How to use the slice syntax

[eagle-k]

How can I write code equivalent to the following python code?

tensor = torch.ones(4, 4)
print('First row: ',tensor[0])
print('First column: ', tensor[:, 0])
print('Last column:', tensor[..., -1])
tensor[:,1] = 0
print(tensor)

[NiklasGustafsson]

That depends on whether you're using .NET FX or .NET Core, since we don't have System.Range in the former. Assuming that you're using .NET Core:

var tensor = torch.ones(4, 4);
Console.WriteLine($"First row: {tensor[0].str()}");
Console.WriteLine($"First column: {tensor[.., 0].str()}");
Console.WriteLine($"Last column: {tensor[TensorIndex.Ellipsis, ^1].str()");
tensor[..,1] = 0;
Console.WriteLine(tensor.str());

The str() is necessary, because the default ToString() function does not print all the values, just the metadata. There's an overloaded ToString() that does the right thing, but I chose the str() extension method just for brevity's sake.

See the TorchSharp C# tutorials for more details:

https://github.com/dotnet/TorchSharpExamples/tree/main/tutorials/CSharp

[eagle-k]

I see that I can use TensorIndex.

By the way, there is a typo in the Ellipsis line, so I'll provide a modified version just in case for others.

Console.WriteLine($"Last column: {tensor[torch.TensorIndex.Ellipsis, 1].str()}");

The C# code worked fine on VSCode's Polyglot Notebook.
Thank you for teaching me how to do this.

I also tried to write this in F# as well as the C# version.

open TorchSharp
open type TorchSharp.torch

let tensor = torch.ones(4, 4)
printfn $"First row: {tensor[0].str()}"
printfn $"First column: {tensor[TensorIndex.Colon, TensorIndex.Single(0)].str()}"
printfn $"Last column: {tensor[TensorIndex.Ellipsis, TensorIndex.Single(-1)].str()}"
tensor[TensorIndex.Colon, TensorIndex.Single(1)] <- torch.tensor(0)
printfn $"{tensor.str()}"

The following F# slice syntax did not seem to work well.

tensor[*, 0] // syntax error

[NiklasGustafsson]

The '^1' on the Ellipsis line was intentional. I believe that's how C# would denote a -1 in an index. However, I noticed that there's no conversion from System.Index to TensorIndex, so that will have to be fixed. For now, use shape[n]-1 where n is the dimension you use a negative index in.

I just filed a bug.

[NiklasGustafsson]

That said, I and didn't realize this, but you can just use -1 instead of ^1. So:

Console.WriteLine($"Last column: {tensor[TensorIndex.Ellipsis, -1].str()}");

[NiklasGustafsson]

The System.Index issue has been addressed in 0.99.6

[eagle-k]

It seems that F#'s slice syntax can be managed by implementing GetSlice and SetSlice separately at the user level. However, it would be even more convenient if the library could provide support for it.

type torch.Tensor with
    member t.GetSlice(startIdx: int option, endIdx: int option) =
        match startIdx, endIdx with
        | Some s, Some e -> t[torch.TensorIndex.Slice(s, e)]
        | Some s, None -> t[torch.TensorIndex.Slice(s)]
        | None, Some e -> t[torch.TensorIndex.Slice(System.Nullable(), e)]
        | None, None -> t[torch.TensorIndex.Slice()]

    member t.SetSlice(startIdx: int option, endIdx: int option, v: torch.Tensor) =
        match startIdx, endIdx with
        | Some s, Some e -> t[torch.TensorIndex.Slice(s, e)] <- v
        | Some s, None -> t[torch.TensorIndex.Slice(s)] <- v
        | None, Some e -> t[torch.TensorIndex.Slice(System.Nullable(), e)] <- v
        | None, None -> t[torch.TensorIndex.Slice()] <- v

let t = torch.arange(24).view(4, 3, 2)
t[1..3] <- torch.tensor(1)
printfn $"{t.str()}"