1. Nested lists: processing and printingIn real-world Often tasks have to store rectangular data table. [say more on this!] Such tables are called matrices or two-dimensional arrays. In Python any table can be represented as a list of lists (a list, where each element is in turn a list). For example, here's the program that creates a numerical table with two rows and three columns, and then makes some manipulations with it: Show a = [[1, 2, 3], [4, 5, 6]] print(a[0]) print(a[1]) b = a[0] print(b) print(a[0][2]) a[0][1] = 7 print(a) print(b) b[2] = 9 print(a[0]) print(b) The first element of To process 2-dimensional array, you typically use nested loops. The first loop iterates through the row number, the second loop runs through the elements inside of a row. For example, that's how you display two-dimensional numerical list on the screen line by line, separating the numbers with spaces: a = [[1, 2, 3, 4], [5, 6], [7, 8, 9]] for i in range(len(a)): for j in range(len(a[i])): print(a[i][j], end=' ') print() We have already tried to explain that a for-loop variable in Python can iterate not only over a a = [[1, 2, 3, 4], [5, 6], [7, 8, 9]] for row in a: for elem in row: print(elem, end=' ') print() Naturally, to output a single line you can use method for row in a: print(' '.join([str(elem) for elem in row])) This is how you can use 2 nested loops to calculate the sum of all the numbers in the 2-dimensional list: a = [[1, 2, 3, 4], [5, 6], [7, 8, 9]] s = 0 for i in range(len(a)): for j in range(len(a[i])): s += a[i][j] print(s) Or the same with iterating by elements, not by the variables
a = [[1, 2, 3, 4], [5, 6], [7, 8, 9]] s = 0 for row in a: for elem in row: s += elem print(s) Advertising by Google, may be based on your interests 2. Nested lists: creatingSuppose that two numbers are given: the number of rows of The obvious solution appears to be wrong: a = [[0] * m] * n This can be easily seen if you set the value of Using our visualizer, keep track of the id of lists. If two lists have the same id number, it's actually the same list in memory. n = 3 m = 4 a = [[0] * m] * n a[0][0] = 5 print(a[1][0]) Thus, a two-dimensional list cannot be created simply by repeating a string. What to do?.. A possible way: you can create a list of
n = 3 m = 4 a = [0] * n for i in range(n): a[i] = [0] * m Another (but similar) way: create an empty list and then n = 3 m = 4 a = [] for i in range(n): a.append([0] * m) But the easiest way
is to use generator, creating a list of n = 3 m = 4 a = [[0] * m for i in range(n)] In this case each element is created independently from the others. The list Advertising by Google, may be based on your interests 3. How do you input a two-dimensional array?Say, a program takes on input two-dimensional array in the form of # the first line of input is the number of rows of the array n = int(input()) a = [] for i in range(n): a.append([int(j) for j in input().split()]) Or, without using sophisticated nested calls: # the first line of input is the number of rows of the array n = int(input()) a = [] for i in range(n): row = input().split() for i in range(len(row)): row[i] = int(row[i]) a.append(row) You can do the same with generators: # the first line of input is the number of rows of the array n = int(input()) a = [[int(j) for j in input().split()] for i in range(n)] Advertising by Google, may be based on your interests 4. Processing a two-dimensional array: an exampleSuppose you are given a square array (an array of 1 0 0 0 2 1 0 0 2 2 1 0 2 2 2 1 (In this case you can do it manually by setting We are eager to show you several ways of solving this problem. First, note that elements that lie above the main diagonal – are elements n = 4 a = [[0] * n for i in range(n)] for i in range(n): for j in range(n): if i < j: a[i][j] = 0 elif i > j: a[i][j] = 2 else: a[i][j] = 1 for row in a: print(' '.join([str(elem) for elem in row])) This algorithm is slow: it uses two loops and for each pair First, fill the main diagonal, for which we will need one loop: for i in range(n): a[i][i] = 1 Then fill with zeros all the elements above the main diagonal. To make this, for each row with the number for i in range(n): for j in range(i + 1, n): a[i][j] = 0 By analogy, for for i in range(n): for j in range(0, i): a[i][j] = 2 You can combine all this code and receive another solution: n = 4 a = [[0] * n for i in range(n)] for i in range(n): for j in range(0, i): a[i][j] = 2 a[i][i] = 1 for j in range(i + 1, n): a[i][j] = 0 for row in a: print(' '.join([str(elem) for elem in row])) Here's another solution, which repeats lists to build the next rows of the list. The n = 4 a = [0] * n for i in range(n): a[i] = [2] * i + [1] + [0] * (n - i - 1) for row in a: print(' '.join([str(elem) for elem in row])) As usual, you can replace the loop with the generator: n = 4 a = [0] * n a = [[2] * i + [1] + [0] * (n - i - 1) for i in range(n)] for row in a: print(' '.join([str(elem) for elem in row])) Advertising by Google, may be based on your interests 5. Two-dimensional arrays: nested generatorsYou can use nested generators to create two-dimensional arrays, placing the generator of the list which is a string, inside the generator of all the strings. Recall that you can create a list of [[0] * m for i in range(n)] But the internal list can also be created using, for example, such generator: [[0 for j in range(m)] for i in range(n)] How is it related to our problem? The thing is, if the number 0 is replaced by some expression that depends on For example, suppose you need to initialize the following array (for convenience, extra spaces are added between items): 0 0 0 0 0 0 0 1 2 3 4 5 0 2 4 6 8 10 0 3 6 9 12 15 0 4 8 12 16 20 In this array there are As always, you could use generator to create such an array: [[i * j for j in range(m)] for i in range(n)] Advertising by Google, may be based on your interests Is list oneThere is only one type of array which should be thought of as a standard list. These are one dimensional arrays. They are called one-dimensional because the data can be viewed entirely in one dimension.
Is a list oneOne-Dimensional Lists - Is when items in the list are linear and you could access them with one index only. One-Dimensional Lists - Is when items in the list are linear and you could access them with more then one index.
Is list multi dimensional?Lists are a very widely use data structure in python. They contain a list of elements separated by comma. But sometimes lists can also contain lists within them. These are called nested lists or multidimensional lists.
What is a 1 dimension array?A One-Dimensional Array is a group of elements having the same data type which are stored in a linear arrangement under a single variable name.
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