Lets say my ArrayList got 20 elements. Well, sometime in runtime I will want it to only have 10 elements, so the other 10 indexes(from 9 to 19) don't exist at all. How do I do that?
Why are you using non-generic collections at all? Use List<T> instead. And you can use the Remove() method on it.
Since your working with ArrayLists, and not Arrays, just use ArrayList.Remove() for a single part, and ArrayList.RemoveRange(x, list.Count-x) for a range.
If you're looking to avoid using memory space for the elements that are not present, you can try with the TrimToSize method:
http://msdn.microsoft.com/en-us/library/system.collections.arraylist.trimtosize.aspx
It will minimize the array overhead, by limiting the capacity to the current items.
If they already exist, you'll need to remove them and then, call this method.
I'm not sure what you're looking for exactly, so more details would be great.
If you just want to remove the elements without changing the capacity, you can use ArrayList.RemoveRange:
http://msdn.microsoft.com/en-us/library/system.collections.arraylist.removerange.aspx
arrayList.RemoveRange(10, list.Count-10)
This method can be used with any other valid range, if you e.g. want to delete the first ten elements instead.
Related
I'm trying to clear an array after each iteration of a for loop in LabVIEW, but the way I've implemented it has the values not going directly to what I want, but it changes with previous values in other parts of the array.
It isn't shown, but this code is inside of a for-loop that iterates through another numeric array.
I know that if I get the array to clear properly after each loop iteration, this should work. How do I do that? I'm a beginner at Labview but have been coding for awhile - help is appreciated!!!
[![labview add to array][2]][2]
It looks as if you're not quite used to how LabVIEW passes data around yet. There's no need to use lots of value property nodes for the same control or indicator within one structure; if you want to use the same data in more than one place, just branch the wire. Perhaps you're thinking that a LabVIEW control or indicator is equivalent to a variable in text languages, and you need to use a property node to get or set it. Instead, think of the wire as the variable. If you want to pass the output of one operation to the input of another, just wire the output to the input.
The indicators with terminals inside your loop will be updated with new values every loop iteration, and the code inside the loop should execute faster than a human can read those values, so once the loop has finished all the outputs except the final values will be lost. Is that what you intended, or do you want to accumulate or store them in some way?
I can see that in each loop iteration you're reading two values from a config file, and the section is specified by the string value of one element of the numeric array Array. You're displaying the two values in the indicators PICKERING and SUBUNIT. If you can describe in words (or pseudocode, or a text language you're used to) what manipulation of data you're actually trying to do in the rest of this code, we may be able to make more specific suggestions.
First of all, I'm assuming that the desired order of operations is the following:
Putting the value of Pickering into Array 2
Extracting from Array 2 the values to put in Pickering 1 and Pickering 2
Putting Array 2 back to its original value
If this is the case, with your current code you can't be sure that operation 1 will be executed be fore operation 2. In fact, the order of these operations can't be pre-determined. You must force the dataflow, for example by creating a sequence structure. You will put the code related to 1 in the first frame, then code related to operation 2 in the second.
Then, to put Array 2 back to it's original value I would add a third frame, where you force an empty array into the Value property node of Array 2 (the tool you use for pickering, but as input and not as output).
The sequence structure has to be inside the for loop.
I have never used the property node Reinit to default, so I can't help you with that.
Unfortunately I can't run Labview on this PC but I hope my explanation was clear enough, if not tell me and I will try to be more specific.
I would like to know what is preferred...
Dim sLines() As String = s.Split(NewLine)
For each:
For Each sLines_item As String In sLines
.GetUpperBound:
For i As Integer = 0 To sLines.GetUpperBound(0)
I have no idea why the "For Each" was introduced for such cases. Until now I have only used .GetUpperBound, and I don't see any PRO for the "For Each".
Thank you
ps: When I use ."GetUpperBound(0)", I do know that I am iterating over the vector.
The "For Each" in contrast sounds like "I don't care in which order the vector is given to me". But that is just personal gusto, I guess.
Short answer: Do not use GetUpperBound(). The only advantage of GetUpperBound() is that it works for multi-dimensional arrays, where Length doesn't work. However, even that usage is outdated since there is Array.GetLength() available that takes the dimension parameter. For all other uses, For i = 0 to Array.Length - 1 is better and probably the fastest option.
It's largely a personal preference.
If you need to alter the elements of the array, you should use For i ... because changing sLines_item will not affect the corresponding array element.
If you need to delete elements of the array, you can iterate For i = ubound(sLines) to 0 step -1 (or the equivalent).
Short answer
You should always use For Each on IEnumerable types unless you have no other choice.
Long answer
Contrary to the popular understanding, For Each is not a syntactic sugar on top of For Next. It will not necessarily iterate over every element of its source. It is a syntactic sugar on top of IEnumerable.GetEnumerator(). For Each will first get an enumerator to its source then loop until it cannot enumerate further. Basically, it will be replaced by the following code. Keep in mind that this is an oversimplification.
' Ask the source for a way to enumerate its content in a forward only manner.
