Strings in Arkouda

Like NumPy, Arkouda supports arrays of strings, but whereas in NumPy arrays of strings are still ndarray objects, in Arkouda the array of strings is its own class: Strings.

In order to efficiently store strings with a wide range of lengths, Arkouda uses a “segmented array” data structure, comprising:

  • bytes: A uint8 array containing the concatenated bytes of all the strings, separated by null (0) bytes.

  • offsets: A int64 array with the start index of each string

Performance

Because strings are a variable-width data type, and because of the way Arkouda represents strings, operations on strings are considerably slower than operations on numeric data. Use numeric data whenever possible. For example, if your raw data contains string data that could be represented numerically, consider setting up a processing pipeline performs the conversion (and stores the result in HDF5 format) on ingest.

I/O

Arrays of strings can be transferred between the Arkouda client and server using the arkouda.array and Strings.to_ndarray functions (see Data I/O). The former converts a Python list or NumPy ndarray of strings to an Arkouda Strings object, whereas the latter converts an Arkouda Strings object to a NumPy ndarray. As with numeric arrays, if the size of the data exceeds the threshold set by ak.client.maxTransferBytes, the client will raise an exception.

Arkouda currently only supports the HDF5 file format for disk-based I/O. In order to read an array of strings from an HDF5 file, the strings must be stored in an HDF5 group containing two datasets: segments (an integer array corresponding to offsets above) and values (a uint8 array corresponding to bytes above). See Supported File Formats for more information and guidelines.

Iteration

Iterating directly over a Strings with for x in string is not supported to discourage transferring all the Strings object’s data from the arkouda server to the Python client since there is almost always a more array-oriented way to express an iterator-based computation. To force this transfer, use the to_ndarray function to return the Strings as a numpy.ndarray. See I/O for more details about using to_ndarray with Strings

arkouda.Strings.to_ndarray(self)

Convert the array to a np.ndarray, transferring array data from the arkouda server to Python. If the array exceeds a built-in size limit, a RuntimeError is raised.

Returns:

A numpy ndarray with the same strings as this array

Return type:

np.ndarray

Notes

The number of bytes in the array cannot exceed ak.client.maxTransferBytes, otherwise a RuntimeError will be raised. This is to protect the user from overflowing the memory of the system on which the Python client is running, under the assumption that the server is running on a distributed system with much more memory than the client. The user may override this limit by setting ak.client.maxTransferBytes to a larger value, but proceed with caution.

See also

array, to_list

Examples

>>> a = ak.array(["hello", "my", "world"])
>>> a.to_ndarray()
array(['hello', 'my', 'world'], dtype='<U5')
>>> type(a.to_ndarray())
numpy.ndarray

Operations

Arkouda Strings objects support the following operations:

String-Specific Methods

Splitting and joining

Strings.peel(delimiter, times=1, includeDelimiter=False, keepPartial=False, fromRight=False, regex=False)[source]

Peel off one or more delimited fields from each string (similar to string.partition), returning two new arrays of strings. Warning: This function is experimental and not guaranteed to work.

Parameters:

  • delimiter (Union[bytes, str_scalars]) – The separator where the split will occur

  • times (Union[int, np.int64]) – The number of times the delimiter is sought, i.e. skip over the first (times-1) delimiters

  • includeDelimiter (bool) – If true, append the delimiter to the end of the first return array. By default, it is prepended to the beginning of the second return array.

  • keepPartial (bool) – If true, a string that does not contain <times> instances of the delimiter will be returned in the first array. By default, such strings are returned in the second array.

  • fromRight (bool) – If true, peel from the right instead of the left (see also rpeel)

  • regex (bool) – Indicates whether delimiter is a regular expression Note: only handles regular expressions supported by re2 (does not support lookaheads/lookbehinds)

Returns:

left: Strings

The field(s) peeled from the end of each string (unless fromRight is true)

right: Strings

The remainder of each string after peeling (unless fromRight is true)

Return type:

Tuple[Strings, Strings]

Raises:

  • TypeError – Raised if the delimiter parameter is not byte or str_scalars, if times is not int64, or if includeDelimiter, keepPartial, or fromRight is not bool

  • ValueError – Raised if times is < 1 or if delimiter is not a valid regex

  • RuntimeError – Raised if there is a server-side error thrown

See also

rpeel, stick, lstick

Examples

>>> s = ak.array(['a.b', 'c.d', 'e.f.g'])
>>> s.peel('.')
(array(['a', 'c', 'e']), array(['b', 'd', 'f.g']))
>>> s.peel('.', includeDelimiter=True)
(array(['a.', 'c.', 'e.']), array(['b', 'd', 'f.g']))
>>> s.peel('.', times=2)
(array(['', '', 'e.f']), array(['a.b', 'c.d', 'g']))
>>> s.peel('.', times=2, keepPartial=True)
(array(['a.b', 'c.d', 'e.f']), array(['', '', 'g']))
Strings.rpeel(delimiter, times=1, includeDelimiter=False, keepPartial=False, regex=False)[source]

Peel off one or more delimited fields from the end of each string (similar to string.rpartition), returning two new arrays of strings. Warning: This function is experimental and not guaranteed to work.

