I have a List of strings. Its being generated elsewhere but i will generate it below to help describe this simplified example
var list = new List<string>();
list.Add("Joe");
list.Add("");
list.Add("Bill");
list.Add("Bill");
list.Add("");
list.Add("Scott");
list.Add("Joe");
list.Add("");
list.Add("");
list = TrimList(list);
I would like a function that "trims" this list and by trim I want to remove all items at the end of the array that are blank strings (the final two in this case).
NOTE: I still want to keep the blank one that is the second item in the array (or any other one that is just not at the end) so I can't do a .Where(r=> String.isNullOrEmpty(r))
I would just write it without any LINQ, to be honest- after all, you're modifying a collection rather than just querying it:
void TrimList(List<string> list)
{
int lastNonEmpty = list.FindLastIndex(x => !string.IsNullOrEmpty(x));
int firstToRemove = lastNonEmpty + 1;
list.RemoveRange(firstToRemove, list.Count - firstToRemove);
}
If you actually want to create a new list, then the LINQ-based solutions are okay... although potentially somewhat inefficient (as Reverse has to buffer everything).
Take advantage of Reverse and SkipWhile.
list = list.Reverse().SkipWhile(s => String.IsNullOrEmpty(s)).Reverse().ToList();
List<T> (not the interface) has a FindLastIndex method. Therefore you can wrap that in a method:
static IList<string> TrimList(List<string> input) {
return input.Take(input.FindLastIndex(x => !string.IsNullOrEmpty(x)) + 1)
.ToList();
}
This produces a copy, whereas Jon's modifies the list.
The only solution I can think of is to code a loop that starts at the end of the list and searches for an element that is not an empty string. Don't know of any library functions that would help. Once you know the last good element, you know which ones to remove.
Be careful not to modify the collection while you are iterating over it. Tends to break the iterator.
I always like to come up with the most generic solution possible. Why restrict yourself with lists and strings? Let's make an algorithm for generic enumerable!
public static class EnumerableExtensions
{
public static IEnumerable<T> TrimEnd<T>(this IEnumerable<T> enumerable, Predicate<T> predicate)
{
if (predicate == null)
{
throw new ArgumentNullException("predicate");
}
var accumulator = new LinkedList<T>();
foreach (var item in enumerable)
{
if (predicate(item))
{
accumulator.AddLast(item);
}
else
{
foreach (var accumulated in accumulator)
{
yield return accumulated;
}
accumulator.Clear();
yield return item;
}
}
}
}
Use it like this:
var list = new[]
{
"Joe",
"",
"Bill",
"Bill",
"",
"Scott",
"Joe",
"",
""
};
foreach (var item in list.TrimEnd(string.IsNullOrEmpty))
{
Console.WriteLine(item);
}
Related
This may be considered bad programming, but prior to .net 4, I used to heavily use code similar to this:
enemyList.ForEach(delegate(Enemy e)
{
e.Update();
if (someCondition)
enemyList.Remove(e);
});
Now, I'm going through an updating some old projects, and there are a LOT of code thats going to have to be changed since ForEach was removed.. Now, I do have an extension to allow me to use the ForEach :
public static void ForEach<T>(this IEnumerable<T> sequence, Action<T> action)
{
if (sequence == null) throw new ArgumentNullException("sequence");
if (action == null) throw new ArgumentNullException("action");
foreach (T item in sequence)
action(item);
}
I know I can do this:
var index = 0;
while(index < enemyList.Count)
{
if(condition)
enemyList.RemoveAt(index);
else
index++;
}
But some of those would be a pain to rewrite like that.. Is there any way to add that functionality back so that I can iterate through that list, remove the items I need without having to go back and rewrite and edit all of those functions ? I still consider myself a newbie to coding, and I just can't figure this one out.. Any help would be appreciated!
********* EDIT *********
So I guess it boils down to rewriting a lot of code.. I have a lot of code such as this that I just pulled out of a project:
GameplayScreen.gameWorld.shipList.ForEach(delegate(Ship s)
{
if (eS.originalShipsID == s.shipID)
{
if (!eS.Alive || eS.health <= 0)
{
// this one sunk...
string log = "0" + s.shipName + " was sunk in battle.. All crew and cargo were lost.";
AddLogEntry(log);
totalCrewLost += s.currentCrew;
GameplayScreen.gameWorld.shipList.Remove(s);
}
}
});
I was just hoping there was a way to not have to rewrite all of that.. So time to update and change the way I code apparently. Thanks!
