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NumPy Part 4


Get The Index Locations That Satisfy A Particular Condition

  • We use where() function to get the index location that satisfy a particular condition

  • When we apply boolean indexing we can extract the elements from the array satisfying the condition

  • But to get the location of these arrays satisfying a condition we use where() function

Example 1


  • Once you have the positions, you can extract them using the array’s take() method

Example 2


  • In np.where() we can also pass two more arguments(optional) they are x, and y

  • This arguments works like an if-else statement. if condition is true then x is yielded or else y is yielded

Example 3


Saving and Loading NumPy objects

  • Sometimes we want to save this NumPy objects and load it back

  • For saving the NumPy objects we use np.save() function

  • For loading the NumPy objects we use np.load() function

  • When we want to store a single array store it as .npy file in np.save() function

  • When we want to store multiple array in a single file store it as .npz file in np.savez() function

  • np.save(filename to be stored,array)

  • np.savez(filename to be stored,array)

  • np.load(stored file name)

  • Use file attribute to load .npz files

Example 4






Concatenating two ndarrays

  • NumPy provides two different ways to do concatenation

  1. np.concatenate()

  2. np.vstack() or np.hstack()

np.concatenate([ndarray1,ndarry2],axis)

  • By using np.concatenate() we can concatenate or combine two ndarrays

  • By using np.concatenate() we can concatenate or combine two ndarrays column wise or row wise by changing the axis parameter to 0 and 1

Example 5

np.vstack() or np.hstack()

  • By using np.vstack() we can stack vertically (rows wise concatenate similar to np.concatenate(axis=0))

  • By using np.hstack() we can stack horizontally (column wise concatenate similar to np.concatenate(axis=1))

Example 6


How To Apply a function column wise or row wise

  • To apply a user defined or built in function we can use apply_along_axis() function provided by numpy module

  • apply_along_axis(function,axis,array)

    • function-- function to apply

    • axis- apply row wise or column wise

    • array - which array this function should be applied

Example 7

Example 8



How to Add A New Axis To A NumPy Array

  • We can add a new axis to ndarray by using newaxis() function

  • Whenever we need to convert a 1d to 2d or 2d to 3d ... we can use newaxis to convert to a higher dimensional ndarray

  • we can convert by adding a newaxis column wise or row wise

Example 9


How To Sort An Array

  • We can use sort() to sort an array column wise or row wise

  • To sort an array column wise or row wise use axis = 0 or 1

Example 10

  • np.argsort() can be used to return the index positions that would sort the array

Example 11

Extras:-

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