python save array to csv with different length

user_16354938 0
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this is my code

import numpy
a = numpy.asarray([ [1,2,3,4],[5,6],[7,8,9,10] ])
numpy.savetxt("a.csv", a, fmt="%d", delimiter=",")

I run it, but it report error

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/numpy/lib/npyio.py", line 1422, in savetxt
    v = format % tuple(row) + newline
TypeError: %d format: a number is required, not list

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/pi/Desktop/csvTest.py", line 3, in <module>
    numpy.savetxt("foo.csv", a,fmt="%d", delimiter=",")
  File "/usr/lib/python3/dist-packages/numpy/lib/npyio.py", line 1426, in savetxt
    % (str(X.dtype), format))
TypeError: Mismatch between array dtype ('object') and format specifier ('%d')

after i try this code

import numpy
a = numpy.asarray([ [1,2,3],[4,5,6],[7,8,9] ])
numpy.savetxt("a.csv", a, fmt="%d", delimiter=",")

it can work, so how do i to solve this problem? can i save different array length with csv?

csv python-3.x numpy arrays python
2 Answers

Saving the array as string can resolve the issue.

a = numpy.asarray([ [1,2,3,4],[5,6],[7,8,9,10] ])
numpy.savetxt("a.csv", a, fmt="%s", delimiter=",")
user_19130170 
In [98]: alist=[[1,2,3,4],[5,6],[7,8,9,10] ]
In [99]: alist
Out[99]: [[1, 2, 3, 4], [5, 6], [7, 8, 9, 10]]

formatting a display string:

In [100]: astr = []
     ...: for row in alist:
     ...:     col = ', '.join(['%d'%i for i in row])
     ...:     astr.append(col)
     ...: astr = '\n'.join(astr)
In [101]: astr
Out[101]: '1, 2, 3, 4\n5, 6\n7, 8, 9, 10'
In [102]: print(astr)
1, 2, 3, 4
5, 6
7, 8, 9, 10

and with a file write:

In [104]: with open('test.txt','w') as f:
     ...:     for row in alist:
     ...:         print(', '.join(['%d'%i for i in row]), file=f)
     ...: 
In [105]: cat test.txt
1, 2, 3, 4
5, 6
7, 8, 9, 10

If we try to write it as an array:

In [106]: arr = np.array(alist)
<ipython-input-106-3fd8e9bd05a9>:1: VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences (which is a list-or-tuple of lists-or-tuples-or ndarrays with different lengths or shapes) is deprecated. If you meant to do this, you must specify 'dtype=object' when creating the ndarray.
  arr = np.array(alist)
In [107]: arr
Out[107]: 
array([list([1, 2, 3, 4]), list([5, 6]), list([7, 8, 9, 10])],
      dtype=object)

This is a 1d array containing lists. If we write with '%s' format the lists will be formatted as standard lists:

In [108]: np.savetxt('test.csv',arr, fmt='%s')
In [109]: cat test.csv
[1, 2, 3, 4]
[5, 6]
[7, 8, 9, 10]

Neither is good csv reading material. The [] interfere with parsing a line. The unequal lines require special handling, either rejection as being the wrong length, or filled with extra values.

user_9019250
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