I have a numpy array that contains strings.
index1 = ['level4','level3','level2','UNKNOWN','level1']
I want to arrange this numpy array such that 'UNKNOWN' is always the first string in the array.
Desired array:
index1 = ['UNKNOWN','level4','level3','level2','level1']
Sorting of level is not desirable
If you literally just want 'UNKNOWN' to be in front, then the easiest and most computationally efficient way to do so is to just swap it with the 0th element:
index1 = np.array(['level4','level3','level2','UNKNOWN','level1'])
# find the index of 'UNKNOWN'
ix = np.flatnonzero(index1 == 'UNKNOWN')[0]
# swap values
index1[[0, ix]] = index1[[ix, 0]]
print(index1)
# output:
# ['UNKNOWN' 'level3' 'level2' 'level4' 'level1']
level elementsIf you want the level elements to have the same order in the output as in the input, here's a (slightly more complicated but still pretty simple) way to make that happen:
index1 = np.array(['level4','level3','level2','UNKNOWN','level1'])
# find the index of 'UNKNOWN'
ix = np.flatnonzero(index1 == 'UNKNOWN')[0]
# shift values
u = index1[ix]
index1[1:ix + 1] = index1[:ix]
index1[0] = u
print(index1)
# output:
# ['UNKNOWN' 'level4' 'level3' 'level2' 'level1']
partitionIf you don't want the results to be sorted, then you shouldn't use numpy.partition:
index1 = np.array(['level4','level3','level2','UNKNOWN','level1'])
index1.partition(np.flatnonzero(index1 == 'UNKNOWN'))
print(index1)
# output:
# ['UNKNOWN' 'level1' 'level2' 'level3' 'level4']
You can use np.roll:
import numpy as np
index1 = np.array(['level4','level3','level2','UNKNOWN','level1'])
position = np.where(index1 == 'UNKNOWN')[0][0]
index1[:position + 1] = np.roll(index1[:position + 1], 1)
print(index1)
array(['UNKNOWN', 'level4', 'level3', 'level2', 'level1'], dtype='<U7')
level elements was (4, 3, 2, 1), which is the same order that the level elements have in Tomothy's output.partition() will permit to reorder your array, but will be sorted.
index1.partition(np.where(index1=='UNKNOWN')[0][0])
partition doesn't answer the question. The result will be sorted.