# Embeddings

TensorFlow provides library support for looking up values in embedding tensors.

### tf.nn.embedding_lookup(params, ids, partition_strategy='mod', name=None, validate_indices=True)

Looks up ids in a list of embedding tensors.

This function is used to perform parallel lookups on the list of tensors in params. It is a generalization of tf.gather(), where params is interpreted as a partition of a larger embedding tensor.

If len(params) > 1, each element id of ids is partitioned between the elements of params according to the partition_strategy. In all strategies, if the id space does not evenly divide the number of partitions, each of the first (max_id + 1) % len(params) partitions will be assigned one more id.

If partition_strategy is "mod", we assign each id to partition p = id % len(params). For instance, 13 ids are split across 5 partitions as: [[0, 5, 10], [1, 6, 11], [2, 7, 12], [3, 8], [4, 9]]

If partition_strategy is "div", we assign ids to partitions in a contiguous manner. In this case, 13 ids are split across 5 partitions as: [[0, 1, 2], [3, 4, 5], [6, 7, 8], [9, 10], [11, 12]]

The results of the lookup are concatenated into a dense tensor. The returned tensor has shape shape(ids) + shape(params)[1:].

##### Args:
• params: A list of tensors with the same type and which can be concatenated along dimension 0. Each Tensor must be appropriately sized for the given partition_strategy.
• ids: A Tensor with type int32 or int64 containing the ids to be looked up in params.
• partition_strategy: A string specifying the partitioning strategy, relevant if len(params) > 1. Currently "div" and "mod" are supported. Default is "mod".
• name: A name for the operation (optional).
• validate_indices: Whether or not to validate gather indices.
##### Returns:

A Tensor with the same type as the tensors in params.

##### Raises:
• ValueError: If params is empty.

### tf.nn.embedding_lookup_sparse(params, sp_ids, sp_weights, partition_strategy='mod', name=None, combiner=None)

Computes embeddings for the given ids and weights.

This op assumes that there is at least one id for each row in the dense tensor represented by sp_ids (i.e. there are no rows with empty features), and that all the indices of sp_ids are in canonical row-major order.

It also assumes that all id values lie in the range [0, p0), where p0 is the sum of the size of params along dimension 0.

##### Args:
• params: A single tensor representing the complete embedding tensor, or a list of P tensors all of same shape except for the first dimension, representing sharded embedding tensors.
• sp_ids: N x M SparseTensor of int64 ids (typically from FeatureValueToId), where N is typically batch size and M is arbitrary.
• sp_weights: either a SparseTensor of float / double weights, or None to indicate all weights should be taken to be 1. If specified, sp_weights must have exactly the same shape and indices as sp_ids.
• partition_strategy: A string specifying the partitioning strategy, relevant if len(params) > 1. Currently "div" and "mod" are supported. Default is "mod". See tf.nn.embedding_lookup for more details.
• name: Optional name for the op.
• combiner: A string specifying the reduction op. Currently "mean", "sqrtn" and "sum" are supported. "sum" computes the weighted sum of the embedding results for each row. "mean" is the weighted sum divided by the total weight. "sqrtn" is the weighted sum divided by the square root of the sum of the squares of the weights.
##### Returns:

A dense tensor representing the combined embeddings for the sparse ids. For each row in the dense tensor represented by sp_ids, the op looks up the embeddings for all ids in that row, multiplies them by the corresponding weight, and combines these embeddings as specified.

In other words, if shape(combined params) = [p0, p1, ..., pm] and shape(sp_ids) = shape(sp_weights) = [d0, d1, ..., dn] then shape(output) = [d0, d1, ..., dn-1, p1, ..., pm].

For instance, if params is a 10x20 matrix, and sp_ids / sp_weights are

[0, 0]: id 1, weight 2.0
[0, 1]: id 3, weight 0.5
[1, 0]: id 0, weight 1.0
[2, 3]: id 1, weight 3.0


with combiner="mean", then the output will be a 3x20 matrix where output[0, :] = (params[1, :] * 2.0 + params[3, :] * 0.5) / (2.0 + 0.5) output[1, :] = params[0, :] * 1.0 output[2, :] = params[1, :] * 3.0

##### Raises:
• TypeError: If sp_ids is not a SparseTensor, or if sp_weights is neither None nor SparseTensor.
• ValueError: If combiner is not one of {"mean", "sqrtn", "sum"}.