5.6 array -- Efficient arrays of numeric values
Keywords: lib

# 5.6 `array` -- Efficient arrays of numeric values

This module defines a new object type which can efficiently represent an array of basic values: characters, integers, floating point numbers. Arrays are sequence types and behave very much like lists, except that the type of objects stored in them is constrained. The type is specified at object creation time by using a type code, which is a single character. The following type codes are defined:

Type code | C Type | Minimum size in bytes
--- | --- | ---
`'c'` | character | 1
`'b'` | signed int | 1
`'B'` | unsigned int | 1
`'h'` | signed int | 2
`'H'` | unsigned int | 2
`'i'` | signed int | 2
`'I'` | unsigned int | 2
`'l'` | signed int | 4
`'L'` | unsigned int | 4
`'f'` | float | 4
`'d'` | double | 8

The actual representation of values is determined by the machine architecture (strictly speaking, by the C implementation). The actual size can be accessed through the `itemsize` attribute. The values stored for `'L'` and `'I'` items will be represented as Python long integers when retrieved, because Python's plain integer type cannot represent the full range of C's unsigned (long) integers.

The module defines the following function and type object:

`array` (typecode[, initializer])
  Return a new array whose items are restricted by typecode, and initialized from the optional initializer value, which must be a list or a string. The list or string is passed to the new array's `fromlist()` or `fromstring()` method (see below) to add initial items to the array.

`ArrayType`
  Type object corresponding to the objects returned by `array()`.

Array objects support the following data items and methods:

`typecode`
  The typecode character used to create the array.

`itemsize`
  The length in bytes of one array item in the internal representation.

`append` (x)
  Append a new item with value x to the end of the array.

`buffer_info` ()
  Return a tuple `(address, length)` giving the current memory address and the length in bytes of the buffer used to hold array's contents. This is occasionally useful when working with low-level (and inherently unsafe) I/O interfaces that require memory addresses, such as certain `ioctl()` operations. The returned numbers are valid as long as the array exists and no length-changing operations are applied to it.

`byteswap` ()
  ``Byteswap'' all items of the array. This is only supported for values which are 1, 2, 4, or 8 bytes in size; for other types of values, `RuntimeError` is raised. It is useful when reading data from a file written on a machine with a different byte order.

`count` (x)
  Return the number of occurences of x in the array.

`extend` (a)
  Append array items from a to the end of the array.

`fromfile` (f, n)
  Read n items (as machine values) from the file object f and append them to the end of the array. If less than n items are available, `EOFError` is raised, but the items that were available are still inserted into the array. f must be a real built-in file object; something else with a `read()` method won't do.

`fromlist` (list)
  Append items from the list. This is equivalent to "`for x in list: a.append(x)`"except that if there is a type error, the array is unchanged.

`fromstring` (s)
  Appends items from the string, interpreting the string as an array of machine values (i.e. as if it had been read from a file using the `fromfile()` method).

`index` (x)
  Return the smallest i such that i is the index of the first occurence of x in the array.

`insert` (i, x)
  Insert a new item with value x in the array before position i.

`pop` ([i])
  Removes the item with the index i from the array and returns it. The optional argument defaults to `-1`, so that by default the last item is removed and returned.

`read` (f, n)
  Deprecated since release 1.5.1. Use the `fromfile()` method.

  Read n items (as machine values) from the file object f and append them to the end of the array. If less than n items are available, `EOFError` is raised, but the items that were available are still inserted into the array. f must be a real built-in file object; something else with a `read()` method won't do.

`remove` (x)
  Remove the first occurence of x from the array.

`reverse` ()
  Reverse the order of the items in the array.

`tofile` (f)
  Write all items (as machine values) to the file object f.

`tolist` ()
  Convert the array to an ordinary list with the same items.

`tostring` ()
  Convert the array to an array of machine values and return the string representation (the same sequence of bytes that would be written to a file by the `tofile()` method.)

`write` (f)
  Deprecated since release 1.5.1. Use the `tofile()` method.

  Write all items (as machine values) to the file object f.

When an array object is printed or converted to a string, it is represented as `array(typecode, initializer)`. The initializer is omitted if the array is empty, otherwise it is a string if the typecode is `'c'`, otherwise it is a list of numbers. The string is guaranteed to be able to be converted back to an array with the same type and value using reverse quotes (````), so long as the `array()` function has been imported using "`from array import array`". Examples:

      array('l')
      array('c', 'hello world')
      array('l', [1, 2, 3, 4, 5])
      array('d', [1.0, 2.0, 3.14])

See Also:

Module `struct`:
  packing and unpacking of heterogeneous binary data.

Module `xdrlib`:
  packing and unpacking of XDR data.

The Numeric Python extension (NumPy) defines another array type; see The Numerical Python Manual for additional information (available online at ftp://ftp-icf.llnl.gov/pub/python/numericalpython.pdf). Further information about NumPy is available at http://www.python.org/topics/scicomp/numpy.html.