Parameters:

  • delimiter (Union[bytes, str_scalars]) – The separator where the split will occur

  • times (Union[int, np.int64]) – The number of times the delimiter is sought, i.e. skip over the last (times-1) delimiters

  • includeDelimiter (bool) – If true, prepend the delimiter to the start of the first return array. By default, it is appended to the end of the second return array.

  • keepPartial (bool) – If true, a string that does not contain <times> instances of the delimiter will be returned in the second array. By default, such strings are returned in the first array.

  • regex (bool) – Indicates whether delimiter is a regular expression Note: only handles regular expressions supported by re2 (does not support lookaheads/lookbehinds)

Returns:

left: Strings

The remainder of the string after peeling

right: Strings

The field(s) that were peeled from the right of each string

Return type:

Tuple[Strings, Strings]

Raises:

  • TypeError – Raised if the delimiter parameter is not bytes or str_scalars or if times is not int64

  • ValueError – Raised if times is < 1 or if delimiter is not a valid regex

  • RuntimeError – Raised if there is a server-side error thrown

See also

peel, stick, lstick

Examples

>>> s = ak.array(['a.b', 'c.d', 'e.f.g'])
>>> s.rpeel('.')
(array(['a', 'c', 'e.f']), array(['b', 'd', 'g']))
# Compared against peel
>>> s.peel('.')
(array(['a', 'c', 'e']), array(['b', 'd', 'f.g']))
Strings.stick(other, delimiter='', toLeft=False)[source]

Join the strings from another array onto one end of the strings of this array, optionally inserting a delimiter. Warning: This function is experimental and not guaranteed to work.

Parameters:

  • other (Strings) – The strings to join onto self’s strings

  • delimiter (str) – String inserted between self and other

  • toLeft (bool) – If true, join other strings to the left of self. By default, other is joined to the right of self.

Returns:

The array of joined strings

Return type:

Strings

Raises:

  • TypeError – Raised if the delimiter parameter is not bytes or str_scalars or if the other parameter is not a Strings instance

  • ValueError – Raised if times is < 1

  • RuntimeError – Raised if there is a server-side error thrown

See also

lstick, peel, rpeel

Examples

>>> s = ak.array(['a', 'c', 'e'])
>>> t = ak.array(['b', 'd', 'f'])
>>> s.stick(t, delimiter='.')
array(['a.b', 'c.d', 'e.f'])
Strings.lstick(other, delimiter='')[source]

Join the strings from another array onto the left of the strings of this array, optionally inserting a delimiter. Warning: This function is experimental and not guaranteed to work.

Parameters:

  • other (Strings) – The strings to join onto self’s strings

  • delimiter (Union[bytes,str_scalars]) – String inserted between self and other

Returns:

The array of joined strings, as other + self

Return type:

Strings

Raises:

  • TypeError – Raised if the delimiter parameter is neither bytes nor a str or if the other parameter is not a Strings instance

  • RuntimeError – Raised if there is a server-side error thrown

See also

stick, peel, rpeel

Examples

>>> s = ak.array(['a', 'c', 'e'])
>>> t = ak.array(['b', 'd', 'f'])
>>> s.lstick(t, delimiter='.')
array(['b.a', 'd.c', 'f.e'])

Flattening

Given an array of strings where each string encodes a variable-length sequence delimited by a common substring, flattening offers a method for unpacking the sequences into a flat array of individual elements. A mapping between original strings and new array elements can be preserved, if desired. This method can be used in pipe

Strings.flatten(delimiter, return_segments=False, regex=False)[source]

Unpack delimiter-joined substrings into a flat array.

Parameters:

  • delimiter (str) – Characters used to split strings into substrings

  • return_segments (bool) – If True, also return mapping of original strings to first substring in return array.

