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

The output tensor is populated by values computed 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>