Say I have the size (2,3,2) array a and the size (2) array b below.
import numpy as np
a = np.array([[[1, 2], [3, 4], [5, 6]], [[7, 8], [9, 10], [11, 12]]])
b = np.array([0.2, 0.8])
Array a looks like this:
I'd like to use numpy routines to concatenate b to the first row of each 2d arrray in a to make the array
I can't seem to make concatenate, vstack, append, etc. work.
Try this:
np.concatenate(([[b]]*2,a),axis=1)
# Result:
array([[[ 0.2, 0.8],
[ 1. , 2. ],
[ 3. , 4. ],
[ 5. , 6. ]],
[[ 0.2, 0.8],
[ 7. , 8. ],
[ 9. , 10. ],
[ 11. , 12. ]]])
This works:
np.insert(a.astype(float), 0, b, 1)
Output:
array([[[ 0.2, 0.8],
[ 1. , 2. ],
[ 3. , 4. ],
[ 5. , 6. ]],
[[ 0.2, 0.8],
[ 7. , 8. ],
[ 9. , 10. ],
[ 11. , 12. ]]])
If you don't cast with astype() first, you just end up prepending [0, 0]
Note, this is slower than the concatenate():
$ python test.py
m1: 8.20246601105 sec
m2: 43.8010189533 sec
Code:
#!/usr/bin/python
import numpy as np
import timeit
a = np.array([[[1, 2], [3, 4], [5, 6]], [[7, 8], [9, 10], [11, 12]]])
b = np.array([0.2, 0.8])
def m1():
np.concatenate(([[b]]*2,a),axis=1)
def m2():
np.insert(a.astype(float), 0, b, 1)
print "m1: %s sec" % timeit.timeit(m1)
print "m2: %s sec" % timeit.timeit(m2)
