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POndim of structured arrays in numpy?
 

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  1. POndim of structured arrays in numpy?
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    <p>This is similarly worded question as <a href="https://stackoverflow.com/questions/10542263/ndim-in-numpy-array-loaded-with-scipy-io-loadmat">ndim in numpy array loaded with scipy.io.loadmat?</a> - but it's actually a lot more basic. </p> <p>Say I have this structured array:</p> <pre class="lang-python prettyprint-override"><code>import sys import numpy as np from pprint import pprint a = np.array([(1.5,2.5),(3.,4.),(1.,3.)], dtype=[('x','f4'),('y',np.float32)]) pprint(a) # array([(1.5, 2.5), (3.0, 4.0), (1.0, 3.0)], # dtype=[('x', '&lt;f4'), ('y', '&lt;f4')]) </code></pre> <p>I see that as a table of 3 rows and 2 columns, so 3x2. However, trying to use <code>ndim</code> here, I see:</p> <pre class="lang-python prettyprint-override"><code>print(".ndim", a.ndim) print(".shape", a.shape) print("asarray.ndim", np.asarray(a).ndim) # ('.ndim', 1) # ('.shape', (3,)) # ('asarray.ndim', 1) </code></pre> <p>... and this is what puzzles me - what makes <code>numpy</code> think this should be a 1d array, when there are clearly fields/columns defined ?!</p> <p>Given that output, no wonder that reshape doesn't work:</p> <pre class="lang-python prettyprint-override"><code>pprint(a.reshape(3,2)) # ValueError: total size of new array must be unchanged </code></pre> <p>Now, I can bruteforce the structured array into a ("normal" numpy, I guess?) array:</p> <pre class="lang-python prettyprint-override"><code>b = np.column_stack((a['x'], a['y'])) pprint(b) # array([[ 1.5, 2.5], # [ 3. , 4. ], # [ 1. , 3. ]], dtype=float32) print(".ndim", b.ndim) print(".shape", b.shape) print("asarray.ndim", np.asarray(b).ndim) # ('.ndim', 2) # ('.shape', (3, 2)) # ('asarray.ndim', 2) </code></pre> <p>... and so I get the information that I expect. </p> <p>But I am wondering - why does numpy behave like this with structured arrays - and is there a way to retrieve the 3x2 shape information from the original structured array (<code>a</code>) directly, without "casting" to a "normal" array?</p>
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