Accessing required number of indices in an array
Question:
I have an array like:
a=np.array([20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67
68 69 70 71 72 73 74 75 76 77 78 79])
requirement:
I would to like to access 10 indices in an array
the above array length is 60,60/10=6. So, i need every 6th indices in an array a.
required output:[0,6,12,18,24,30,36,42,48,64,60]
Answers:
Numpy is powerful i would recommend to read the Documentation about indexing in numpy
everySixthEntry=a[np.arange(0,a.shape[0],6)]
I think you meant to say:
The required index values are [0,6,12,18,24,30,36,42,48,64,60]
Corresponding output values are [20, 26, 32, 38, 44, 50, 56, 62, 68, 74]
The code below should give you the values for every 6th index.
a=np.array([20,21,22,23,24,25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43,
44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67,
68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,])
Out=[]
for i in range(10):
Out.append(a[6*i])
print(Out)
Output is :
[20, 26, 32, 38, 44, 50, 56, 62, 68, 74]
If the Index values are required: Do the following
Out1=[]
for i in range(0,11): #for only 10 indices (going from 0 to 10)
print(6*i)
Out1.append(6*i)
print("The required index values is : {}".format(Out1))
This gives an output :
0
6
12
18
24
30
36
42
48
54
60
The required index values is : [0, 6, 12, 18, 24, 30, 36, 42, 48, 54, 60]
You can generate the indexes for any array a with np.arange(len(a)). To access every 6th index use the a slice a[start:stop:step]. Jack posted one way, here a bit more detailed.
import numpy as np
# define data
a = np.arange(60) + 20
# generate indexes for the array, index start at 0 till len(a)-1
indexes = np.arange(len(a))[::6] # [start:stop:step]
# the same result a bit different
indexes = np.arange(0, len(a), 6) # (start,stop,step)
indexes
# -> array([ 0, 6, 12, 18, 24, 30, 36, 42, 48, 54])
# access the values of your original array
a[indexes]
# -> array([20, 26, 32, 38, 44, 50, 56, 62, 68, 74])
Basics of slicing
a[start:stop:step] is equivalent to a[slice(start, stop, step)]. If you don’t want to specify any of start, stop, step set it to None. start and stop takes values from 0 to len(a)-1 and negative represents the position from the end of the array.
Some Slice Examples:
step = 20
a[slice(None, None, step)], a[slice(0, -1, step)], a[0: -1: step], a[::step]
# all -> array([20, 40, 60])
# the first 4 elements
step = 1
start = 0 # or None
end = 5
a[slice(start, end, step)], a[slice(start, end)] , a[start: end: step] , a[start:end]
# all -> array([20, 21, 22, 23])
# the last 4 elements
step = 1
start = -4
end = None # -1 will cute the last entry
a[slice(start, end, step)], a[slice(start, end)] , a[start: end: step] , a[start:end]
# all -> array([76, 77, 78, 79]
I have an array like:
a=np.array([20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67
68 69 70 71 72 73 74 75 76 77 78 79])
requirement:
I would to like to access 10 indices in an array
the above array length is 60,60/10=6. So, i need every 6th indices in an array a.
required output:[0,6,12,18,24,30,36,42,48,64,60]
Numpy is powerful i would recommend to read the Documentation about indexing in numpy
everySixthEntry=a[np.arange(0,a.shape[0],6)]
I think you meant to say:
The required index values are [0,6,12,18,24,30,36,42,48,64,60]
Corresponding output values are [20, 26, 32, 38, 44, 50, 56, 62, 68, 74]
The code below should give you the values for every 6th index.
a=np.array([20,21,22,23,24,25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43,
44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67,
68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,])
Out=[]
for i in range(10):
Out.append(a[6*i])
print(Out)
Output is :
[20, 26, 32, 38, 44, 50, 56, 62, 68, 74]
If the Index values are required: Do the following
Out1=[]
for i in range(0,11): #for only 10 indices (going from 0 to 10)
print(6*i)
Out1.append(6*i)
print("The required index values is : {}".format(Out1))
This gives an output :
0
6
12
18
24
30
36
42
48
54
60
The required index values is : [0, 6, 12, 18, 24, 30, 36, 42, 48, 54, 60]
You can generate the indexes for any array a with np.arange(len(a)). To access every 6th index use the a slice a[start:stop:step]. Jack posted one way, here a bit more detailed.
import numpy as np
# define data
a = np.arange(60) + 20
# generate indexes for the array, index start at 0 till len(a)-1
indexes = np.arange(len(a))[::6] # [start:stop:step]
# the same result a bit different
indexes = np.arange(0, len(a), 6) # (start,stop,step)
indexes
# -> array([ 0, 6, 12, 18, 24, 30, 36, 42, 48, 54])
# access the values of your original array
a[indexes]
# -> array([20, 26, 32, 38, 44, 50, 56, 62, 68, 74])
Basics of slicing
a[start:stop:step] is equivalent to a[slice(start, stop, step)]. If you don’t want to specify any of start, stop, step set it to None. start and stop takes values from 0 to len(a)-1 and negative represents the position from the end of the array.
Some Slice Examples:
step = 20
a[slice(None, None, step)], a[slice(0, -1, step)], a[0: -1: step], a[::step]
# all -> array([20, 40, 60])
# the first 4 elements
step = 1
start = 0 # or None
end = 5
a[slice(start, end, step)], a[slice(start, end)] , a[start: end: step] , a[start:end]
# all -> array([20, 21, 22, 23])
# the last 4 elements
step = 1
start = -4
end = None # -1 will cute the last entry
a[slice(start, end, step)], a[slice(start, end)] , a[start: end: step] , a[start:end]
# all -> array([76, 77, 78, 79]