeException count is less than 0. How do I distinguish between chords going 'up' and chords going 'down' when writing a harmony? Find centralized, trusted content and collaborate around the technologies you use most. Making statements based on opinion; back them up with references or personal experience. I was a little surprised no one suggested using Except for this approach as it is more robust(can be applied to string and int). Convert a 0 V / 3.3 V trigger signal into a 0 V / 5V trigger signal (TTL). so 't' prefix means temporary? Difference between machine language and machine code, maybe in the C64 community? Returns the number of elements in a sequence. Should I sell stocks that are performing well or poorly first? This allows you to do a foreach on it without explicitly having to check for null. True. Use MathJax to format equations. I wouldn't call this LINQ query "simple". Order is a custom object. - (so e.g: List, Stack, Queue, ). It means that you do not have to convert or cast anything. Enumerable.ElementAt Method (System.Linq) | Microsoft Learn The number of elements in source is larger than Int32.MaxValue. This method is implemented by using deferred execution. Why would the Bank not withdraw all of the money for the check amount I wrote? Return random subset of N elements of a given array. The number 6 does not occur in the source but in the result. Does this change how I list it on my CV? Making statements based on opinion; back them up with references or personal experience. If you're using an implementation of System.Collections.IEnumerable you can do like following to convert it to a List. international train travel in Europe for European citizens. Unfortunately, I do not have control over the list (I have to "play it as it lies") Ah - that sucks. Although SelectMany works similarly to Select, it differs in that the transform function returns a collection that is then expanded by SelectMany before it is returned. static IEnumerable<SomeType> PickSomeInRandomOrder<SomeType>( IEnumerable<SomeType> someTypes, int maxCount) { Random random = new Random(DateTime.Now.Millisecond); Dictionary<double, SomeType> randomSortTable = new Dictionary<double,SomeType>(); foreach . To learn more, see our tips on writing great answers. Making statements based on opinion; back them up with references or personal experience. Select is a projection or a conversion for each item. But doesn't this enumerate and sort the whole list? Are MSO formulae expressible as existential SO formulae over arbitrary structures? The following code example demonstrates how to use Count(IEnumerable) to count the elements in an array. Based on Kyle's answer, here's my c# implementation. Does this change how I list it on my CV? This is actually a harder problem than it sounds like, mainly because many mathematically-correct solutions will fail to actually allow you to hit all the possibilities (more on this below). Curiously enough, this O(n) time complexity issue is true even if you only touch the list when you actually return an item, like I do in my implementation. The former performs many more calls to the random number generator, plus iterates more times over the items. type? (NOTE: there are multiple values for the key "qwer" and multiple keys for the value "asdf".). Are there good reasons to minimize the number of keywords in a language? Asking for help, clarification, or responding to other answers. March 29 Saint Feast Day,
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