Use the list's RemoveAll method.
You can refactor the code to:
enemyList.RemoveAll(enemy => enemy.SomeCondition);
Not only is it better than the while loop, I'd argue it's quite a bit better than the Foreach method.
You can't. The only way would be to add the items to remove to another list and then iterate over that list and remove them after the initial iteration.
A better option would be to use a reverse for loop to iterate over the values. You can then safely remove the items during the initial iteration:
for (var i = enemyList.Count() - 1; i >= 0; i--) {
{
if(condition) enemyList.RemoveAt(i);
}
Since you said you do it a lot, why not do something like this:
public static void RemoveIfTrue<T>(this ICollection<T> list, Func<T, bool> condition)
{
List<T> itemsToRemove = list.Where(condition).ToList();
foreach (var item in itemsToRemove)
{
list.Remove(item);
}
}
Then you could use it like:
myList.RemoveIfTrue(x => YourConditionIsTrue)
That way you don't have a bunch of duplication of logic.
If you're using a List<T>, you can use List<T>.RemoveAll(Predicate<T> match)
So there is a built-in thing to do this already.
Even better - the built-in one knows exactly how to avoid problems modifying the collection while iterating over it. And because it has access to the private internals, it's more efficient too.
So, just using the List class itself you can write code like this:
enemies.RemoveAll(enemy => (enemy.Health <= 0));
This is possible with a small tweak. Here's an example:
public static class Extensions
{
public static void ForEach<T>(this IList<T> list, Action<T> action)
{
for (int i = 0; i < list.Count; i++)
{
action(list[i]);
}
}
}
class Program
{
static void Main(string[] args)
{
List<string> vals = new List<string>(new string[] { "a", "bc", "de", "f", "gh", "i", "jk" });
vals.ToList().ForEach<string>(delegate(string value)
{
if (value.Length > 1)
{
vals.Remove(value);
}
});
vals.ToList().ForEach<string>(delegate(string value)
{
Console.WriteLine(value);
});
Console.ReadKey();
}
}
Now, there are a couple of things worth mentioning here: first, normally elements would be skipped. However, a separate copy of the list is made by calling ToList(). Second, you should be careful to do this only with reference types - i.e. not with primitive types - otherwise you'll remove more than a single element with the remove method.
EDIT
I'd also like to add that probably any of the posted alternatives are better - but I thought it was interesting that this could be done; it's less performant but probably quicker to chuck into existing code.
I have an MVC3 C#.Net web app. I have the below string array.
public static string[] HeaderNamesWbs = new[]
{
WBS_NUMBER,
BOE_TITLE,
SOW_DESCRIPTION,
HARRIS_WIN_THEME,
COST_BOGEY
};
I want to find the Index of a given entry when in another loop. I thought the list would have an IndexOf. I can't find it. Any ideas?
Well you can use Array.IndexOf:
int index = Array.IndexOf(HeaderNamesWbs, someValue);
Or just declare HeaderNamesWbs as an IList<string> instead - which can still be an array if you want:
public static IList<string> HeaderNamesWbs = new[] { ... };
Note that I'd discourage you from exposing an array as public static, even public static readonly. You should consider ReadOnlyCollection:
public static readonly ReadOnlyCollection<string> HeaderNamesWbs =
new List<string> { ... }.AsReadOnly();
If you ever want this for IEnumerable<T>, you could use:
var indexOf = collection.Select((value, index) => new { value, index })
.Where(pair => pair.value == targetValue)
.Select(pair => pair.index + 1)
.FirstOrDefault() - 1;
(The +1 and -1 are so that it will return -1 for "missing" rather than 0.)
I'm late to the thread here. But I wanted to share my solution to this. Jon's is awesome, but I prefer simple lambdas for everything.
You can extend LINQ itself to get what you want. It's fairly simple to do. This will allow you to use syntax like:
// Gets the index of the customer with the Id of 16.
var index = Customers.IndexOf(cust => cust.Id == 16);
This is likely not part of LINQ by default because it requires enumeration. It's not just another deferred selector/predicate.
