Iterating Without For Loop In Numpy Array
Solution 1:
Here's an approach using a combination of np.maximum.accumulate
and np.where
to create stepped indices that are to be stopped at certain intervals and then simply indexing into b
would give us the desired output.
Thus, an implementation would be -
mask = a!="b"idx = np.maximum.accumulate(np.where(mask,np.arange(mask.size),0))
out = b[idx]
Sample step-by-step run -
In [656]: # Inputs
...: a = np.array(['a', 'b', 'a', 'a', 'b', 'a'])
...: b = np.array([150, 154, 147, 126, 148, 125])
...:
In [657]: mask = a!="b"
In [658]: mask
Out[658]: array([ True, False, True, True, False, True], dtype=bool)
# Crux of the implmentation happens here :
In [696]: np.where(mask,np.arange(mask.size),0)
Out[696]: array([0, 0, 2, 3, 0, 5])
In [697]: np.maximum.accumulate(np.where(mask,np.arange(mask.size),0))
Out[697]: array([0, 0, 2, 3, 3, 5])# Stepped indices "intervaled" at masked places
In [698]: idx = np.maximum.accumulate(np.where(mask,np.arange(mask.size),0))
In [699]: b[idx]
Out[699]: array([150, 150, 147, 126, 126, 125])
Solution 2:
You could use a more vectorized approach Like so:
np.where(a == "b", np.roll(c, 1), b)
np.where
will take the elements from np.roll(c, 1)
if the condition is True
or it will take from b
if the condition is False
. np.roll(c, 1)
will "roll" forward all the elements of c
by 1 so that each element refers to c[i-1]
.
These type of operations are what make numpy so invaluable. Looping should be avoided if possible.
Solution 3:
If you don't need to wrap around the margin there is a very simply solution:
a = np.array(['a', 'b', 'a', 'a', 'b', 'a'])
b = np.array([150, 154, 147, 126, 148, 125])
c = b.copy() #removes necessity of elsecase
c[a[:-1]=='b'] = c[a[1:]=='b']
or equally:
a = np.array(['a', 'b', 'a', 'a', 'b', 'a'])
b = np.array([150, 154, 147, 126, 148, 125])
c = b.copy() #removes necessity of elsecase
mask = a == 'b'
c[mask[:-1]] = c[mask[1:]]
If you want to wrap around the margin (a[0]=='b'
) then it gets a little more complicated, you either need to use roll
or catch this case first with and if
.
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