syntax

Question mark before dot in Dart

由 大兔子大兔子 提交于 2020-01-30 11:22:28
问题 What's the meaning of the question mark in this piece of code? And when am I supposed to use it? My code functions the same way with or without the question mark. void dispose(){ bloc?.dispose(); super.dispose(); } 回答1: The question mark is one of the null-aware operators in Dart. In this example it says: call the dispose() method only if bloc is not null. Without the question mark, if bloc was null when it tried to call dispose() a NoSuchMethodError would be thrown. There is a useful section

Strange valid python syntax with string [duplicate]

由 戏子无情 提交于 2020-01-30 10:31:26
问题 This question already has answers here : String concatenation without '+' operator (6 answers) Closed 2 years ago . I am running on Python 3.6.1 and today I missed a comma, as follows: nt = namedtuple('Record', ['c', 'a' 'b']) # instead the following is what I actually want nt = namedtuple('Record', ['c', 'a', 'b']) But I just wonder why the first way is valid Python in any way? Should not it complains with syntax error? I just tried in 3.5.2 and 2.7.11. Seems all valid. But it is valid? 回答1:

Parse error: syntax error, unexpected T_ECHO

由 浪子不回头ぞ 提交于 2020-01-30 08:51:11
问题 I've been working on something for the past few days but this one bit of code perpetually throws an unexpected T_ECHO. My friends can't seem to find anything wrong with it and I'm at the edge of my patience. Even with the nested while loop removed it still throws an error and I switched to the while: endwhile; syntax as well and I'm still getting it. I'm sure the answer is staring me in the face but I probably can't see it. while ($row = mysql_fetch_array($result, MYSQL_ASSOC)): echo "<tr>";

MySQL set database name dynamically and select db.schematable

由 泄露秘密 提交于 2020-01-30 06:51:46
问题 I am creating a stored procedure, which takes a number of input parameters. I would like to be able to select which database to use, whereafter depending on the conditions as shown below, I would like to use the correct where conditions. I know you cannot use "USE" keyword in a stored proc and I have also tried to set @ds in the query but with not much success. I'm using Navicat and have defined my parameters in the IDE, my input params are : icustomername varchar(255) ipostcode varchar(255)

Why can't I access a member of this class? [duplicate]

由 最后都变了- 提交于 2020-01-30 05:57:05
问题 This question already has answers here : My attempt at value initialization is interpreted as a function declaration, and why doesn't A a(()); solve it? (5 answers) Closed 6 years ago . I have the following three class definitions: class String { public: String() {} String(const char *) {} }; class ClassA { public: ClassA(const String &) {} }; class ClassB { public: ClassB(const ClassA &, const String & = String()) {} void method() {} }; Now suppose I want to create an instance of ClassB :

Haskell import declaration

由 ▼魔方 西西 提交于 2020-01-30 04:17:14
问题 I started to read about monad transformers and what puzzles me is Control.Monad.CatchIO 's import declaration which I see in many code examples: import "MonadCatchIO-transformers" Control.Monad.CatchIO (finally) What does this quoted token mean? I took a look at the Haskell 98 Report's section on import declarations, but this didn't help me understand. 回答1: Its a package-qualified import, which is a GHC extension. The string is a package name. See Package-qualified imports, from the ghc docs,

python smtp发送邮件:500 Error: bad syntax

由 核能气质少年 提交于 2020-01-28 11:27:16
reply: b'220 126.com Anti-spam GT for Coremail System (126com[20140526])\r\n' reply: retcode (220); Msg: b'126.com Anti-spam GT for Coremail System (126com[20140526])' connect: b'126.com Anti-spam GT for Coremail System (126com[20140526])' send: 'ehlo TOMAS-PC.DHCP HOST\r\n' reply: b'500 Error: bad syntax\r\n' reply: retcode (500); Msg: b'Error: bad syntax' send: 'helo TOMAS-PC.DHCP HOST\r\n' reply: b'500 Error: bad syntax\r\n' reply: retcode (500); Msg: b'Error: bad syntax' (500, b'Error: bad syntax') fqdn = socket.getfqdn() if '.' in fqdn: self.local_hostname = fqdn else: # We can't find an

Is it possible to turn off support for “and” / “or” boolean operator usage in gcc?

由 坚强是说给别人听的谎言 提交于 2020-01-28 02:16:25
问题 GCC seems to allow "and" / "or" to be used instead of "&&" / "||" in C++ code; however, as I expected, many compilers (notably MSVC 7) do not support this. The fact that GCC allows this has caused some annoyances for us in that we have different developers working on the same code base on multiple platforms and occasionally, these "errors" slip in as people are switching back and forth between Python and C++ development. Ideally, we would all remember to use the appropriate syntax, but for

What is the formal difference in Scala between braces and parentheses, and when should they be used?

由 旧时模样 提交于 2020-01-26 05:23:42
问题 What is the formal difference between passing arguments to functions in parentheses () and in braces {} ? The feeling I got from the Programming in Scala book is that Scala's pretty flexible and I should use the one I like best, but I find that some cases compile while others don't. For instance (just meant as an example; I would appreciate any response that discusses the general case, not this particular example only): val tupleList = List[(String, String)]() val filtered = tupleList
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