Also, please note that this returns the first index only. If you want indexes (plural), you should return an IEnumerable<int> and yield return index inside the method. And of course don't return -1. That would be useful where you are not filtering by a primary key.
public static int IndexOf<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) {
var index = 0;
foreach (var item in source) {
if (predicate.Invoke(item)) {
return index;
}
index++;
}
return -1;
}
If you want to search List with a function rather than specifying an item value, you can use List.FindIndex(Predicate match).
See https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.findindex?view=netframework-4.8
Right List has IndexOf(), just declare it as ILIst<string> rather than string[]
public static IList<string> HeaderNamesWbs = new List<string>
{
WBS_NUMBER,
BOE_TITLE,
SOW_DESCRIPTION,
HARRIS_WIN_THEME,
COST_BOGEY
};
int index = HeaderNamesWbs.IndexOf(WBS_NUMBER);
MSDN: List(Of T).IndexOf Method (T)
I have an existing collection of objects with two properties of interest. Both properties are of the same type. I want to create a new sequence consisting of the property values. Here's one way (I'm using tuples instead of my custom type for simplicity):
var list = new List<Tuple<string, string>>
{ Tuple.Create("dog", "cat"), Tuple.Create("fish", "frog") };
var result =
list.SelectMany(x => new[] {x.Item1, x.Item2});
foreach (string item in result)
{
Console.WriteLine(item);
}
Results in:
dog
cat
fish
frog
This gives me the results I want, but is there a better way to accomplish this (in particular, without the need to create arrays or collections)?
Edit:
This also works, at the cost of iterating over the collection twice:
var result = list.Select(x => x.Item1).Concat(list.Select(x => x.Item2));
If you want to avoid creating another collection, you could yield the results instead.
void Main()
{
var list = new List<Tuple<string, string>>
{ Tuple.Create("dog", "cat"), Tuple.Create("fish", "frog") };
foreach (var element in GetSingleList(list))
{
Console.WriteLine (element);
}
}
// A reusable extension method would be a better approach.
IEnumerable<T> GetSingleList<T>(IEnumerable<Tuple<T,T>> list) {
foreach (var element in list)
{
yield return element.Item1;
yield return element.Item2;
}
}
I think your approach is fine and I would stick with that. The use of the array nicely gets the job done when using SelectMany, and the final result is an IEnumerable<string>.
There are some alternate approaches, but I think they're more verbose than your approach.
Aggregate approach:
var result = list.Aggregate(new List<string>(), (seed, t) =>
{
seed.Add(t.Item1);
seed.Add(t.Item2);
return seed;
});
result.ForEach(Console.WriteLine);
ForEach approach:
var result = new List<string>();
list.ForEach(t => { result.Add(t.Item1); result.Add(t.Item2); });
result.ForEach(Console.WriteLine);
In both cases a new List<string> is created.
I am having trouble remembering how (but not why) to use IEnumerators in C#. I am used to Java with its wonderful documentation that explains everything to beginners quite nicely. So please, bear with me.
I have tried learning from other answers on these boards to no avail. Rather than ask a generic question that has already been asked before, I have a specific example that would clarify things for me.
Suppose I have a method that needs to be passed an IEnumerable<String> object. All the method needs to do is concatenate the letters roxxors to the end of every String in the iterator. It then will return this new iterator (of course the original IEnumerable object is left as it was).
How would I go about this? The answer here should help many with basic questions about these objects in addition to me, of course.
Here is the documentation on IEnumerator. They are used to get the values of lists, where the length is not necessarily known ahead of time (even though it could be). The word comes from enumerate, which means "to count off or name one by one".
IEnumerator and IEnumerator<T> is provided by all IEnumerable and IEnumerable<T> interfaces (the latter providing both) in .NET via GetEnumerator(). This is important because the foreach statement is designed to work directly with enumerators through those interface methods.
So for example:
IEnumerator enumerator = enumerable.GetEnumerator();
while (enumerator.MoveNext())
{
object item = enumerator.Current;
// Perform logic on the item
}
Becomes:
foreach(object item in enumerable)
{
// Perform logic on the item
}
As to your specific scenario, almost all collections in .NET implement IEnumerable. Because of that, you can do the following:
public IEnumerator Enumerate(IEnumerable enumerable)
{
// List implements IEnumerable, but could be any collection.
