indexing

问题 When I add an item (an instance of a class) to a list, I need to know the index of the new item. Is it possible with any function? Example code: MapTiles.Add(new Class1(num, x * 32 + cameraX, y * 32 + cameraY)); 回答1: MapTiles.Count will give you the index of next item that will be added to the list Something like: Console.WriteLine("Adding " + MapTiles.Count + "th item to MapTiles List"); MapTiles.Add(new Class1(num, x * 32 + cameraX, y * 32 + cameraY)); 回答2: Class1 newTile = new Class1(num,

问题 I am using MATLAB. I have a question about how to sort an matrix in descending order along one column of a matrix and have the rest of the row values follow suit. For example, A = [1 30; 2 40; 3 10; 4 50; 5 20] becomes B = [4 50; 2 40; 1 30; 5 20; 3 10] Thanks 回答1: Use the SORTROWS function, specifying that you want to sort using the second column in descending order... B = sortrows(A, -2); Here, the 2 means "sort in column 2," and the negative sign in front of the 2 means "in de -scending

问题 I have big set of urls and I want to implement an autocompletion. I don't like the complexity of the naive approach as it is linear with the set size: for(String url: urls) if(url.startsWith(input) {doSomething();} Now I know that in a Hash Set, the function "contains()" works in "O(1)" but there is no "containsPrefix()". Is there a simple way without using a big library like Lucene or coding it myself? I would have no problem doing it but it seems overkill for such a simple problem so I want

问题 I have big set of urls and I want to implement an autocompletion. I don't like the complexity of the naive approach as it is linear with the set size: for(String url: urls) if(url.startsWith(input) {doSomething();} Now I know that in a Hash Set, the function "contains()" works in "O(1)" but there is no "containsPrefix()". Is there a simple way without using a big library like Lucene or coding it myself? I would have no problem doing it but it seems overkill for such a simple problem so I want

问题 I am trying to list all the book_sales information for a particular book author. So I have a query and it's not using Index to lookup records. The following is my tables structure: -- Table structure for table `books` CREATE TABLE IF NOT EXISTS `books` ( `book_id` int(11) NOT NULL auto_increment, `author_id` int(11) unsigned NOT NULL, `book_type_id` int(11) NOT NULL, `book_title` varchar(50) NOT NULL, `book_price` smallint(4) NOT NULL, `in_stock` char(1) NOT NULL, PRIMARY KEY (`book_id`), KEY

问题 I have discovered a huge performance problem in Linq to SQL. When selecting from a table using strings, the parameters passed to sql server are always nvarchar, even when the sql table is a varchar. This results in table scans instead of seeks, a massive performance issue. var q = ( from a in tbl where a.index == "TEST" select a) var qa = q.ToArray(); The parameter is passed through as a nvarchar, which results in the entire index being converted from varchar to nvarchar before being used. If

问题 Can I modify existing index in MongoDB without dropping it ? I don't see anything about it in documentation. I have an non-unique index on String field. Collection is ~6M documents. It's replica set. I know I can delete index and add new one. But it's problematic due to two reasons: 1) at time while index doesn't exist some queries will be very very slow. 2) adding new index (in my project) creates very high load on DB which visibly slows down my web-site. 回答1: There is no way to alter an

问题 I have a Pandas DataFrame with a DatetimeIndex and one column MSE Loss the index is formatted as follows: DatetimeIndex(['2015-07-16 07:14:41', '2015-07-16 07:14:48', '2015-07-16 07:14:54', '2015-07-16 07:15:01', '2015-07-16 07:15:07', '2015-07-16 07:15:14',...] It includes several days. I want to select all the rows (all times) of a particular days without specifically knowing the actual time intervals. For example: Between 2015-07-16 07:00:00 and 2015-07-16 23:00:00 I tried the approach

问题 What would be the best way to find the index of a specified character in a list containing multiple characters? 回答1: >>> ['a', 'b'].index('b') 1 If the list is already sorted, you can of course do better than linear search. 回答2: Probably the index method? a = ["a", "b", "c", "d", "e"] print a.index("c") 回答3: As suggested by others, you can use index . Other than that you can use enumerate to get both the index as well as the character for position,char in enumerate(['a','b','c','d']): if char

问题 I am considering adding a column to a database table, and that column will be null possibly for most rows however I do want to be able to query on that column for particular non-null values. Therefore an index would help with retrieval time, however if null values are included that would ruin the selectivity of my index. I have done some reading and I have got the impression that SQL Server doesn't necessarily adhere to standards when it comes to NULL values and indexes, but I can find no