1: In a numpy array replace all odd position numbers with -1?

    
arr = np.array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
arr[arr % 2 == 1] = -1
print(arr)
    

	
    o/p:

   #>  array([ 0, -1,  2, -1,  4, -1,  6, -1,  8, -1])
   
   

2: In a numpy array replace all odd position numbers with -1 without affecting the original array?

arr = np.arange(10)
out = np.where(arr % 2 == 1, -1, arr)
print('Original : ',arr)
print('Changed ':out)

#> Original :  [0 1 2 3 4 5 6 7 8 9]
   Changed : array([ 0, -1,  2, -1,  4, -1,  6, -1,  8, -1])

3: Reshape a given 1-D array in 4*3 shape?

array([ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

arr = np.arange(12)
arr.reshape(4, 3)  # Setting to -1 means (4,-1) automatically decides the number of cols
print(arr)

#>	array([[0, 1, 2, 3],
		[4, 5, 6, 7]
		[8, 9, 10, 11]])

Question 4: let's we have two numpy arrays,now stack them vertically?

	
        
Solution
a = np.arange(10).reshape(2,5)
b = np.repeat(2, 10).reshape(2,5)

# Method 2:
np.concatenate([a, b], axis=0)

# Method 2:
np.vstack([a, b])

# Method 3:
np.r_[a, b]
    

#> array([[0, 1, 2, 3, 4],
#>        [5, 6, 7, 8, 9],
#>        [2, 2, 2, 2, 2],
#>        [2, 2, 2, 2, 2]])

Question 5: Take any two arrays and stack them horizontally?

Solution
a = np.arange(10).reshape(2,5)
b = np.repeat(2, 10).reshape(2,5)

# Method 2:
np.concatenate([a, b], axis=2)

# Method 2:
np.hstack([a, b])

# Method 3:
np.c_[a, b]

#> array([[0, 1, 2, 3, 4, 2, 2, 2, 2, 2],
#>        [5, 6, 7, 8, 9, 2, 2, 2, 2, 2]])

Question 6: How to create custom sequences in numpy without hardcoding using given array?

a = np.array([1,2,3])`
Desired Output:

#> array([1, 1, 1, 2, 2, 2, 3, 3, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3])

    
        
Solution
        np.r_[np.repeat(a, 3), np.tile(a, 3)]
#> array([1, 1, 1, 2, 2, 2, 3, 3, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3])

Question 7: How to get the common items between two python numpy arrays?

Get the common items between a and b

Input:

a = np.array([1,2,3,2,3,4,3,4,5,6])
b = np.array([7,2,10,2,7,4,9,4,9,8])

Desired Output:

array([2, 4])

Solution
a = np.array([1,2,3,2,3,4,3,4,5,6])
b = np.array([7,2,10,2,7,4,9,4,9,8])
np.intersect1d(a,b)
#> array([2, 4])

Question 8: How to remove from one array those items that exist in another?

Note: From array a remove all items present in array b.

Input:

a = np.array([1,2,3,4,5])
b = np.array([5,6,7,8,9])
Desired Output:

array([1,2,3,4])

Solution
a = np.array([1,2,3,4,5])
b = np.array([5,6,7,8,9])

# From 'a' remove all of 'b'
np.setdiff1d(a,b)
#> array([1, 2, 3, 4])

Question 9: How to get the positions where elements of two arrays match?

Note: Get the positions where elements of a and b match.

Input:

a = np.array([1,2,3,2,3,4,3,4,5,6])
b = np.array([7,2,10,2,7,4,9,4,9,8])
Desired Output:

#> (array([1, 3, 5, 7]),)

Solution
a = np.array([1,2,3,2,3,4,3,4,5,6])
b = np.array([7,2,10,2,7,4,9,4,9,8])

np.where(a == b)
#> (array([1, 3, 5, 7]),)

Question 10: How to extract all numbers between a given range from a numpy array?

Note: Get all items between 5 and 10 from a.

Input:

a = np.array([2, 6, 1, 9, 10, 3, 27])
Desired Output:

(array([6, 9, 10]),)

Solution
a = np.arange(15)

# Method 1
index = np.where((a >= 5) & (a <= 10))
a[index]

# Method 2:
index = np.where(np.logical_and(a>=5, a<=10))
a[index]
#> (array([6, 9, 10]),)

# Method 3: (thanks loganzk!)
a[(a >= 5) & (a <= 10)]