List<string> list = new List<string>();
foreach(string value in enumerable)
{
list.Add(value + "roxxors");
}
return list.GetEnumerator();
}
public IEnumerable<string> Appender(IEnumerable<string> strings)
{
List<string> myList = new List<string>();
foreach(string str in strings)
{
myList.Add(str + "roxxors");
}
return myList;
}
or
public IEnumerable<string> Appender(IEnumerable<string> strings)
{
foreach(string str in strings)
{
yield return str + "roxxors";
}
}
using the yield construct, or simply
var newCollection = strings.Select(str => str + "roxxors"); //(*)
or
var newCollection = from str in strings select str + "roxxors"; //(**)
where the two latter use LINQ and (**) is just syntactic sugar for (*).
If i understand you correctly then in c# the yield return compiler magic is all you need i think.
e.g.
IEnumerable<string> myMethod(IEnumerable<string> sequence)
{
foreach(string item in sequence)
{
yield return item + "roxxors";
}
}
I'd do something like:
private IEnumerable<string> DoWork(IEnumerable<string> data)
{
List<string> newData = new List<string>();
foreach(string item in data)
{
newData.Add(item + "roxxors");
}
return newData;
}
Simple stuff :)
Also you can use LINQ's Select Method:
var source = new[] { "Line 1", "Line 2" };
var result = source.Select(s => s + " roxxors");
Read more here Enumerable.Select Method
What is the best way to approach removing items from a collection in C#, once the item is known, but not it's index. This is one way to do it, but it seems inelegant at best.
//Remove the existing role assignment for the user.
int cnt = 0;
int assToDelete = 0;
foreach (SPRoleAssignment spAssignment in workspace.RoleAssignments)
{
if (spAssignment.Member.Name == shortName)
{
assToDelete = cnt;
}
cnt++;
}
workspace.RoleAssignments.Remove(assToDelete);
What I would really like to do is find the item to remove by property (in this case, name) without looping through the entire collection and using 2 additional variables.
If RoleAssignments is a List<T> you can use the following code.
workSpace.RoleAssignments.RemoveAll(x =>x.Member.Name == shortName);
If you want to access members of the collection by one of their properties, you might consider using a Dictionary<T> or KeyedCollection<T> instead. This way you don't have to search for the item you're looking for.
Otherwise, you could at least do this:
foreach (SPRoleAssignment spAssignment in workspace.RoleAssignments)
{
if (spAssignment.Member.Name == shortName)
{
workspace.RoleAssignments.Remove(spAssignment);
break;
}
}
#smaclell asked why reverse iteration was more efficient in in a comment to #sambo99.
Sometimes it's more efficient. Consider you have a list of people, and you want to remove or filter all customers with a credit rating < 1000;
We have the following data
"Bob" 999
"Mary" 999
"Ted" 1000
If we were to iterate forward, we'd soon get into trouble
for( int idx = 0; idx < list.Count ; idx++ )
{
if( list[idx].Rating < 1000 )
{
list.RemoveAt(idx); // whoops!
}
}
At idx = 0 we remove Bob, which then shifts all remaining elements left. The next time through the loop idx = 1, but
list[1] is now Ted instead of Mary. We end up skipping Mary by mistake. We could use a while loop, and we could introduce more variables.
Or, we just reverse iterate:
for (int idx = list.Count-1; idx >= 0; idx--)
{
if (list[idx].Rating < 1000)
{
list.RemoveAt(idx);
}
}
All the indexes to the left of the removed item stay the same, so you don't skip any items.
The same principle applies if you're given a list of indexes to remove from an array. In order to keep things straight you need to sort the list and then remove the items from highest index to lowest.
