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Hi all,
Can someone let me know what is happening with the following line of code? I don't understand the syntax used in after j == (arr[i].Length - 1)
Specifically what's happening with ? "" :
I do know what " " is doing at the very end.
System.Console.Write("{0}{1}", arr[i][j], j == (arr[i].Length - 1) ? "" : " ");
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the ternary operator ?: is used to choose between an empty string and a space.
x?y:z is an expression that evaluates to y or to z, depending on the boolean value of x.
another example would be:
bool equals = a==b;
Console.WriteLine(equals ? "a==b" : "a!=b");
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? is the ternary if operator. if the condition ( j == (arr[i].Length - 1) is true, it outputs"" (the null string), if the condition is false, it outputs " " (one space.
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Just in case you didn't understand the answers in the previous two replies.
System.Console.Write("{0}{1}", arr[i][j], j == (arr[i].Length - 1) ? "" : " ");
is the same as
string result = string.Empty;
if (j == (arr[i].Length - 1))
{
result = "";
}
else
{
result = " ";
}
System.Console.Write("{0}{1}", arr[i][j], result);
The ternary operator ?: is a shorthand version of if - else. See here[^].
Henry Minute
Do not read medical books! You could die of a misprint. - Mark Twain
Girl: (staring) "Why do you need an icy cucumber?"
“I want to report a fraud. The government is lying to us all.”
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What happens with the sequence points though? For the ternary operator there is a sequence point between the condition and the second/third operand (whichever it chooses), but AFAIK not after the second/third operand (unless you close it with a semicolon, of course)
Or did MS sneakily add some sequence points there, like they did with method arguments?
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It is beyond my capabilities to give a definitive answer to that although I suspect that Luc might be able to.
As a guess though, and knowing how sneaky M$ are, I'd go for the latter.
Henry Minute
Do not read medical books! You could die of a misprint. - Mark Twain
Girl: (staring) "Why do you need an icy cucumber?"
“I want to report a fraud. The government is lying to us all.”
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Disclaimer: I don't know the finer details as I don't really care; with 20+ years of C under the belt (using all kinds of compilers) I have learned to stay well away from the borders. Defensive programming is what I call it. Besides, doing so improves readability and robustness of the code.
But then I decided to browse the C# 3.0 spec anyway, and here are some findings:
1.
the notion of "sequence point" does not appear in the C# standard.
2.
"The order of evaluation of operators in an expression is determined by the precedence and associativity of the operators (§7.2.1)."
3.
"If the array creation expression contains an array initializer, then each expression in the array initializer is evaluated and assigned to its corresponding array element. The evaluations and assignments are performed in the order the expressions are written in the array initializer"
4.
"A method decorated with the Conditional attribute is a conditional method. The Conditional attribute indicates a condition by testing a conditional compilation symbol. Calls to a conditional method are either included or omitted depending on whether this symbol is defined at the point of the call. If the symbol is defined, the call is included; otherwise, the call (including evaluation of the parameters of the call) is omitted."
I did not find:
- a single thing about side-effects
- evaluation order of method call arguments
which does not mean it isn't covered, there are over 500 pages in the spec.
Note: as 4.0 supports named arguments (and hence accepts them in any order), the problem potentially gets larger.
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I don't think there can be any side effects in C#, because objects (and everything is an object), are essentially static in nature. For example in C, an operation like x++?: could introduce some ambiguity, while in C#, x and x++ are distinct objects.
Am I out of my depth here? Absolutely
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There always is some potential for side-effects.
First example:
int v = ...;
int y = (v++==100) ? 0 : v;
will the auto-increment happen before or after the evaluation of the second/third operand?
Second example:
int y = someFlag ? method1() : method2();
We all hope this causes exactly one method to get executed; in general it is not equivalent to:
int v1=method1();
int v2=method2();
int y = someFlag ? v1 : v2;
as those methods could have all kinds of side-effects. However I did not see that stated explicitly in the spec.
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Luc Pattyn wrote: will the auto-increment happen before or after the evaluation of the second/third operand?
Neither, because the code won't compile. "int v = ...;" 
I know the language. I've read a book. - _Madmatt
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you're right, you'd better insert some expression that the compiler can't evaluate right away.
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Someone disliked your message enough to down-vote it. The topic is interesting enough IMO, albeit a little OT but that's OK.
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Luc Pattyn wrote: I have learned to stay well away from the borders. Defensive programming is what I call it.
Couldn't agree more, and liberal use of parentheses
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I didn't find much, but I did find "The expressions of an argument list are always evaluated in the order they are written. " on page 182/553 of the C# 2 spec (4th edition / June 2006)
I'm not sure whether that still applies to C# 4 - I'll download the new spec (I don't really know why I still had that old one)
And in 14.2 (page 172) of the same document I found "The order in which operands in an expression are evaluated, is left to right. [Example: In F(i) + G(i++) * H(i), method F is called using the old value of i, then method G is called with the old value of i, and, finally, method H is called with the new value of i. This is separate from and unrelated to operator precedence. end example] "
edit: IMO that [the first quote] supports my claim that MS had inserted sequence points in method argument evaluation, they're just not calling it that..
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Thanks Harold for "The expressions of an argument list are always evaluated in the order they are written"; that is what I knew had to be there, and I now found it in C# 3.0 too.
I'm afraid there is some semantic ambiguity involved; to evaluate can either be "determine the value" or "process the expression yielding a value and execute all side-effects", and I'm pretty sure the latter is what was meant for argument lists.
harold aptroot wrote: And in 14.2 (page 172)
the numbers clearly have changed since. I now found that sentence too, it is under 7.2; and it supports the broader meaning of "to evaluate".
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The second meaning is also what's used by the compiler, so it's probably that
But I can't really find any good "C# 4 language specification" documents, do you know one? Google didn't seem in the mood to give me useful results..
I found some drafts, but well, they're drafts.. or does that mean that the C# 4 specs aren't "official" yet?
modified on Monday, April 26, 2010 5:31 PM
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This[^] is what Google is willing to offer; I see two candidates, nothing final though.
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Thanks Michel. I have been expecting Bing to become better at finding MS stuff. They wouldn't dare putting in some meta tags to fool other search engines, would they?
Michel Godfroid wrote: Google is NOT your friend
Yes he is, he is my best mate; he knows almost everything I don't know (and that is a lot). Can't do without him.
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harold aptroot wrote: the C# 4 specs aren't "official" yet?
yes, I expect that is part of the game: propose a standard, get it accepted, then propose extensions and implement them faster than anyone else can, giving you de facto control.
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The sentence "The expressions of an argument list are always evaluated in the order they are written" is still present in the 4.0 document Michel provided a link for. And it now has an example with named parameters. Great.
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Thanks everyone. I googled ?:, but didn't see anything and am not experienced enough to have looked at msdn's operators. Will know for next time.
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Hi,
I want to overlay controls on my main window that fade away and dissapear when the mouse is outside the window. Very much like in Windows Media Player.
Is there a control to do that for me, or can anyone give me some starting information on how that might be achieved.
thanks
jon
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I guess it depends on what version of Media Player you are using but I thought the current version uses WPF which has more graphics capabilities then standard WinForms. Using WinForms though you could try handling the mouse move event or the Deactivated event
I know the language. I've read a book. - _Madmatt
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I don't know about any control buy but,
You can put your controls inside a panel, and then handle your main form's active / deactivate , enter / leave events and set the opacity of the panel from 0 to 100 and 100 to 0, using a timer control to fade out and fade in...
(I think Mark has suggested you the same thing...)
modified on Monday, April 26, 2010 1:27 PM
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