<\/span><\/h2>\n\n\n\nBefore we understand slicing, it is essential to learn about indexing in Python. Slicing completely depends on indexing in Python. Index is the position of a character or element in an indexable data type such as list, array, tuple, or string. It is automatically assigned by Python when you define a data variable. It cannot be explicitly assigned or modified by us. <\/p>\n\n\n\n
Indexes can be positive or negative. The positive index begin from 0 assigned to the first character, 1 to next character and so on. The negative indexes start from 0 for the last character, -1 for the penultimate character and so on. Positive and negative indexes allow you to access elements from start or the end respectively. In fact, every character of a data can be accessed using both positive as well as negative indexes. Here is an example of positive as well as negative indexes.<\/p>\n\n\n\n
0 1 2 3 4 # positive index
h e l l o # string characters
-5 -4 -3 -2 -1 # negative index<\/pre>\n\n\n\nAs the data is modified the element indexes are also modified. So they are dynamic. For example, if you remove the first character of the above string, the indexes will become different for the other characters.<\/p>\n\n\n\n
0 1 2 3 # positive index
e l l o # string characters
-4 -3 -2 -1 # negative index<\/pre>\n\n\n\n<\/span>Slicing in Python<\/span><\/h2>\n\n\n\nNow that we have a good understanding of indexing in Python, let us learn how to use it for slicing. Slicing is basically accessing one or group of elements in an iterable data such as string, array, list or tuple. It can be to insert, modify, or delete these elements. There are several ways to slice data in Python. Bu they all use 3 main parameters – start, stop and step. Start is the starting index of a slice. Stop is the ending index of the slice. Step is a parameter that means the number of elements to be skipped at a time, between start and stop indexes. Each of these parameters is optional and we will see how to use them below.<\/p>\n\n\n\n
<\/span>How Slicing Works in Python<\/span><\/h2>\n\n\n\nThere are 3 different ways to slice in Python<\/p>\n\n\n\n
<\/span>1. Using Index<\/span><\/h2>\n\n\n\nWe typically use index to access a single element from a list or array. Here is an example.<\/p>\n\n\n\n
>>> data = 'hello'
>>> data[1]
'e'<\/pre>\n\n\n\nExtending this approach, you can also extract a slice of a list. Here is the syntax to do this.<\/p>\n\n\n\n
data[start:stop:step]<\/pre>\n\n\n\nIn the above code, we specify the start index, end index and step. Each of these are optional. Therefore, here are several simple ways to slice your string.<\/p>\n\n\n\n
data[start:stop] # from start to stop-1
data[start:] # start till end
data[:stop] # from first item to stop-1
data[:] # copy of whole array
data[start:stop:step] # start to stop, skipping step each time<\/pre>\n\n\n\nHere are some examples to demonstrate it. Let us say you have the following string.<\/p>\n\n\n\n
data = 'hello'<\/pre>\n\n\n\nHere are the different slices, using the previously mentioned syntaxes.<\/p>\n\n\n\n
>>> data[2:3] # start at index=2 and stop and index=3
'l'
>>> data[2:] # from index=2 till end
'llo'
>>> data[:3] # from start till index=3
'hel'
>>> data[:] # copy full array
'hello'
>>> data[1:4:2] # start at index=1 and stop and index=4 with step=2
'el'<\/pre>\n\n\n\nYou can also use negative indexes.<\/p>\n\n\n\n
>>> data[-4:-2]
'el'<\/pre>\n\n\n\nYou can also use negative step as shown below. When the start index is after the stop index, with a negative index, then Python will traverse the string in reverse order. Here -1 is penultimate character and -4 is the first character, and step=-2 so Python starts displaying slice characters in reverse order.<\/p>\n\n\n\n
>>> data[-1:-4:-2]
'ol'<\/pre>\n\n\n\n<\/span>2. Using slice() function<\/span><\/h2>\n\n\n\nPython also provides a constructor slice() to slice a sequence of characters. It also requires the same 3 arguments used above – start, stop and step. Here is its syntax.<\/p>\n\n\n\n
slice(start, stop, step)<\/pre>\n\n\n\nYou need to assign it to a variable and then use it use it directly inside […]. Here are both examples for your reference.<\/p>\n\n\n\n
>>> data='hello'\n\n>>> x=slice(2,3)\n>>> data[x]\n'l'\n\n>>> data[slice(2,3)]\n'l'<\/pre>\n\n\n\nLet us look at the above examples using slice() function.<\/p>\n\n\n\n
>>> data[slice(2,3)] # from index=2 to index=3
'l'
>>> data[slice(2,)] # from index=2 till end
'he'
>>> data[slice(2)] # from index=2 till end
'he'
>>> data[slice(0,3)] # from index=0 till index=3
'hel'
>>> data[slice(0,4,2)] # from index=0 to index=4 with step=2
'hl'<\/pre>\n\n\n\nAs you can see, slice() offers numerous ways to extract data.<\/p>\n\n\n\n