Now you can just use Linq and declare what you're doing in a straightforward manner.
list.RemoveAll(o => o.Rating < 1000);
For this case of removing a single item, it's no more efficient iterating forwards or backwards. You could also use Linq for this.
int removeIndex = list.FindIndex(o => o.Name == "Ted");
if( removeIndex != -1 )
{
list.RemoveAt(removeIndex);
}
If it's an ICollection then you won't have a RemoveAll method. Here's an extension method that will do it:
public static void RemoveAll<T>(this ICollection<T> source,
Func<T, bool> predicate)
{
if (source == null)
throw new ArgumentNullException("source", "source is null.");
if (predicate == null)
throw new ArgumentNullException("predicate", "predicate is null.");
source.Where(predicate).ToList().ForEach(e => source.Remove(e));
}
Based on:
http://phejndorf.wordpress.com/2011/03/09/a-removeall-extension-for-the-collection-class/
For a simple List structure the most efficient way seems to be using the Predicate RemoveAll implementation.
Eg.
workSpace.RoleAssignments.RemoveAll(x =>x.Member.Name == shortName);
The reasons are:
The Predicate/Linq RemoveAll method is implemented in List and has access to the internal array storing the actual data. It will shift the data and resize the internal array.
The RemoveAt method implementation is quite slow, and will copy the entire underlying array of data into a new array. This means reverse iteration is useless for List
If you are stuck implementing this in a the pre c# 3.0 era. You have 2 options.
The easily maintainable option. Copy all the matching items into a new list and and swap the underlying list.
Eg.
List<int> list2 = new List<int>() ;
foreach (int i in GetList())
{
if (!(i % 2 == 0))
{
list2.Add(i);
}
}
list2 = list2;
Or
The tricky slightly faster option, which involves shifting all the data in the list down when it does not match and then resizing the array.
If you are removing stuff really frequently from a list, perhaps another structure like a HashTable (.net 1.1) or a Dictionary (.net 2.0) or a HashSet (.net 3.5) are better suited for this purpose.
What type is the collection? If it's List, you can use the helpful "RemoveAll":
int cnt = workspace.RoleAssignments
.RemoveAll(spa => spa.Member.Name == shortName)
(This works in .NET 2.0. Of course, if you don't have the newer compiler, you'll have to use "delegate (SPRoleAssignment spa) { return spa.Member.Name == shortName; }" instead of the nice lambda syntax.)
Another approach if it's not a List, but still an ICollection:
var toRemove = workspace.RoleAssignments
.FirstOrDefault(spa => spa.Member.Name == shortName)
if (toRemove != null) workspace.RoleAssignments.Remove(toRemove);
This requires the Enumerable extension methods. (You can copy the Mono ones in, if you are stuck on .NET 2.0). If it's some custom collection that cannot take an item, but MUST take an index, some of the other Enumerable methods, such as Select, pass in the integer index for you.
This is my generic solution
public static IEnumerable<T> Remove<T>(this IEnumerable<T> items, Func<T, bool> match)
{
var list = items.ToList();
for (int idx = 0; idx < list.Count(); idx++)
{
if (match(list[idx]))
{
list.RemoveAt(idx);
idx--; // the list is 1 item shorter
}
}
return list.AsEnumerable();
}
It would look much simpler if extension methods support passing by reference !
usage:
var result = string[]{"mike", "john", "ali"}
result = result.Remove(x => x.Username == "mike").ToArray();
Assert.IsTrue(result.Length == 2);
EDIT: ensured that the list looping remains valid even when deleting items by decrementing the index (idx).
Here is a pretty good way to do it
http://support.microsoft.com/kb/555972
System.Collections.ArrayList arr = new System.Collections.ArrayList();
arr.Add("1");
arr.Add("2");
arr.Add("3");
/*This throws an exception
foreach (string s in arr)
{
arr.Remove(s);
}
*/
//where as this works correctly
Console.WriteLine(arr.Count);
foreach (string s in new System.Collections.ArrayList(arr))
{
arr.Remove(s);
}
Console.WriteLine(arr.Count);
Console.ReadKey();
There is another approach you can take depending on how you're using your collection. If you're downloading the assignments one time (e.g., when the app runs), you could translate the collection on the fly into a hashtable where:
shortname => SPRoleAssignment
If you do this, then when you want to remove an item by short name, all you need to do is remove the item from the hashtable by key.
Unfortunately, if you're loading these SPRoleAssignments a lot, that obviously isn't going to be any more cost efficient in terms of time. The suggestions other people made about using Linq would be good if you're using a new version of the .NET Framework, but otherwise, you'll have to stick to the method you're using.