  • regex (bool) – Indicates whether delimiter is a regular expression Note: only handles regular expressions supported by re2 (does not support lookaheads/lookbehinds)

Return type:

Union[Strings, Tuple]

Returns:

  • Strings – Flattened substrings with delimiters removed

  • pdarray, int64 (optional) – For each original string, the index of first corresponding substring in the return array

See also

peel, rpeel

Examples

>>> orig = ak.array(['one|two', 'three|four|five', 'six'])
>>> orig.flatten('|')
array(['one', 'two', 'three', 'four', 'five', 'six'])
>>> flat, map = orig.flatten('|', return_segments=True)
>>> map
array([0, 2, 5])
>>> under = ak.array(['one_two', 'three_____four____five', 'six'])
>>> under_flat, under_map = under.flatten('_+', return_segments=True, regex=True)
>>> under_flat
array(['one', 'two', 'three', 'four', 'five', 'six'])
>>> under_map
array([0, 2, 5])

Regular Expressions

Strings implements behavior similar to the re python library applied to every element. This functionality is based on Chapel’s regex module which is built on google’s re2. re2 sacrifices some functionality (notably lookahead/lookbehind) in exchange for guarantees that searches complete in linear time and in a fixed amount of stack space

Strings.search(pattern)[source]

Returns a match object with the first location in each element where pattern produces a match. Elements match if any part of the string matches the regular expression pattern

Parameters:

pattern (str) – Regex used to find matches

Returns:

Match object where elements match if any part of the string matches the regular expression pattern

Return type:

Match

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.search('_+')
<ak.Match object: matched=True, span=(1, 2); matched=True, span=(0, 4);
matched=False; matched=True, span=(0, 2); matched=False>
Strings.match(pattern)[source]

Returns a match object where elements match only if the beginning of the string matches the regular expression pattern

Parameters:

pattern (str) – Regex used to find matches

Returns:

Match object where elements match only if the beginning of the string matches the regular expression pattern

Return type:

Match

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.match('_+')
<ak.Match object: matched=False; matched=True, span=(0, 4); matched=False;
matched=True, span=(0, 2); matched=False>
Strings.fullmatch(pattern)[source]

Returns a match object where elements match only if the whole string matches the regular expression pattern

Parameters:

pattern (str) – Regex used to find matches

Returns:

Match object where elements match only if the whole string matches the regular expression pattern

Return type:

Match

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.fullmatch('_+')
<ak.Match object: matched=False; matched=True, span=(0, 4); matched=False;
matched=False; matched=False>
Strings.split(pattern, maxsplit=0, return_segments=False)[source]

Returns a new Strings split by the occurrences of pattern. If maxsplit is nonzero, at most maxsplit splits occur

Parameters:

  • pattern (str) – Regex used to split strings into substrings

  • maxsplit (int) – The max number of pattern match occurences in each element to split. The default maxsplit=0 splits on all occurences

  • return_segments (bool) – If True, return mapping of original strings to first substring in return array.

Return type:

Union[Strings, Tuple]

Returns:

  • Strings – Substrings with pattern matches removed

  • pdarray, int64 (optional) – For each original string, the index of first corresponding substring in the return array

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.split('_+', maxsplit=2, return_segments=True)
(array(['1', '2', '', '', '', '3', '', '4', '5____6___7', '']), array([0 3 5 6 9]))
Strings.findall(pattern, return_match_origins=False)[source]

Return a new Strings containg all non-overlapping matches of pattern

Parameters:

  • pattern (str_scalars) – Regex used to find matches

  • return_match_origins (bool) – If True, return a pdarray containing the index of the original string each pattern match is from

Return type:

Union[Strings, Tuple]

Returns:

  • Strings – Strings object containing only pattern matches

  • pdarray, int64 (optional) – The index of the original string each pattern match is from

Raises:

  • TypeError – Raised if the pattern parameter is not bytes or str_scalars

  • ValueError – Raised if pattern is not a valid regex

  • RuntimeError – Raised if there is a server-side error thrown

See also

Strings.find_locations

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.findall('_+', return_match_origins=True)
(array(['_', '___', '____', '__', '___', '____', '___']), array([0 0 1 3 3 3 3]))
Strings.sub(pattern, repl, count=0)[source]

Return new Strings obtained by replacing non-overlapping occurrences of pattern with the replacement repl. If count is nonzero, at most count substitutions occur

Parameters:

  • pattern (str_scalars) – The regex to substitue

  • repl (str_scalars) – The substring to replace pattern matches with

  • count (int) – The max number of pattern match occurences in each element to replace. The default count=0 replaces all occurences of pattern with repl

Returns:

Strings with pattern matches replaced

Return type:

Strings

Raises:

