![]() Here we omit start/ stop parameters and use only step. ![]() What if we want to have only every 2-nd element of nums? This is where the step parameter comes into play: > nums = We take all but the last two elements of original list. We can freely mix negative and positive indexes in start and stop positions: > nums = Īnother good usage of negative indexes: > nums = We start from the third element from the end (value 70 with index -3) and take everything to the end. That means you take from the start position, till the end of the list. ![]() Negative indexes allow us to easily take n-last elements of a list: > nums = This combination is a handy shortcut to take n first elements of a list. If we skip the start number then it starts from 0 index: > nums = Slice notation allows you to skip any element of the full syntax. Here we start from the first element(index 0) and take a list till the element with index 4. With slices we can extract an arbitrary part of a list, e.g.: > nums = We did not use step in our slice, so we didn’t skip any element and obtained all values within the range. In the end, slice creates a new list(we named it some_nums) with selected elements. stop is 7, so the last element of the slice is 70 with index 6. In our example start equals 2, so our slice starts from value 30. step allows you to take each nth-element within a start:stop range. stop refers to the index of the element we should stop just before to finish our slice. start refers to the index of the element which is used as a start of our slice. The full slice syntax is: start:stop:step. So, here is our first example of a slice: 2:7. What if we want to take a sublist from the nums list? This is a snap when using slice: > nums = Those and lots of other cool tricks can be done with slice notation. Or we want to update a bunch of cells at once? Or we want to go on a frenzy and extend a list with an arbitrary number of new cells in any position? What if we want to get a sublist of the list. Slice NotationĪs it was shown, indexing allows you to access/change/delete only a single cell of a list. ![]() But assignment and deletion operations are not applicable to immutable sequential types. Read-only indexing operations work perfectly well for all sequential types. We can also easily delete any element from the list by using indexing and del statement: > basket = We can freely use positive or negative indexing for assignment. But it’s also possible to change cell content using an assignment operation: > basket = > colors = īefore we used indexing only for accessing the content of a list cell. In negative indexing system -1 corresponds to the last element of the list(value ‘black’), -2 to the penultimate (value ‘white’), and so on. So, instead of using indexes from zero and above, we can use indexes from -1 and below. To address this requirement there is negative indexing. But what if we want to take the last element of a list? Or the penultimate element? In this case, we want to enumerate elements from the tail of a list. This is handy if we use position from the head of a list. Using indexing we can easily get any element by its position. To access an element by its index we need to use square brackets: > colors = That means, the first element(value ‘red’) has an index 0, the second(value ‘green’) has index 1, and so on. Each item in the list has a value(color name) and an index(its position in the list). It allows you to store an enumerated set of items in one place and access an item by its position – index. In Python, list is akin to arrays in other scripting languages(Ruby, JavaScript, PHP). ![]() Before discussing slice notation, we need to have a good grasp of indexing for sequential types. ![]()
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