Strings

Note: Functions taking Tensor arguments can also take anything accepted by tf.convert_to_tensor.

[TOC]

Hashing

String hashing ops take a string input tensor and map each element to an integer.

tf.string_to_hash_bucket_fast(input, num_buckets, name=None)

Converts each string in the input Tensor to its hash mod by a number of buckets.

The hash function is deterministic on the content of the string within the process and will never change. However, it is not suitable for cryptography. This function may be used when CPU time is scarce and inputs are trusted or unimportant. There is a risk of adversaries constructing inputs that all hash to the same bucket. To prevent this problem, use a strong hash function with tf.string_to_hash_bucket_strong.

Args:

• input: A Tensor of type string. The strings to assign a hash bucket.
• num_buckets: An int that is >= 1. The number of buckets.
• name: A name for the operation (optional).

Returns:

A Tensor of type int64. A Tensor of the same shape as the input string_tensor.

tf.string_to_hash_bucket_strong(input, num_buckets, key, name=None)

Converts each string in the input Tensor to its hash mod by a number of buckets.

The hash function is deterministic on the content of the string within the process. The hash function is a keyed hash function, where attribute key defines the key of the hash function. key is an array of 2 elements.

A strong hash is important when inputs may be malicious, e.g. URLs with additional components. Adversaries could try to make their inputs hash to the same bucket for a denial-of-service attack or to skew the results. A strong hash prevents this by making it dificult, if not infeasible, to compute inputs that hash to the same bucket. This comes at a cost of roughly 4x higher compute time than tf.string_to_hash_bucket_fast.

Args:

• input: A Tensor of type string. The strings to assign a hash bucket.
• num_buckets: An int that is >= 1. The number of buckets.
• key: A list of ints. The key for the keyed hash function passed as a list of two uint64 elements.
• name: A name for the operation (optional).

Returns:

A Tensor of type int64. A Tensor of the same shape as the input string_tensor.

tf.string_to_hash_bucket(string_tensor, num_buckets, name=None)

Converts each string in the input Tensor to its hash mod by a number of buckets.

The hash function is deterministic on the content of the string within the process.

Note that the hash function may change from time to time. This functionality will be deprecated and it's recommended to use tf.string_to_hash_bucket_fast() or tf.string_to_hash_bucket_strong().

Args:

• string_tensor: A Tensor of type string.
• num_buckets: An int that is >= 1. The number of buckets.
• name: A name for the operation (optional).

Returns:

A Tensor of type int64. A Tensor of the same shape as the input string_tensor.

Joining

String joining ops concatenate elements of input string tensors to produce a new string tensor.

tf.reduce_join(inputs, reduction_indices, keep_dims=None, separator=None, name=None)

Joins a string Tensor across the given dimensions.

Computes the string join across dimensions in the given string Tensor of shape [d_0, d_1, ..., d_n-1]. Returns a new Tensor created by joining the input strings with the given separator (default: empty string). Negative indices are counted backwards from the end, with -1 being equivalent to n - 1. Passing an empty reduction_indices joins all strings in linear index order and outputs a scalar string.

For example:

# tensor `a` is [["a", "b"], ["c", "d"]]
tf.reduce_join(a, 0) ==> ["ac", "bd"]
tf.reduce_join(a, 1) ==> ["ab", "cd"]
tf.reduce_join(a, -2) = tf.reduce_join(a, 0) ==> ["ac", "bd"]
tf.reduce_join(a, -1) = tf.reduce_join(a, 1) ==> ["ab", "cd"]
tf.reduce_join(a, 0, keep_dims=True) ==> [["ac", "bd"]]
tf.reduce_join(a, 1, keep_dims=True) ==> [["ab"], ["cd"]]
tf.reduce_join(a, 0, separator=".") ==> ["a.c", "b.d"]
tf.reduce_join(a, [0, 1]) ==> ["acbd"]
tf.reduce_join(a, [1, 0]) ==> ["abcd"]
tf.reduce_join(a, []) ==> ["abcd"]

Args:

• inputs: A Tensor of type string. The input to be joined. All reduced indices must have non-zero size.
• reduction_indices: A Tensor of type int32. The dimensions to reduce over. Dimensions are reduced in the order specified. Omitting reduction_indices is equivalent to passing [n-1, n-2, ..., 0]. Negative indices from -n to -1 are supported.
• keep_dims: An optional bool. Defaults to False. If True, retain reduced dimensions with length 1.
• separator: An optional string. Defaults to "". The separator to use when joining.
• name: A name for the operation (optional).

