The slice notation is a very powerful concept in Python. It allows us to extract parts of a sequence (such as a list or string) effortlessly.
The notation takes three values.
Suppose we have a string called myString.
The notation myString[a:b:c] indicates that we want to extract a slice from myString that starts from index a, up to (but not including) index b.
a and b are both optional. If a is omitted, the slice starts from the first element. If b is omitted, the slice ends at the last element.
Finally, we have c. This is the stepper; it allows us to skip elements. For instance, if the stepper is 2, we’ll get every 2nd element in the string.
If c is positive, the slice steps from left to right. If it is negative, it steps from right to left.
c is also optional. If omitted, the default value is 1.
Confusing? Let’s look at some examples.
Examples:
Suppose
myString = 'abcdefg'
myString[1:5:1]
gives us elements from index 1 up to (but not including) index 5. In other words, it gives us elements from index 1 to 4. Hence, we get the string 'bcde'.
myString[:5:1]
gives us elements from the start (as a is omitted) of the string to index 4. Hence, we get the string 'abcde'.
myString[1::1]
gives us elements from index 1 to the end of the string (as b is omitted). Hence, we get the string 'bcdefg'.
myString[::1]
gives us 'abcdefg'. In other words, as the stepper is 1, we simply get every element in the string.
On the other hand, myString[::3]
gives us 'adg' (i.e. every 3rd character in the string) as the stepper is 3.
myString[::-1]
gives us 'gfedcba' as the stepper is negative one (i.e. every character in the string starting from the right)
myString[::-2]
gives us 'geca' (i.e. every 2nd character in the string starting from the right)
myString[-1:-5:-2]
gives us 'ge'. This is the hardest to understand if you are not familiar with using negative indices for lists.
Recall that we can use negative indices when accessing the elements in a list? Negative indices start from the back of the list.
Index -1 refers to the last element, -2 refers to the second last and so on. In other words, myString[-1] gives us 'g' while myString[-2] gives us 'f'.
myString[-1:-5:-2] means we want to extract elements from index -1 ('g'), up to but not including index -5 ('c'). In other words, the elements are extracted from 'f' to 'd'. As the stepper is -2, we get every second element (from right to left). Hence, the result is 'ge'.