Szathmáry László honlapja @ DEIK

string slicing

Warning! C#'s list.GetRange() differs from Python's list slicing!

csharp> var li = new List<char> {'a', 'b', 'c', 'd', 'e', 'f', 'g'}

csharp> li.GetRange(0, 2)
{ 'a', 'b' } // Seems good, right?

csharp> li.GetRange(2, 4)
{ 'c', 'd', 'e', 'f' } // Ooops! What da hell?

list.GetRange() has 2 parameters: start position and count. Thus list.GetRange(2, 4) means to start at position 2 and take 4 characters altogether.

To simulate Python's list slicing, I have this solution:

// slice support for lists (like Python)
public static List<T> Slice<T>(this List<T> li, int start, int end)
{
if (start < 0) // support negative indexing
{
start = li.Count + start;
}
if (end < 0) // support negative indexing
{
end = li.Count + end;
}
if (start > li.Count) // if the start value is too high
{
start = li.Count;
}
if (end > li.Count) // if the end value is too high
{
end = li.Count;
}
var count = end - start; // calculate count (number of elements)
if (count < 0)
{
count = 0;
}
return li.GetRange(start, count); // return a shallow copy of li of count elements
}

For usage examples, here are some tests:

[Fact]
public void Slice_list()
{
var li1 = new List<char> {'a', 'b', 'c', 'd', 'e', 'f', 'g'};
Assert.Equal(new List<char> {'c', 'd'}, li1.Slice(2, 4));
Assert.Equal(new List<char> {'b', 'c', 'd', 'e', 'f', 'g'}, li1.Slice(1, li1.Count));
Assert.Equal(new List<char> {'a', 'b', 'c'}, li1.Slice(0, 3));
Assert.Equal(li1, li1.Slice(0, 4).Concat(li1.Slice(4, li1.Count)));
Assert.Equal(li1, li1.Slice(0, 100));
Assert.Equal(new List<char>(), li1.Slice(100, 200));

Assert.Equal(new List<char> {'g'}, li1.Slice(-1, li1.Count));
Assert.Equal(new List<char> {'f', 'g'}, li1.Slice(-2, li1.Count));
Assert.Equal(new List<char> {'a', 'b', 'c', 'd', 'e', 'f'}, li1.Slice(0, -1));

Assert.Equal(new List<char> {'c', 'd', 'e'}, li1.Slice(2, -2));
}

See the current version here.

20180405e

string slicing