Dim enumerator As IEnumerator = sLines.GetEnumerator()
' Loop until there is no more element in front of us.
While enumerator.Next() Then
' Invoke back the content of the for each block by passing
' the currently enumerated element.
forEachContent.Invoke(enumerator.Current)
End While
The major difference between this and a classical For Next loop is that it does not depend on any length. This fixes two limitations in modern .NET languages. The first one has to do with the Count method. IEnumerable provides a Count method, but the implementation might not be able to keep track of the actual amount of elements it stores. Because of this, calling IEnumerable.Count might cause the source to be iterated over to actually count the amount of element it contains. Moreover, doing this as the end value for traditional For Next loop will cause this process to be done for every element in the loop. This is very slow. Here is an illustration of this process:
For i As Integer = 0 To source.Count() ' This here will cause Count to be
' evaluated for every element in source.
DoSomething(source(i))
Next
The use of For Each fixes this by never requesting the length of the source.
The second limitation it fixes is the lack of a concept for arrays with infinite amount of elements. An example of such cases would be an array containing every digit of PI where each digit is only calculated when you request them. This is where LINQ makes its entrance and really shines because it enables you to write the following code:
Dim piWith10DigitPrecision = From d In InfinitePiSource
Take 10
Dim piWith250DigitPrecision = From d In InfinitePiSource
Take 250
Dim infite2PiSource = From d In InfinitePiSource
Select d * 2
Now, in an infinite source, you cannot depend on a length to iterate over all of its elements. It has an infinite length thus making a traditional For Next loop an infinite loop. This does not change anything for the first two examples I have given with pi because we explicitly provides the amount of elements we want, but it does for the third one. When would you stop iterating? For Each, when combined with Yield (used by the Take operator), makes sure that you never iterate until you actually requests a specific value.
You might have already figured it out by now but these two things means that For Each effectively have no concept of bounds because it simply does not require them. The only use for GetLowerBound and GetUpperBound are for non-zero-indexed arrays. For instance, you might have an array that indexes values from 1 instead of zero. Even then, you only need GetLowerBound and Length. Obviously, this is only if the position of the element in the source actually matters. If it does not, you can still use For Each to iterate over all elements as it is bound agnostic.
Also, as already mentioned, GetLength should be used for zero-indexed multi-dimensional arrays, again, only if the position of the element matters and not just the element itself.
the idea is to take a word and sub out all specified letters for another letter.
Any help on how to make this kind of function work?
The last array_push is not being called because you're returning before. Change it to:
array_push($stepsinchain, $subed);
return $subed;
Since $subed is never stored in the $stepsinchain array, due to the return being before, you're not able to access previous alternations.
array_push is also slower and not recommended when entering one element in an array. Instead, use
$stepsinchain[] = $subed;
It is also much faster as documented at http://www.php.net/manual/en/function.array-push.php#Hcom83388
In String Template one can easily get an element of a Java Map within the template.
Is it possible to get the n-th element of an array in a similar way?
According to the String Template Cheat Sheet you can easily get the first or second element:
You can combine operations to say things like first(rest(names)) to get second element.
but it doesn't seem possible to get the n-th element easily. I usually transform my list into a map with list indexes as keys and do something like
map.("25")
Is there some easier/more straightforward way?
Sorry, there is no mechanism to get a[i].
There is no easy way getting n-th element of the list.
In my opinion this indicates that your view and business logic are not separated enough: knowledge of what magic number 25 means is spread in both tiers.
One possible solution might be converting list of values to object which provides meaning to the elements. For example, lets say list of String represents address lines, in which case instead of map.("3") you would write address.street.
I have a model:
class List:
data = ...
previous = models.ForeignKey('List', related_name='r1')
obj = models.ForeignKey('Obj', related_name='nodes')
This is one direction list containing reference to some obj of Obj class. I can reverse relation and get some list's all elements refering to obj by:
obj.nodes
But how Can I get the very last node? Without using raw sql, genering as little SQL queries by django as can.
obj.nodes is a RelatedManager, not a list. As with any manager, you can get the last queried element by
obj.nodes.all().reverse()[0]
This makes sense anyway only if there is any default order defined on the Node's Meta class, because otherwise the semantic of 'reverse' don't make any sense. If you don't have any specified order, set it explicitly:
obj.nodes.order_by('-pk')[0]
len(obj.nodes)-1
should give you the index of the last element (counting from 0) of your list
so something like
obj.nodes[len(obj.nodes)-1]
should give the last element of the list
i'm not sure it's good for your case, just give it a try :)
I see this question is quite old, but in newer versions of Django there are first() and last() methods on querysets now.
Well, you just can use [-1] index and it will return last element from the list. Maybe this question are close to yours:
Getting the last element of a list in Python
for further reading, Django does not support negative indexing and using something like
obj.nodes.all()[-1]
will raise an error.
in newer versions of Django you can use last() function on queryset to get the last item of your list.
obj.nodes.last()
another approach is to use len() function to get the index of last item of a list
obj.nodes[len(obj.nodes)-1]