Set/extend List<T> length in c#


Set/extend List<T> length in c#



Given a List<T> in c# is there a way to extend it (within its capacity) and set the new elements to null? I'd like something that works like a memset. I'm not looking for sugar here, I want fast code. I known that in C the operation could be done in something like 1-3 asm ops per entry.

The best solution I've found is this:

list.AddRange(Enumerable.Repeat(null, count-list.Count)); 

however that is c# 3.0 (<3.0 is preferred) and might be generating and evaluating an enumerator.

My current code uses:

while(list.Count < lim) list.Add(null); 

so that's the starting point for time cost.

The motivation for this is that I need to set the n'th element even if it is after the old Count.




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The simplest way is probably by creating a temporary array:.
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list.AddRange(new T[size - count]); 
Where size is the required new size, and count is the count of items in the list.


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However, for relatively large values of size - count, this can have bad performance, since it can cause the list to reallocate multiple times.(*) It also has the disadvantage of allocating an additional temporary array, which, depending on your requirements, may not be acceptable.


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You could mitigate both issues at the expense of more explicit code, by using the following methods:.
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public static class CollectionsUtil {     public static List<T> EnsureSize<T>(this List<T> list, int size)     {         return EnsureSize(list, size, default(T));     }      public static List<T> EnsureSize<T>(this List<T> list, int size, T value)     {         if (list == null) throw new ArgumentNullException("list");         if (size < 0) throw new ArgumentOutOfRangeException("size");          int count = list.Count;         if (count < size)         {             int capacity = list.Capacity;             if (capacity < size)                 list.Capacity = Math.Max(size, capacity * 2);              while (count < size)             {                 list.Add(value);                 ++count;             }         }          return list;     } } 
The only C# 3.0 here is the use of the "this" modifier to make them extension methods.


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Remove the modifier and it will work in C# 2.0..
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Unfortunately, I never compared the performance of the two versions, so I don't know which one is better.. Oh, and did you know you could resize an array by calling Array.Resize<T>? I didn't know that.

:). Update:
(*) Using list.AddRange(array) will not cause an enumerator to be used.

Looking further through Reflector showed that the array will be casted to ICollection<T>, and the Count property will be used so that allocation is done only once..


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static IEnumerable<T> GetValues<T>(T value, int count) {    for (int i = 0; i < count; ++i)       yield return value; }  list.AddRange(GetValues<object>(null, number_of_nulls_to_add)); 
This will work with 2.0+.


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Why do you want to do that ? The main advantage of a List is that it can grow as needed, so why do you want to add a number of null or default elements to it ?. Isn't it better that you use an array in this case ?.



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