Precision of numpy array lost after tolist

numpy

Solution

Something goes wrong on your conversion to strings. With just numbers:

>>> import numpy as np
>>> a = np.random.random(10)*30
>>> a
array([ 27.30713434,  10.25895255,  19.65843272,  23.93161555,
        29.08479175,  25.69713898,  11.90236158,   5.41050686,
        18.16481691,  14.12808414])
>>> 
>>> b = np.round(a, decimals=1)
>>> b
array([ 27.3,  10.3,  19.7,  23.9,  29.1,  25.7,  11.9,   5.4,  18.2,  14.1])
>>> b.tolist()
[27.3, 10.3, 19.7, 23.9, 29.1, 25.7, 11.9, 5.4, 18.2, 14.1]

Notice that `np.round` does not work in-place:

>>> a
array([ 27.30713434,  10.25895255,  19.65843272,  23.93161555,
        29.08479175,  25.69713898,  11.90236158,   5.41050686,
        18.16481691,  14.12808414])

If all you need is to convert numbers to strings:

>>> " ".join(str(_) for _ in np.round(a, 1)) 
'27.3 10.3 19.7 23.9 29.1 25.7 11.9 5.4 18.2 14.1'

EDIT: Apparently,`np.round` does not play nice with `float32` (other answers give reasons for this). A simple workaround is to cast your array explicitly to either `np.float` or `np.float64` or just `float`:

>>> # prepare an array of float32 values
>>> a32  = (np.random.random(10) * 30).astype(np.float32)
>>> a32.dtype
dtype('float32')
>>> 
>>> # notice the use of .astype(np.float32)
>>> np.round(a32.astype(np.float64), 1)
array([  5.5,   8.2,  29.8,   8.6,  15.5,  28.3,   2. ,  24.5,  18.4,   8.3])
>>>

EDIT2: As demonstrated by Warren in his answer, string formatting actually rounds things properly (try `"%.1f" % (4.79,)`). Thus there's no need to cast between float types. I'll leave my answer mainly as a reminder that using `np.around` is not the right thing to do in these circumstances.

Problem

I have a numpy array in which every number has a certain designated precision(using around(x,1). ``` [[ 3. 15294.7 32977.7 4419.5 978.4 504.4 123.6] [ 4. 14173.8 31487.2 3853.9 967.8 410.2 107.1] [ 5. 15323.5 34754.5 3738.7 1034.7 376.1 105.5] [ 6. 17396.7 41164.5 3787.4 1103.2 363.9 109.4] [ 7. 19665.5 48967.6 3900.9 1161. 362.1 115.8] [ 8. 21839.8 56922.5 4037.4 1208.2 365.9 123.5] [ 9. 23840.6 64573.8 4178.1 1247. 373.2 131.9] [ 10. 25659.9 71800.2 4314.8 1279.5 382.7 140.5] [ 11. 27310.3 78577.7 4444.3 1307.1 393.7 149.1] [ 12. 28809.1 84910.4 4565.8 1331. 405.5 157.4]] ``` I'm trying to convert every number into a string so that I can write them into a word table using python-docx. But the result of tolist() function is a total mess. The precision of the numbers are lost, resulting very long output. ``` [['3.0', '15294.7001953', '32977.6992188', '4419.5', '978.400024414', '504.399993896', '123.599998474'], ['4.0', '14173.7998047', '31487.1992188', '3853.89990234', '967.799987793', '410.200012207', '107.099998474'], ....... ``` Besides the tolist() function, I also tried [[str(e) for e in a] for a in m]. The result is the same. This is very annoying. How can I convert to string easily while maintaining the precision? Thanks!

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