  • TypeError – Raised if pattern or repl are not bytes or str_scalars

  • ValueError – Raised if pattern is not a valid regex

  • RuntimeError – Raised if there is a server-side error thrown

See also

Strings.subn

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.sub(pattern='_+', repl='-', count=2)
array(['1-2-', '-', '3', '-4-5____6___7', ''])
Strings.subn(pattern, repl, count=0)[source]

Perform the same operation as sub(), but return a tuple (new_Strings, number_of_substitions)

Parameters:

  • pattern (str_scalars) – The regex to substitue

  • repl (str_scalars) – The substring to replace pattern matches with

  • count (int) – The max number of pattern match occurences in each element to replace. The default count=0 replaces all occurences of pattern with repl

Return type:

Tuple

Returns:

  • Strings – Strings with pattern matches replaced

  • pdarray, int64 – The number of substitutions made for each element of Strings

Raises:

  • TypeError – Raised if pattern or repl are not bytes or str_scalars

  • ValueError – Raised if pattern is not a valid regex

  • RuntimeError – Raised if there is a server-side error thrown

See also

Strings.sub

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.subn(pattern='_+', repl='-', count=2)
(array(['1-2-', '-', '3', '-4-5____6___7', '']), array([2 1 0 2 0]))
Strings.find_locations(pattern)[source]

Finds pattern matches and returns pdarrays containing the number, start postitions, and lengths of matches

Parameters:

pattern (str_scalars) – The regex pattern used to find matches

Return type:

Tuple[pdarray, pdarray, pdarray]

Returns:

  • pdarray, int64 – For each original string, the number of pattern matches

  • pdarray, int64 – The start positons of pattern matches

  • pdarray, int64 – The lengths of pattern matches

Raises:

  • TypeError – Raised if the pattern parameter is not bytes or str_scalars

  • ValueError – Raised if pattern is not a valid regex

  • RuntimeError – Raised if there is a server-side error thrown

See also

Strings.findall, Strings.match

Examples

>>> strings = ak.array([f'{i} string {i}' for i in range(1, 6)])
>>> num_matches, starts, lens = strings.find_locations('\d')
>>> num_matches
array([2, 2, 2, 2, 2])
>>> starts
array([0, 9, 0, 9, 0, 9, 0, 9, 0, 9])
>>> lens
array([1, 1, 1, 1, 1, 1, 1, 1, 1, 1]))

Match Object

search, match, and fullmatch return a Match object which supports the following methods

Match.matched()[source]

Returns a boolean array indiciating whether each element matched

Returns:

True for elements that match, False otherwise

Return type:

pdarray, bool

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.search('_+').matched()
array([True True False True False])
Match.start()[source]

Returns the starts of matches

Returns:

The start positions of matches

Return type:

pdarray, int64

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.search('_+').start()
array([1 0 0])
Match.end()[source]

Returns the ends of matches

Returns:

The end positions of matches

Return type:

pdarray, int64

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.search('_+').end()
array([2 4 2])
Match.match_type()[source]

Returns the type of the Match object

Returns:

MatchType of the Match object

Return type:

str

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.search('_+').match_type()
'SEARCH'
Match.find_matches(return_match_origins=False)[source]

Return all matches as a new Strings object

Parameters:

return_match_origins (bool) – If True, return a pdarray containing the index of the original string each pattern match is from

Returns:

  • Strings – Strings object containing only matches

  • pdarray, int64 (optional) – The index of the original string each pattern match is from

Raises:

RuntimeError – Raised if there is a server-side error thrown

Examples

>>> strings = ak.array(['1_2___', '____', '3', '__4___5____6___7', ''])
>>> strings.search('_+').find_matches(return_match_origins=True)
(array(['_', '____', '__']), array([0 1 3]))
Match.group(group_num=0, return_group_origins=False)[source]

Returns a new Strings containing the capture group corresponding to group_num. For the default, group_num=0, return the full match

Parameters:

  • group_num (int) – The index of the capture group to be returned

  • return_group_origins (bool) – If True, return a pdarray containing the index of the original string each capture group is from

Returns:

  • Strings – Strings object containing only the capture groups corresponding to group_num

  • pdarray, int64 (optional) – The index of the original string each group is from

Examples

>>> strings = ak.array(["Isaac Newton, physics", '<-calculus->', 'Gottfried Leibniz, math'])
>>> m = strings.search("(\w+) (\w+)")
>>> m.group()
array(['Isaac Newton', 'Gottfried Leibniz'])
>>> m.group(1)
array(['Isaac', 'Gottfried'])
>>> m.group(2, return_group_origins=True)
(array(['Newton', 'Leibniz']), array([0 2]))