here is with numpy 2d array Show Python provides many ways to create 2-dimensional lists/arrays. However one must know the differences between these ways because they can create complications in code that can be very difficult to trace out. Let’s start by looking at common ways of creating 1d array of size N initialized with 0s. Using 2D arrays/lists the right wayMethod 1: Creating a 1-D listExample 1: Creating 1d list Using Naive methods Python3Example 2: creating 1d list using List Comprehension Python3
Explanation: Here we are multiplying the number of rows to the empty list and hence a entire list is created with every element is zero. Method 2 Creating a 2-D listExample 1: Naive Method Python3
Output [[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]] Explanation: Here we are multiplying the number of columns and hence we are getting the 1-D list of size equal to number of columns and then multiplying it with the number of rows which results in the creation of a 2-D list. Example 2: Using List Comprehension Python3
Output [[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]] Explanation: Here we are basically using the concept of list comprehension and applying loop for list inside a list and hence creating a 2-D list. Example 3: Using empty list Python3
Output [[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]] Explanation: Here we are appending zeros as elements for number of columns times and then appending this 1-D list into the empty row list and hence creating the 2-D list. Python3
Output [1, 0, 0, 0, 0] [1, 0, 0, 0, 0] [1, 0, 0, 0, 0] [1, 0, 0, 0, 0] [1, 0, 0, 0, 0] [1, 0, 0, 0, 0] [0, 0, 0, 0, 0] [0, 0, 0, 0, 0] [0, 0, 0, 0, 0] [0, 0, 0, 0, 0] Explanation:We expect only the first element of first row to change to 1 but the first element of every row gets changed to 1 in method 2a. This peculiar functioning is because Python uses shallow lists which we will try to understand. If we assign the 0th index to a another integer say 1, then a new integer object is created with the value of 1 and then the 0th index now points to this new int object as shown below Similarly, when we create a 2d array as “arr = [[0]*cols]*rows” we are essentially the extending the above analogy.
The above setup can be visualized in the image below. Now lets change the first element in first row of “arr” as arr[0][0] = 1
This can be clearly seen in the below image. So when 2d arrays are created like this, changing values at a certain row will effect all the rows since there is essentially only one integer object and only one list object being referenced by the all the rows of the array. As you would expect, tracing out errors caused by such usage of shallow lists is difficult. Hence the better way to declare a 2d array is Python3
Output [[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]] This method creates 5 separate list objects unlike method 2a. One way to check this is using the ‘is’ operator which checks if the two operands refer to the same object. Python3
How do you update a 2D array in python?Updating Values in Python 2D Array
We can update a single element in a 2D array, by specifying its row and column index, and assigning that array position a new value. Assigning a new value overwrites the old value, thus updating it.
Can you change values in an array python?In Python, it is also possible to change multiple elements in an array at once. To do this, you will need to make use of the slice operator and assign the sliced values a new array to replace them.
How do you assign a value to a 2D array in python?Insert.py. # Write a program to insert the element into the 2D (two dimensional) array of Python.. from array import * # import all package related to the array.. arr1 = [[1, 2, 3, 4], [8, 9, 10, 12]] # initialize the array elements.. print("Before inserting the array elements: "). print(arr1) # print the arr1 elements.. |