The ternary operator has a shorthand version, i.e.
var = exp ?: exp2
I know it works in PHP. Other languages may have also picked it up. C# has similar functionality (for the context of this question) - ??.
When the condition passes, is the expression evaluated again, or is the result stored somewhere?
http://msdn.microsoft.com/en-us/library/ms173224.aspx
The ?? operator is called the null-coalescing operator and is used to define a default value for nullable value types or reference types. It returns the left-hand operand if the operand is not null; otherwise it returns the right operand.
It is stored, not computed twice.
Related
Using the sqlite3 library in Python 3, I'm looking to filter a column by whether or not it matches a regex.
I found a question here that seems to do exactly that: Problem with regexp python and sqlite
The function in the top answer there takes arguments expr, item, where expr is given in the parametrized value and item is the cell given from iterating over the column bar. I can't find any documentation that would suggest that the value of the cell in the WHERE clause is passed into the last parameter of the custom function - it seems that it should be WHERE REGEXP(?,bar)?
Is there anywhere that documents this interaction explicitly?
When you use x REGEXP y in sqlite, it's turned into regexp(y, x) if a function by that name exists, and an error otherwise. (LIKE and GLOB are treated the same way, except those are provided by sqlite instead of relying on an extension module or user defined function).
From the documentation:
The REGEXP operator is a special syntax for the regexp() user function. No regexp() user function is defined by default and so use of the REGEXP operator will normally result in an error message. If an application-defined SQL function named "regexp" is added at run-time, then the "X REGEXP Y" operator will be implemented as a call to "regexp(Y,X)".
I came across this error message in another question and I would like to know what is the meaning of the $: part of the signature, please?
Cannot resolve caller index(Str: Str, Any); none of these signatures match:
(Str:D $: Cool:D $needle, *%_)
(Str:D $: Str:D $needle, *%_)
(Str:D $: Cool:D $needle, Cool:D $pos, *%_)
(Str:D $: Str:D $needle, Int:D $pos, *%_)
The $: is two distinct unrelated tokens, a $ and a :, that have been smooshed together.
The $ represents a single item1 aka a scalar2.
The single item / scalar hasn't been given a name, so there's no way to reference it. And there's already enough of a parameter (the Str:D) to let you and the compiler know that this parameter is a scalar (a single string). Either way, the $ is redundant and Elizabeth has made a related change.
The : is a special parameter separator. (The usual parameter separator is a comma ,.)
It may only be used immediately after the first parameter of a method or standalone signature. It may not be used in the signature of a routine that is not a method.
If used as a parameter separator after the first parameter in a signature, it marks that parameter as corresponding to a method's "invocant".
(If not used, the invocant parameter is implicit.)
The corresponding invocant argument will arrive anyway, and be aliased to self, whether or not the signature explicitly lists an invocant parameter. But if the invocant parameter is explicitly specified, it's possible to give it an additional/alternate name and/or explicitly constrain its type.
Crazy over-the-top footnotes for added entertainment. If they confuse you, just forget you ever read them.
1 A single item refers to data that is naturally a single thing, like the number 42, OR data that is naturally a composite thing (like an array) that is being treated like it's a single thing (like an array). (Did you see what I did there?) I like to point out the mnemonic that a $ symbol is like an S (for single) overlaid with an I (for item), or vice-versa. To me this represents the idea of emphasizing the single item nature of any data, hiding any plural aspect even if it's actually an array or other composite data item.
2 "scalar" is a traditional computing term. Wikipedia's Scalar disambiguation page lists "Variable (computing), or scalar, an atomic quantity that can hold only one value at a time" as a definition. Also, a single item aka scalar (all lowercase) is often/usually a Scalar (uppercase S), a special case of a single item that's a Single Item container that contains a Single Item (which can be composite data being treated as a single thing).
The : mark the first argument as an invocant.
my $word = "bananarama";
say $word.index( "r", 0 );
In this case, it indicates the invocant is going to be treated as an scalar, since it's constrained by a single $
I have this code...
if $hostname in $var_slave {
file {
"/var/spool/cron/mysql":
ensure => present,
owner => $mysqlUser,
group => $mysqlGroup,
mode => "0600",
source => 'puppet:///modules/eikonappdbcron/mysql-slave',
}
}
I want a way of checking if a value is not in a file.
I tried "if $hostname not in $var_master" but this doesn't work. After doing some research I believe I need to use an "!". I can't get the syntax correct.
Puppet does not have a single, combined "not in" operator. Instead, it has the in operator, and it has a general-purpose boolean negation operator (!). You can use these together to write a compound expression that evaluates the condition you want.
If you already realized that, then perhaps you ran into a problem with operator precedence. The negation operator has the higher precedence than does in. Indeed, ! has the highest precedence of any Puppet operator, so if its operand is intended to be a binary expression then you must enclose the operand in parentheses.
Thus, the negation of the boolean expression
$hostname in $var_master
is
! ($hostname in $var_master)
Just a straight forward beginner question, I am coding Lua stuff for Garrys Mod, learning by reading wiki and other codings.
if (self.Owner:SteamID( ) == "STEAM_0:1:44037488" ) then
the above is the code I want to use, to check to see if the STEAM ID (which I believe is a string) is equal to my exact string.
Is this viable? Or is there another way I should do it?
This should work exactly as you expect it to. In lua '==' for string will return true if contents of the strings are equal.
As it was pointed out in the comments, lua strings are interned, which means that any two strings that have the same value are actually the same string.
One thing to consider while learning Lua (from www.lua.org/source/5.2/lstring.h.html):
/*
** as all string are internalized, string equality becomes
** pointer equality
*/
#define eqstr(a,b) ((a) == (b))
String comparison in Lua is cheap, string creation may be not.
According to http://wiki.garrysmod.com/page/Player/SteamID, SteamID() returns a string so you should be able to write
if self.Owner:SteamID() == "STEAM_0:1:44037488" then
...do stuff...
end
If you ever need to confirm the type of an object, use type and print, like in this case print('type is', type(self.Owner:SteamID())) should print 'type is string'.
In lua, as answered above, '==' checks for equality.
Not saying you did this, because you didnt, but a common mistake is thinking that '=' is equality. '=' is assignment, '==' is equality.
I have a mathematical expression in string form like:
string strExpression = "10+100+Math.Sin(90)";
I want to simply assign this expression (at run time) to a float variable (say result), so that it becomes the following code statement:
float result = 10+100+Math.Sin(90);
How can I do this?
You have to compile the expression within a syntactically correct code block. See http://devreminder.wordpress.com/net/net-framework-fundamentals/c-dynamic-math-expression-evaluation/ as an example.
Edit: Or alternatively write your own expression parser if the expression is going to be VERY simple (I wouldn't recommend this though)
You could use CS-Script to dynamically make a class with a method that you can run, if you don't want to write your own parser but rather use C# which you allready know..