empty-list

Print empty list in Haksell

由 生来就可爱ヽ(ⅴ＜●) 提交于 2019-12-19 11:52:27
问题 Here is the piece of code: import System.Environment myReverse :: [a] -> [a] myReverse [] = [] main = print (myReverse []) When I compile that with GHC I get the following error: [1 of 1] Compiling Main ( problem5_myReverse.hs, problem5_myReverse.o ) problem5_myReverse.hs:6:8: No instance for (Show a0) arising from a use of print' The type variable a0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Show

When does Python create new list objects for empty lists?

由 心已入冬 提交于 2019-12-18 03:57:20
问题 The following makes sense to me: >>> [] is [] False Given that lists are mutable, I would expect [] to be a new empty list object every time it appears in an expression. Using this explanation however, the following surprises me: id([]) == id([]) True Why? What is the explanation? 回答1: In the first example, [] is not [] precisely because the lists are mutable. If they weren't, they could safely map to the same one without issue. In the second example, id([]) creates a list, gets the id, and

MASS WHERE/IN statement, how to handle empty/returned data as well matches?

由 主宰稳场 提交于 2019-12-12 00:32:31
问题 I have a question I hope you can help me with.. I am doing a large SELECT statement using WHERE/IN and passing in an $array for all the 'search terms' to be used. quick example.. (although in the end the $array is much larger) $harNumArray = (0100001943,0100001944,0100002392,0100007414,0100012110,0100015761,0100015835); $harNumArray2 = implode(',', $harNumArray); $results = mysqli_query($mysqli, "SELECT har_id, guar_num FROM placements WHERE har_id IN ($harNumArray2)"); //**outputting the

skipping empty list and continuing with function

由 ぐ巨炮叔叔 提交于 2019-12-11 15:13:24
问题 Background import pandas as pd Names = [list(['Jon', 'Smith', 'jon', 'John']), list([]), list(['Bob', 'bobby', 'Bobs'])] df = pd.DataFrame({'Text' : ['Jon J Smith is Here and jon John from ', '', 'I like Bob and bobby and also Bobs diner '], 'P_ID': [1,2,3], 'P_Name' : Names }) #rearrange columns df = df[['Text', 'P_ID', 'P_Name']] df Text P_ID P_Name 0 Jon J Smith is Here and jon John from 1 [Jon, Smith, jon, John] 1 2 [] 2 I like Bob and bobby and also Bobs diner 3 [Bob, bobby, Bobs] Goal I

In Pyyaml, how to represent empty strings and lists in block style?

由 十年热恋 提交于 2019-12-11 01:38:10
问题 I have added representers for folded strings, literal strings as mentioned in Any yaml libraries in Python that support dumping of long strings as block literals or folded blocks?. I have also added representer to print a list in block style in the dumped yaml content. But the issue is that when the string is empty, i.e., "" or the list is empty, they appear in non-block style in the dumped YAML content. How do force the pyyaml dumper to output "" empty strings with ">" or "|" style and empty

Is it possible to write an empty list as a difference list in Prolog?

由 冷暖自知 提交于 2019-12-10 20:49:33
问题 Empty lists are ... strange, to a Prolog beginner like myself. I would say that it isn't possible to write an empty list [] as a difference list T1-T2 just as it isn't possible to write an atom as a difference list. However, I would guess that to use recursion, there must be a way to use [] in a difference list setting. I have Google'd for this but I cannot find an answer, and Bratko (Prolog Programming for AI) only briefly touches the subject. So, is it possible to write an empty list as a

Generics: Cannot convert from Collections.emptyList() to List<String>

由 末鹿安然 提交于 2019-12-08 17:02:51
问题 Why public List<String> getList(){ if (isMyListOKReady()) return myList; return Collections.emptyList(); } compiles well, but for public List<String> getList(){ return isMyListReady() ? myList : Collections.emptyList(); } Eclipse says "Type mismatch: cannot convert from List<Object> to List<String>" ? 回答1: You need to take care of type safety of empty list. So return the empty list of string like this public List<String> getList(){ return isMyListReady() ? myList : Collections.<String

modification of skipping empty list and continuing with function

由 夙愿已清 提交于 2019-12-04 06:06:28
问题 Background The following code is slightly modified from skipping empty list and continuing with function import pandas as pd Names = [list(['Jon', 'Smith', 'jon', 'John']), list([]), list(['Bob', 'bobby', 'Bobs']), list([]), list([])] df = pd.DataFrame({'Text' : ['Jon J Smith is Here and jon John from ', 'get nothing from here', 'I like Bob and bobby and also Bobs diner ', 'nothing here too', 'same here' ], 'P_ID': [1,2,3, 4,5], 'P_Name' : Names }) #rearrange columns df = df[['Text', 'P_ID',

Is there an “Empty List” singleton in C#?

由 扶醉桌前 提交于 2019-12-03 05:23:34
问题 In C# I use LINQ and IEnumerable a good bit. And all is well-and-good (or at least mostly so). However, in many cases I find myself that I need an empty IEnumerable<X> as a default. That is, I would like for (var x in xs) { ... } to work without needing a null-check. Now this is what I currently do, depending upon the larger context: var xs = f() ?? new X[0]; // when xs is assigned, sometimes for (var x in xs ?? new X[0]) { ... } // inline, sometimes Now, while the above is perfectly fine for

Collections.emptyList() instead of null check?

由 生来就可爱ヽ(ⅴ＜●) 提交于 2019-12-02 23:05:25
If I have a rarely used collection in some class which may be instantiated many times, I may sometimes resort to the following "idiom" in order to save unnecessary object creations: List<Object> list = null; void add(Object object) { if (list == null) list = new ArrayList<Object>(); list.add(object); } // somewhere else if (list != null) for (Object object : list) ; Now I was wondering if I couldn't eliminate those null checks using Collections.emptyList() , however then I would have to alter the if check in add() like so: if (list == Collections.<Object>emptyList()) list = new ArrayList
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