# TopK¶

Versioned name : TopK-1

Category : Sorting and maximization

Short description : TopK computes indices and values of the k maximum/minimum values for each slice along specified axis.

Attributes

• axis

• Description : Specifies the axis along which the values are retrieved.

• Range of values : An integer. Negative value means counting dimension from the end.

• Type : int

• Required : yes

• mode

• Description : Specifies which operation is used to select the biggest element of two.

• Range of values : min, max

• Type : string

• Required : yes

• sort

• Description : Specifies order of output elements and/or indices.

• Range of values : value, index, none

• Type : string

• Required : yes

• index_element_type

• Description : the type of output tensor with indices

• Range of values : “i64” or “i32”

• Type : string

• Default value : “i32”

• Required : no

Inputs :

• 1 : Arbitrary tensor. Required.

• 2 : k scalar specifies how many maximum/minimum elements should be computed

Outputs :

• 1 : Output tensor with top k values from the input tensor along specified dimension axis. The shape of the tensor is [input1.shape[0], ..., input1.shape[axis-1], k, input1.shape[axis+1], ...].

• 2 : Output tensor with top k indices for each slice along axis dimension. It is 1D tensor of shape [k]. The shape of the tensor is the same as for the 1st output, that is [input1.shape[0], ..., input1.shape[axis-1], k, input1.shape[axis+1], ...]

Detailed Description

Output tensor is populated by values computes in the following way:

output[i1, ..., i(axis-1), j, i(axis+1) ..., iN] = top_k(input[i1, ...., i(axis-1), :, i(axis+1), ..., iN]), k, sort, mode)


So for each slice input[i1, ...., i(axis-1), :, i(axis+1), ..., iN] which represents 1D array, top_k value is computed individually.

Sorting and minimum/maximum are controlled by sort and mode attributes:

• mode = max, sort = value - descending by value

• mode = max, sort = index - ascending by index

• mode = max, sort = none - undefined

• mode = min, sort = value - ascending by value

• mode = min, sort = index - ascending by index

• mode = min, sort = none - undefined

If there are several elements with the same value then their output order is not determined.

Example

<layer ... type="TopK" ... >
<data axis="1" mode="max" sort="value"/>
<input>
<port id="0">
<dim>6</dim>
<dim>12</dim>
<dim>10</dim>
<dim>24</dim>
</port>
<port id="1">
<!-- k = 3 -->
</port>
<output>
<port id="2">
<dim>6</dim>
<dim>3</dim>
<dim>10</dim>
<dim>24</dim>
</port>
</output>
</layer>