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reverse foreach
Posted: Fri Dec 16, 2016 1:16 pm
by wriedmann
Hi,
would it possible to have a foreach that works in a reverse sense, i.e. from the last element to the first one?
I have to remove entries from a Dictionary object, and the following code gives a runtime error:
Code: Select all
foreach oItem as KeyValuePair<string,int> in oDict
if oItem:Value == nValue
oDict:Remove( oItem:Key )
endif
next
As solution, I'm using this code:
Code: Select all
oRemove := List<string>{}
foreach oItem as KeyValuePair<string,int> in oDict
if oItem:Value == nValue
oRemove:Add( oItem:Key )
endif
next
foreach cKey as string in oRemove
oDict:Remove( cKey )
next
Wolfgang
reverse foreach
Posted: Fri Dec 16, 2016 2:02 pm
by Phil Hepburn
Hi Wolfgang,
I thought it was "forward and read only" - the enumerator !
But then what do I know.
Removal will have to be by object and not index value - for obvious reasons.
However, since you have collection(s) then why not use LINQ to create the new list to be used, and work from there !?
my 2 cents worth of the top of my head - yes, having a better day at this end, will post soon about my recent stupidity with EF6 ;-0)
Regards,
Phil.
reverse foreach
Posted: Fri Dec 16, 2016 2:12 pm
by wriedmann
Hi Phil,
I need that in my ServiceContainer class: when a tab/window is closed, it should remove all registered services from the ServiceContainer object. Therefore I need to search for value and not for key.
And using LinQ for such a thing seems to be a little bit too much....
Wolfgang
reverse foreach
Posted: Fri Dec 16, 2016 2:12 pm
by NickFriend
Hi Wolfgang,
LINQ could make it neater, something like (I started to put in X# syntax then realised I'd almost certainly get it wrong)....
Code: Select all
oDict = oDict.Where(d => d.Value != nValue).ToDictionary();
Nick
reverse foreach
Posted: Fri Dec 16, 2016 2:26 pm
by wriedmann
Hi Nick,
it is not an overkill when I rebuild a complete dictionary with a lot of entries only to remove some of them?
Wolfgang
reverse foreach
Posted: Fri Dec 16, 2016 2:36 pm
by NickFriend
Hi Wolfgang,
How big is this dictionary?.... unless it's got tens of thousands of entries I can't imagine it having the slightest impact on performance. I would expect the performance of this to be much better than looping through mulitple lists/dictionaries. Also you eliminate the Remove calls, which are probably the slowest part (I haven't got any evidence for that statement!).
I use this sort of thing all over the place without any issues at all, even in class properties to feed data into grid cells.
Nick
reverse foreach
Posted: Fri Dec 16, 2016 2:40 pm
by wriedmann
Hi Nick,
I have absolutely no idea how big this dictionary can be.
It is the central service locator in my application....
Wolfgang
reverse foreach
Posted: Fri Dec 16, 2016 3:45 pm
by robert
Wolfgang,
An answer from StackExchange:
Code: Select all
var itemsToRemove = myDic.Where(f => f.Value == 42).ToArray();
foreach (var item in itemsToRemove)
myDic.Remove(item.Key);
or
Code: Select all
foreach (var item in myDic.ToList())
{
if (item.value == 42)
myDic.remove(item.key);
}
Robert
reverse foreach
Posted: Fri Dec 16, 2016 3:49 pm
by wriedmann
Hi Robert,
thank you. I like the second better. And I don't know what LinQ creates under the hood, so maybe the second one is faster too.
Wolfgang
reverse foreach
Posted: Fri Dec 16, 2016 4:00 pm
by Phil Hepburn
Hi Wolfgang and all,
Contrary to what 'common myth' has to say (and the guys down the 'pub'), LINQ is just a standard part of current .NET coding.
Nothing special is required, and the namespaces are already included by default in the standard templates for Projects.
Nick and Robert have pointed you at some LINQ based solutions to your problem, some more eloquent than others (even if they do contain Lambda clauses). Robert's first one looks a bit neater and nicer, and is almost what Nick suggested.
We should be reaching for LINQ for all collection based processing - that's what it was designed for - COLLECTIONS.
Miss it out, and you are missing a BIG 'trick'.
RULE '1' - change and adapt !
HTH,
Phil.