Returns:

A Tensor of type string. Has shape equal to that of the input with reduced dimensions removed or set to 1 depending on keep_dims.

tf.string_join(inputs, separator=None, name=None)

Joins the strings in the given list of string tensors into one tensor;

with the given separator (default is an empty separator).

Args:

• inputs: A list of at least 1 Tensor objects of type string. A list of string tensors. The tensors must all have the same shape, or be scalars. Scalars may be mixed in; these will be broadcast to the shape of non-scalar inputs.
• separator: An optional string. Defaults to "". string, an optional join separator.
• name: A name for the operation (optional).

Returns:

A Tensor of type string.

Splitting

tf.string_split(source, delimiter=' ')

Split elements of source based on delimiter into a SparseTensor.

Let N be the size of source (typically N will be the batch size). Split each element of source based on delimiter and return a SparseTensor containing the splitted tokens. Empty tokens are ignored.

If delimiter is an empty string, each element of the source is split into individual 1 character strings.

For example: N = 2, source[0] is 'hello world' and source[1] is 'a b c', then the output will be

st.indices = [0, 0; 0, 1; 1, 0; 1, 1; 1, 2] st.shape = [2, 3] st.values = ['hello', 'world', 'a', 'b', 'c']

Args:

• source: 1-D string Tensor, the strings to split.
• delimiter: 0-D string Tensor, the delimiter character, the string should be length 0 or 1.

Returns:

A SparseTensor of rank 2, the strings split according to the delimiter. The first column of the indices corresponds to the row in source and the second column corresponds to the index of the split component in this row.

Raises:

• ValueError: If delimiter is not a character.

Conversion

tf.as_string(input, precision=None, scientific=None, shortest=None, width=None, fill=None, name=None)

Converts each entry in the given tensor to strings. Supports many numeric

types and boolean.

Args:

• input: A Tensor. Must be one of the following types: int32, int64, complex64, float32, float64, bool, int8.
• precision: An optional int. Defaults to -1. The post-decimal precision to use for floating point numbers. Only used if precision > -1.
• scientific: An optional bool. Defaults to False. Use scientific notation for floating point numbers.
• shortest: An optional bool. Defaults to False. Use shortest representation (either scientific or standard) for floating point numbers.
• width: An optional int. Defaults to -1. Pad pre-decimal numbers to this width. Applies to both floating point and integer numbers. Only used if width > -1.
• fill: An optional string. Defaults to "". The value to pad if width > -1. If empty, pads with spaces. Another typical value is '0'. String cannot be longer than 1 character.
• name: A name for the operation (optional).

Returns:

A Tensor of type string.

tf.encode_base64(input, pad=None, name=None)

Encode strings into web-safe base64 format.

Refer to the following article for more information on base64 format: en.wikipedia.org/wiki/Base64. Base64 strings may have padding with '=' at the end so that the encoded has length multiple of 4. See Padding section of the link above.

Web-safe means that the encoder uses - and _ instead of + and /.

Args:

• input: A Tensor of type string. Strings to be encoded.
• pad: An optional bool. Defaults to False. Bool whether padding is applied at the ends.
• name: A name for the operation (optional).

Returns:

A Tensor of type string. Input strings encoded in base64.

tf.decode_base64(input, name=None)

Decode web-safe base64-encoded strings.

Input may or may not have padding at the end. See EncodeBase64 for padding. Web-safe means that input must use - and _ instead of + and /.

Args:

• input: A Tensor of type string. Base64 strings to decode.
• name: A name for the operation (optional).

Returns:

A Tensor of type string. Decoded strings.