`library(torch)`

In this article we describe the indexing operator for torch tensors and how it compares to the R indexing operator for arrays.

Torch’s indexing semantics are closer to numpy’s semantics than R’s. You will find a lot of similarities between this article and the `numpy`

indexing article available here.

Single element indexing for a 1-D tensors works mostly as expected. Like R, it is 1-based. Unlike R though, it accepts negative indices for indexing from the end of the array. (In R, negative indices are used to remove elements.)

```
<- torch_tensor(1:10)
x 1]
x[-1] x[
```

You can also subset matrices and higher dimensions arrays using the same syntax:

```
<- x$reshape(shape = c(2,5))
x
x1,3]
x[1,-1] x[
```

Note that if one indexes a multidimensional tensor with fewer indices than dimensions, one gets an error, unlike in R that would flatten the array. For example:

`1] x[`

It is possible to slice and stride arrays to extract sub-arrays of the same number of dimensions, but of different sizes than the original. This is best illustrated by a few examples:

```
<- torch_tensor(1:10)
x
x2:5]
x[1:(-7)] x[
```

You can also use the `1:10:2`

syntax which means: In the range from 1 to 10, take every second item. For example:

`1:5:2] x[`

Another special syntax is the `N`

, meaning the size of the specified dimension.

`5:N] x[`

Note: the slicing behavior relies on Non Standard Evaluation. It requires that the expression is passed to the

`[`

not exactly the resulting R vector.

To allow dynamic dynamic indices, you can create a new slice using the `slc`

function. For example:

`1:5:2] x[`

is equivalent to:

`slc(start = 1, end = 5, step = 2)] x[`

Like in R, you can take all elements in a dimension by leaving an index empty.

Consider a matrix:

```
<- torch_randn(2, 3)
x x
```

The following syntax will give you the first row:

`1,] x[`

And this would give you the first 2 columns:

`1:2] x[,`

By default, when indexing by a single integer, this dimension will be dropped to avoid the singleton dimension:

```
<- torch_randn(2, 3)
x 1,]$shape x[
```

You can optionally use the `drop = FALSE`

argument to avoid dropping the dimension.

`1,,drop = FALSE]$shape x[`

It’s possible to add a new dimension to a tensor using index-like syntax:

```
<- torch_tensor(c(10))
x $shape
x$shape
x[, newaxis]$shape x[, newaxis, newaxis]
```

You can also use `NULL`

instead of `newaxis`

:

`NULL]$shape x[,`

Sometimes we don’t know how many dimensions a tensor has, but we do know what to do with the last available dimension, or the first one. To subsume all others, we can use `..`

:

```
<- torch_tensor(1:125)$reshape(c(5,5,5))
z 1,..]
z[1] z[..,
```

Vector indexing is also supported but care must be taken regarding performance as, in general its much less performant than slice based indexing.

Note: Starting from version 0.5.0, vector indexing in torch follows R semantics, prior to that the behavior was similar to numpy’s advanced indexing. To use the old behavior, consider using

`?torch_index`

,`?torch_index_put`

or`torch_index_put_`

.

```
<- torch_randn(4,4)
x c(1,3), c(1,3)] x[
```

You can also use boolean vectors, for example:

`c(TRUE, FALSE, TRUE, FALSE), c(TRUE, FALSE, TRUE, FALSE)] x[`

The above examples also work if the index were long or boolean tensors, instead of R vectors. It’s also possible to index with multi-dimensional boolean tensors:

```
<- torch_tensor(rbind(
x c(1,2,3),
c(4,5,6)
))>3] x[x
```