Understand Python Slicing

Python slicing is used to slice lists or tuples. Let us learn Python slicing through examples.

Let us declare a Python list.

In [1]:

x = [1,2,3,4,5,6,7,8,9]

Python slicing can be done in two ways...
x[start:stop:step] -- Select items at every step (index) from Start through stop-1
x[slice(start,stop,step) -- Select items at every step (index) from Start through stop-1
Note step (argument) is optional

In [2]:

x[0:1]

Out[2]:

[1]

In [3]:

x[0:1:1]

Out[3]:

[1]

In [4]:

x[slice(0,1)]

Out[4]:

[1]

In [5]:

x[slice(0,1,1)]

Out[5]:

[1]

Note step parameter is optional, by default it is 1.

Python slice all but first element

In [6]:

x[1:]

Out[6]:

[2, 3, 4, 5, 6, 7, 8, 9]

Python slice all but last element

In [7]:

x[:8]

Out[7]:

[1, 2, 3, 4, 5, 6, 7, 8]

Python slice and copy the whole list

In [8]:

x[:]

Out[8]:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Python slice every other element in the list

The below command means start from index 0 and extract every 2nd element.
index 0 , index (2) 0 +2, index (4) 0 + 2 +2, index (6) 0 + 2 + 2 +2 , index (8) 0 + 2 + 2 + 2 +2

Note two colons in the below syntax. There is nothing between Ist and 2nd colon that means evreything from start to end of list.

In [9]:

x[0::2]

Out[9]:

[1, 3, 5, 7, 9]

Python Slicing - Negative and Positive step values

Negative step of -1 means go from right to left
Positive step of 1 means go from left to right

Python slicing using negative indices

Negative number -1 means last element, -2 means seconda last element so on and so forth.

In [10]:

x[-1]

Out[10]:

The below syntax will print nothing, because index is starting from -1 which means last element in the list then it goes to index 0 because by default step is 1 but our "stop" is -3. To fix this we will have to give step of -1.

In [11]:

x[-1:-3]

Out[11]:

[]

As we can see, the index will start from -1, the last element, then goes to -2 because step is -1, then stops at -3

In [12]:

x[-1:-3:-1]

Out[12]:

[9, 8]

Python Reverse all the items in the array using Slicing

In [13]:

x[::-1]

Out[13]:

[9, 8, 7, 6, 5, 4, 3, 2, 1]

Note in the above, since step is -1, Python considers start:stop as -1:-10, we can re-write above as shown below.

In [14]:

x[-1:-10:-1]

Out[14]:

[9, 8, 7, 6, 5, 4, 3, 2, 1]

print last 3 elements. Note below syntax means -3 to last element (right to left) and step is 1 by default

In [15]:

x[-3:]

Out[15]:

[7, 8, 9]

Below syntax means, start at -3 index, then go to (-5) -3-2 so on and so forth to all the way to start of the list (right to left) because step is -1.

In [16]:

x[-3::-2]

Out[16]:

[7, 5, 3, 1]

Python Slicing - Reverse first two items

In [17]:

x[1::-1]

Out[17]:

[2, 1]

Let us see how did the above slicing syntax works.
The index will start at...
index 1, index (0) 1-1
Note -1 tells it go from right to left

In [18]:

x[1::-1]

Out[18]:

[2, 1]

Python Everything Except the last two items - Reversed

In [19]:

x[-3::-1]

Out[19]:

[7, 6, 5, 4, 3, 2, 1]

Note all the above can be used with slice method. The different is instead of ":" (colon) use None to indicate end or start of list index.

In [20]:

x[slice(-3,None,-1)]

Out[20]:

[7, 6, 5, 4, 3, 2, 1]