Character <-> Int conversion in numpy

char_int_numpy.rst

Character <-> Int conversion in numpy

First thing to do:

>>> import numpy

Char array to int array¹:

>>> ac = numpy.array('ABC', 'c') >>> ac #doctest: +NORMALIZE_WHITESPACE array(['A', 'B', 'C'], dtype='|S1') >>> ac_as_int = ac.view(numpy.uint8) >>> ac_as_int #doctest: +NORMALIZE_WHITESPACE array([65, 66, 67], dtype=uint8) >>> # get new copy >>> numpy.array(ac_as_int) #doctest: +NORMALIZE_WHITESPACE array([65, 66, 67], dtype=uint8)

Unicode array to int array:

>>> au = numpy.array(list('ABC'), 'U1') >>> au #doctest: +NORMALIZE_WHITESPACE array([u'A', u'B', u'C'], dtype='<U1') >>> au_as_int = au.view(numpy.uint32) >>> au_as_int array([65, 66, 67], dtype=uint32) >>> # get new copy >>> numpy.array(au_as_int) #doctest: +NORMALIZE_WHITESPACE array([65, 66, 67], dtype=uint32)

Int array to char array:

>>> ac_as_int.view('c') #doctest: +NORMALIZE_WHITESPACE array(['A', 'B', 'C'], dtype='|S1')

Int array to unicode array:

>>> au_as_int.view('U1') #doctest: +NORMALIZE_WHITESPACE array([u'A', u'B', u'C'], dtype='<U1') >>> # you can't do this >>> ac_as_int.view('U1') #doctest: +NORMALIZE_WHITESPACE Traceback (most recent call last): ... ValueError: new type not compatible with array. >>> # you need explicit conversion >>> ac_as_int.astype(numpy.uint32).view('U1') #doctest: +NORMALIZE_WHITESPACE array([u'A', u'B', u'C'], dtype='<U1')

To use .view, you need to have same size (compatible type) of dtype. This is how to quickly check the size:

>>> numpy.array(0, dtype='c').dtype.itemsize 1 >>> numpy.array(0, dtype=numpy.uint8).dtype.itemsize 1 >>> numpy.array(0, dtype='U1').dtype.itemsize 4 >>> numpy.array(0, dtype=numpy.uint32).dtype.itemsize 4

(Not so many) resources:

• [Numpy-discussion] String to integer array of ASCII values
• Data type objects (dtype) --- NumPy v1.7.dev-72185d3 Manual (DRAFT) <http://docs.scipy.org/doc/numpy/reference/arrays.dtypes.html #specifying-and-constructing-data-types>
• ASCII value of a character in python - Stack Overflow <http://stackoverflow.com/questions/227459/ ascii-value-of-a-character-in-python> (when you want to do it in pure python.)

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1. Stolen from [Numpy-discussion] String to integer array of ASCII values.↩

Thank you for this gist. Also I've just discovered:

>>> np.char.mod('%c', ac_as_int)
array(['A', 'B', 'C'], dtype='<U1')

This is an amazing post!

Very helpful indeed :)