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- NumPy - Indexing & Slicing
- NumPy - Advanced Indexing
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# NumPy - Indexing & Slicing

Contents of ndarray object can be accessed and modified by indexing or slicing, just like Python's in-built container objects.

As mentioned earlier, items in ndarray object follows zero-based index. Three types of indexing methods are available − **field access, basic slicing** and **advanced indexing**.

Basic slicing is an extension of Python's basic concept of slicing to n dimensions. A Python slice object is constructed by giving **start, stop**, and **step** parameters to the built-in **slice** function. This slice object is passed to the array to extract a part of array.

## Example 1

import numpy as np a = np.arange(10) s = slice(2,7,2) print a[s]

Its output is as follows −

[2 4 6]

In the above example, an **ndarray** object is prepared by **arange()** function. Then a slice object is defined with start, stop, and step values 2, 7, and 2 respectively. When this slice object is passed to the ndarray, a part of it starting with index 2 up to 7 with a step of 2 is sliced.

The same result can also be obtained by giving the slicing parameters separated by a colon : (start:stop:step) directly to the **ndarray** object.

## Example 2

import numpy as np a = np.arange(10) b = a[2:7:2] print b

Here, we will get the same output −

[2 4 6]

If only one parameter is put, a single item corresponding to the index will be returned. If a : is inserted in front of it, all items from that index onwards will be extracted. If two parameters (with : between them) is used, items between the two indexes (not including the stop index) with default step one are sliced.

## Example 3

# slice single item import numpy as np a = np.arange(10) b = a[5] print b

Its output is as follows −

5

## Example 4

# slice items starting from index import numpy as np a = np.arange(10) print a[2:]

Now, the output would be −

[2 3 4 5 6 7 8 9]

## Example 5

# slice items between indexes import numpy as np a = np.arange(10) print a[2:5]

Here, the output would be −

[2 3 4]

The above description applies to multi-dimensional **ndarray** too.

## Example 6

import numpy as np a = np.array([[1,2,3],[3,4,5],[4,5,6]]) print a # slice items starting from index print 'Now we will slice the array from the index a[1:]' print a[1:]

The output is as follows −

[[1 2 3] [3 4 5] [4 5 6]] Now we will slice the array from the index a[1:] [[3 4 5] [4 5 6]]

Slicing can also include ellipsis (…) to make a selection tuple of the same length as the dimension of an array. If ellipsis is used at the row position, it will return an ndarray comprising of items in rows.

## Example 7

# array to begin with import numpy as np a = np.array([[1,2,3],[3,4,5],[4,5,6]]) print 'Our array is:' print a print '\n' # this returns array of items in the second column print 'The items in the second column are:' print a[...,1] print '\n' # Now we will slice all items from the second row print 'The items in the second row are:' print a[1,...] print '\n' # Now we will slice all items from column 1 onwards print 'The items column 1 onwards are:' print a[...,1:]

The output of this program is as follows −

Our array is: [[1 2 3] [3 4 5] [4 5 6]] The items in the second column are: [2 4 5] The items in the second row are: [3 4 5] The items column 1 onwards are: [[2 3] [4 5] [5 6]]