When I use resize(int newsize)
in C++ for vector<T>
, it means that the size
of this vector
are set to newsize
and the indexes run in range [0..newsize)
. How to do the same in C# for List<T>
?
Changing the List<T>
property Capacity
only changes the Capacity
but leaves the Count
the same, and furthermore the indexes still are in range [0..Count)
. Help me out, please.
P.S. Imagine I have a vector<T> tmp
with a tmp.size() == 5
I cannot refer to tmp[9]
, but when I then use tmp.resize(10)
I may refer to tmp[9]
. In C# if I have List<T> tmp
with tmp.Count == 5
I cannot refer to tmp[9]
(IndexOutOfRangeException
), but even when I set tmp.Capacity=10
I will not be able to refer to tmp[9]
coz of tmp.Count
is still 5. I want to find some analogy of resize in C#.
C language supports a rich set of built-in operators. An operator is a special symbol that tells the compiler to perform specific mathematical or logical operations.
|= is shorthand for doing an OR operation and assignment. For example,, x |= 3 is equivalent to x = x | 3 . You can also use other operators ( +, -, *, & , etc) in this manner as well.
No, but you can use extension methods to add your own. The following has the same behaviour as std::vector<T>::resize()
, including the same time-complexity. The only difference is that in C++ we can define a default with void resize ( size_type sz, T c = T() )
and the way templates work means that that's fine if we call it without the default for a T
that has no accessible parameterless constructor. In C# we can't do that, so instead we have to create one method with no constraint that matches the non-default-used case, and another with a where new()
constraint that calls into it.
public static class ListExtra { public static void Resize<T>(this List<T> list, int sz, T c) { int cur = list.Count; if(sz < cur) list.RemoveRange(sz, cur - sz); else if(sz > cur) { if(sz > list.Capacity)//this bit is purely an optimisation, to avoid multiple automatic capacity changes. list.Capacity = sz; list.AddRange(Enumerable.Repeat(c, sz - cur)); } } public static void Resize<T>(this List<T> list, int sz) where T : new() { Resize(list, sz, new T()); } }
Now the likes of myList.Resize(23)
or myList.Resize(23, myDefaultValue)
will match what one expects from C++'s vector. I'd note though that sometimes where with C++ you'd have a vector of pointers, in C# you'd have a list of some reference-type. Hence in cases where the C++ T()
produces a null pointer (because it's a pointer), here we're expecting it to call a parameterless constructor. For that reason you might find it closer to the behaviour you're used to to replace the second method with:
public static void Resize<T>(this List<T> list, int sz) { Resize(list, sz, default(T)); }
This has the same effect with value types (call parameterless constructor), but with reference-types, it'll fill with nulls. In which case, we can just rewrite the entire class to:
public static class ListExtra { public static void Resize<T>(this List<T> list, int sz, T c = default(T)) { int cur = list.Count; if(sz < cur) list.RemoveRange(sz, cur - sz); else if(sz > cur) list.AddRange(Enumerable.Repeat(c, sz - cur)); } }
Note that this isn't so much about differences between std::vector<T>
and List<T>
as about the differences in how pointers are used in C++ and C#.
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