In Python, you can slice arrays (or lists) using the following syntax: list[start:stop:step]. Its a quick and very useful syntax for extracting data from arrays in an easy format, but its not a common technique in other languages, so generally something new for python programmers to learn.
But its powerful and easy once you learn the **technique.
Here's what each part of that syntax: list[start:stop:step] means:
start: The index where the slice starts. If not provided, it defaults to the start of the list.
stop: The index where the slice ends. This index is not included in the slice. If not provided, it defaults to the end of the list.
step: The amount by which the index increases during the slice. If not provided, it defaults to 1.
Here's an example:
numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(numbers[1:5]) # Output: [1, 2, 3, 4]
print(numbers[::2]) # Output: [0, 2, 4, 6, 8]`
In the first print statement, we're getting elements from index 1 to 4 (exclusive). In the second print statement, we're starting at the beginning of the list and taking every second element, often quite useful to reshape arrays or decimate data
Getting a sublist
If you want to get a sublist from a certain index to another, you can do so like this:
numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(numbers[3:7]) # Output: [3, 4, 5, 6]
This will give you the elements from index 3 to 6 (the stop index is exclusive).
Reversing a list
If you want to reverse a list, you can do so like this:
numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(numbers[::-1]) # Output: [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
This will give you the same list, but in reverse order.
Stepping through a list
If you want to step through a list, you can do so like this:
numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(numbers[::2]) # Output: [0, 2, 4, 6, 8]
This will give you every second element in the list, starting from the first element.
Getting elements from the end of a list
If you want to get elements from the end of a list, you can do so like this:
numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(numbers[-3:]) # Output: [7, 8, 9]
This will give you the last three elements in the list.
Selecting Entire Dimensions
To select all elements in a specific dimension, you can use empty slice brackets []. For instance, to select all elements from the first dimension of a multidimensional array, you would use the slice [:, :].
More Advanced Slicing Techniques
Slicing Nested Arrays
When dealing with nested arrays, you can slice each dimension separately within the individual square brackets. For example, to slice elements from index 1 to index 3 in the first dimension and from index 2 to index 4 in the second dimension, you would use the slice [1:3, 2:4].
Combining Slicing Techniques
You can combine multiple slicing techniques to create more complex extractions. For example, to extract elements from the second to the fourth row, skipping every other element, you would use the slice [1::2, :].
Slicing arrays is a fundamental operation in Python for manipulating data structures. By understanding these different slicing methods, you can effectively extract specific portions of arrays for various data analysis and manipulation tasks. Try them, learn them, use them.