Similar to Dictionary Collection point of view, I have done this.
Dictionary<string, bool> sourceDict = new Dictionary<string, bool>();
sourceDict.Add("Sai", true);
sourceDict.Add("Sri", false);
sourceDict.Add("SaiSri", true);
sourceDict.Add("SaiSriMahi", true);
var itemsToDelete = sourceDict.Where(DictItem => DictItem.Value == false);
foreach (var item in itemsToDelete)
{
sourceDict.Remove(item.Key);
}
Note:
Above code will fail in .Net Client Profile (3.5 and 4.5) also some viewers mentioned it is
Failing for them in .Net4.0 as well not sure which settings are causing the problem.
So replace with below code (.ToList()) for Where statement, to avoid that error. “Collection was modified; enumeration operation may not execute.”
var itemsToDelete = sourceDict.Where(DictItem => DictItem.Value == false).ToList();
Per MSDN From .Net4.5 onwards Client Profile are discontinued. http://msdn.microsoft.com/en-us/library/cc656912(v=vs.110).aspx
Save your items first, than delete them.
var itemsToDelete = Items.Where(x => !!!your condition!!!).ToArray();
for (int i = 0; i < itemsToDelete.Length; ++i)
Items.Remove(itemsToDelete[i]);
You need to override GetHashCode() in your Item class.
The best way to do it is by using linq.
Example class:
public class Product
{
public string Name { get; set; }
public string Price { get; set; }
}
Linq query:
var subCollection = collection1.RemoveAll(w => collection2.Any(q => q.Name == w.Name));
This query will remove all elements from collection1 if Name match any element Name from collection2
Remember to use: using System.Linq;
To do this while looping through the collection and not to get the modifying a collection exception, this is the approach I've taken in the past (note the .ToList() at the end of the original collection, this creates another collection in memory, then you can modify the existing collection)
foreach (SPRoleAssignment spAssignment in workspace.RoleAssignments.ToList())
{
if (spAssignment.Member.Name == shortName)
{
workspace.RoleAssignments.Remove(spAssignment);
}
}
If you have got a List<T>, then List<T>.RemoveAll is your best bet. There can't be anything more efficient. Internally it does the array moving in one shot, not to mention it is O(N).
If all you got is an IList<T> or an ICollection<T> you got roughly these three options:
public static void RemoveAll<T>(this IList<T> ilist, Predicate<T> predicate) // O(N^2)
{
for (var index = ilist.Count - 1; index >= 0; index--)
{
var item = ilist[index];
if (predicate(item))
{
ilist.RemoveAt(index);
}
}
}
or
public static void RemoveAll<T>(this ICollection<T> icollection, Predicate<T> predicate) // O(N)
{
var nonMatchingItems = new List<T>();
// Move all the items that do not match to another collection.
foreach (var item in icollection)
{
if (!predicate(item))
{
nonMatchingItems.Add(item);
}
}
// Clear the collection and then copy back the non-matched items.
icollection.Clear();
foreach (var item in nonMatchingItems)
{
icollection.Add(item);
}
}
or
public static void RemoveAll<T>(this ICollection<T> icollection, Func<T, bool> predicate) // O(N^2)
{
foreach (var item in icollection.Where(predicate).ToList())
{
icollection.Remove(item);
}
}
Go for either 1 or 2.
1 is lighter on memory and faster if you have less deletes to perform (i.e. predicate is false most of the times).
2 is faster if you have more deletes to perform.
3 is the cleanest code but performs poorly IMO. Again all that depends on input data.
For some benchmarking details see https://github.com/dotnet/BenchmarkDotNet/issues/1505
A lot of good responses here; I especially like the lambda expressions...very clean. I was remiss, however, in not specifying the type of Collection. This is a SPRoleAssignmentCollection (from MOSS) that only has Remove(int) and Remove(SPPrincipal), not the handy RemoveAll(). So, I have settled on this, unless there is a better suggestion.
foreach (SPRoleAssignment spAssignment in workspace.RoleAssignments)
{
if (spAssignment.Member.Name != shortName) continue;
workspace.RoleAssignments.Remove((SPPrincipal)spAssignment.Member);
break;
}