where

问题 I want to filter a list using LINQ with Where extension method. But apart from filtering I also want to update a global variable inside Where . However I cannot do it. Consider this example: var list = new List<string> { "1", "2", "3", "4", "5" }; bool flag = false; var newList = list.Where(item => { flag = true; return item == "2"; }); // Here I expect flag = true, but in fact it's false Console.Write(flag); As you can see I set flag = true , still the value flag == false after execution. It

问题 Not so much a question but something puzzling me. I have a column of dates that looks something like this: 0 NaT 1 1996-04-01 2 2000-03-01 3 NaT 4 NaT 5 NaT 6 NaT 7 NaT 8 NaT I'd like to convert it the NaTs to a static value. (Assume I imported pandas as pd and numpy as np). If I do: mydata['mynewdate'] = mydata.mydate.replace( np.NaN, pd.datetime(1994,6,30,0,0)) All is well, I get: 0 1994-06-30 1 1996-04-01 2 2000-03-01 3 1994-06-30 4 1994-06-30 5 1994-06-30 6 1994-06-30 7 1994-06-30 8 1994

问题 Not so much a question but something puzzling me. I have a column of dates that looks something like this: 0 NaT 1 1996-04-01 2 2000-03-01 3 NaT 4 NaT 5 NaT 6 NaT 7 NaT 8 NaT I'd like to convert it the NaTs to a static value. (Assume I imported pandas as pd and numpy as np). If I do: mydata['mynewdate'] = mydata.mydate.replace( np.NaN, pd.datetime(1994,6,30,0,0)) All is well, I get: 0 1994-06-30 1 1996-04-01 2 2000-03-01 3 1994-06-30 4 1994-06-30 5 1994-06-30 6 1994-06-30 7 1994-06-30 8 1994

问题 I'm trying to make an image database which does not keep a consistent record of ID's. For example it might go 1,2,6,7,12, but as you can see that is only 5 rows. Inside the table I have fileid and filename. I created a PHP script to show me the image when I give the fileid. But if I give it the ID 5 which does not exist I get an error. That's fine as I want an error for that, but not for users who will browse through these images using forward and back buttons. The forward and back buttons

问题 Is it right to expect that, for the typical modern RDBMS, querying by one specific primary key is as fast as querying a hashtable by key? Or is there "actual work" done to traverse the table and track down the primary key value? That seems unthinkably wasteful, even if there are automatic indexes for primary keys. 回答1: Database operation involves access of secondary memory unit (Disk). And to achieve efficiency important is to reduce block access time(not operations). Complexity of Select

问题 Is it right to expect that, for the typical modern RDBMS, querying by one specific primary key is as fast as querying a hashtable by key? Or is there "actual work" done to traverse the table and track down the primary key value? That seems unthinkably wasteful, even if there are automatic indexes for primary keys. 回答1: Database operation involves access of secondary memory unit (Disk). And to achieve efficiency important is to reduce block access time(not operations). Complexity of Select

问题 I have been searching a lot about my current problem but I could not find a real answer to solve that issue. I am trying to build a LINQ Query that produces the following SQL: SELECT * FROM TABLE WHERE (Field1 = X, Field2 = Y ... ) or (Field3 = Z) In a normal situation I would just do this: Object.Where(c => (c.Field1 == X && c.Field2 == Y) || (c.Field3 == Z)) I cannot use this approach because the query is build by using multiple .Where() calls. Having an example: // This is a short example,

问题 I want to add functionality of contains() method in where clause. Right now I was doing something like "address = ?" and providing address in arguments but now I want to use a logic which works as address.contains(?) and then providing the address in arguments. How can I implement it? Till now my code is and need to know the require modification. Uri mSmsinboxQueryUri = Uri.parse("content://sms"); String[] projection = new String[] { "_id", "thread_id", "address", "person", "date", "body",

问题 I want to add functionality of contains() method in where clause. Right now I was doing something like "address = ?" and providing address in arguments but now I want to use a logic which works as address.contains(?) and then providing the address in arguments. How can I implement it? Till now my code is and need to know the require modification. Uri mSmsinboxQueryUri = Uri.parse("content://sms"); String[] projection = new String[] { "_id", "thread_id", "address", "person", "date", "body",

问题 I want to add functionality of contains() method in where clause. Right now I was doing something like "address = ?" and providing address in arguments but now I want to use a logic which works as address.contains(?) and then providing the address in arguments. How can I implement it? Till now my code is and need to know the require modification. Uri mSmsinboxQueryUri = Uri.parse("content://sms"); String[] projection = new String[] { "_id", "thread_id", "address", "person", "date", "body",