Cara menggunakan python 2d list length

Multi-dimensional lists in Python

There can be more than one additional dimension to lists in Python. Keeping in mind that a list can hold other lists, that basic principle can be applied over and over. Multi-dimensional lists are the lists within lists. Usually, a dictionary will be the better choice rather than a multi-dimensional list in Python.

Table of Contents

• Multi-dimensional lists in Python
• Python | Using 2D arrays/lists the right way
• Lesson 9Two-dimensional lists (arrays)
• Python 2D array
• Two-Dimensional Array (2D Array)
• Access Two-Dimensional Array
• Traversing the element in 2D (two dimensional)
• Insert elements in a 2D (Two Dimensional) Array
• Update elements in a 2 -D (Two Dimensional) Array
• Delete values from a 2D (two Dimensional) array in Python
• Size of a 2D array
• Python - 2-D Array
• Graphical Representation
• How to declare/initialize a 2d python list

Accessing a multidimensional list:

Approach 1:

# Python program to demonstrate printing

# of complete multidimensional list

a = [[2, 4, 6, 8, 10], [3, 6, 9, 12, 15], [4, 8, 12, 16, 20]]

print(a)

Output: [[2, 4, 6, 8, 10], [3, 6, 9, 12, 15], [4, 8, 12, 16, 20]]

Approach 2: Accessing with the help of loop.

# Python program to demonstrate printing

# of complete multidimensional list row

# by row.

a = [[2, 4, 6, 8, 10], [3, 6, 9, 12, 15], [4, 8, 12, 16, 20]]

for record in a:

print(record)

Output:

Approach 3: Accessing using square brackets.
Example:

# Python program to demonstrate that we

# can access multidimensional list using

# square brackets

a = [ [2, 4, 6, 8 ],

[ 1, 3, 5, 7 ],

[ 8, 6, 4, 2 ],

[ 7, 5, 3, 1 ] ]

for i in range(len(a)) :

for j in range(len(a[i])) :

print(a[i][j], end=" ")

print()

Output: 2 4 6 8 1 3 5 7 8 6 4 2 7 5 3 1

Creating a multidimensional list with all zeros:

# Python program to create a m x n matrix

# with all 0s

m = 4

n = 5

a = [[0 for x in range(n)] for x in range(m)]

print(a)

Output: [[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]

Methods on Multidimensional lists

1. append(): Adds an element at the end of the list.
Example:

# Adding a sublist

a = [[2, 4, 6, 8, 10], [3, 6, 9, 12, 15], [4, 8, 12, 16, 20]]

a.append([5, 10, 15, 20, 25])

print(a)

Output: [[2, 4, 6, 8, 10], [3, 6, 9, 12, 15], [4, 8, 12, 16, 20], [5, 10, 15, 20, 25]]

2. extend(): Add the elements of a list (or any iterable), to the end of the current list.

# Extending a sublist

a = [[2, 4, 6, 8, 10], [3, 6, 9, 12, 15], [4, 8, 12, 16, 20]]

a[0].extend([12, 14, 16, 18])

print(a)

Output: [[2, 4, 6, 8, 10, 12, 14, 16, 18], [3, 6, 9, 12, 15], [4, 8, 12, 16, 20]]

3. reverse(): Reverses the order of the list.

# Reversing a sublist

a = [[2, 4, 6, 8, 10], [3, 6, 9, 12, 15], [4, 8, 12, 16, 20]]

a[2].reverse()

print(a)

Output: [[2, 4, 6, 8, 10], [3, 6, 9, 12, 15], [20, 16, 12, 8, 4]]

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Python | Using 2D arrays/lists the right way

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. Lets start by looking at common ways of creating 1d array of size N initialized with 0s.
Method 1a

Python3

# First method to create a 1 D array

N = 5

arr = [0]*N

print(arr)

Output: [0, 0, 0, 0, 0]

Method 1b

Python3

# Second method to create a 1 D array

N = 5

arr = [0 for i in range(N)]

print(arr)

Output: [0, 0, 0, 0, 0]

Extending the above we can define 2-dimensional arrays in the following ways.
Method 2a

Python3

# Using above first method to create a

# 2D array

rows, cols = (5, 5)

arr = [[0]*cols]*rows

print(arr)

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]]

Method 2b

Python3

# Using above second method to create a

# 2D array

rows, cols = (5, 5)

arr = [[0 for i in range(cols)] for j in range(rows)]

print(arr)

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]]

Method 2c

Python3

# Using above second method to create a

# 2D array

rows, cols = (5, 5)

arr=[]

for i in range(rows):

col = []

for j in range(cols):

col.append(0)

arr.append(col)

print(arr)

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]]

Both the ways give seemingly same output as of now. Lets change one of the elements in the array of method 2a and method 2b.

Python3

# Python 3 program to demonstrate working

# of method 1 and method 2.

rows, cols = (5, 5)

# method 2a

arr = [[0]*cols]*rows

# lets change the first element of the

# first row to 1 and print the array

arr[0][0] = 1

for row in arr:

print(row)

# outputs the following

#[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]

# method 2b

arr = [[0 for i in range(cols)] for j in range(rows)]

# again in this new array lets change

# the first element of the first row

# to 1 and print the array

arr[0][0] = 1

for row in arr:

print(row)

# outputs the following as expected

#[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]

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]

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.
In method 1a, Python doesn’t create 5 integer objects but creates only one integer object and all the indices of the array arr point to the same int object as shown.

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.
1. Only one integer object is created.
2. A single 1d list is created and all its indices point to the same int object in point 1.
3. Now, arr[0], arr[1], arr[2] …. arr[n-1] all point to the same list object above in point 2.
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
=> arr[0] points to the single list object we created we above.(Remember arr[1], arr[2] …arr[n-1] all point to the same list object too)
=> The assignment of arr[0][0] will create a new int object with the value 1 and arr[0][0] will now point to this new int object.(and so will arr[1][0], arr[2][0] …arr[n-1][0])
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

rows, cols = (5, 5)

arr = [[0 for i in range(cols)] for j in range(rows)]

Output:

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

rows, cols = (5, 5)

# method 2b

arr = [[0 for i in range(cols)] for j in range(rows)]

# check if arr[0] and arr[1] refer to

# the same object

print(arr[0] is arr[1]) # prints False

# method 2a

arr = [[0]*cols]*rows

# check if arr[0] and arr[1] refer to

# the same object

# prints True because there is only one

# list object being created.

print(arr[0] is arr[1])

Output: False True

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Lesson 9Two-dimensional lists (arrays)

• Theory
• Steps
• Problems

Python 2D array

An array is a collection of linear data structures that contain all elements of the same data type in contiguous memory space. It is like a container that holds a certain number of elements that have the same data type. An array's index starts at 0, and therefore, the programmer can easily obtain the position of each element and perform various operations on the array. In this section, we will learn about 2D (two dimensional) arrays in Python.

Two-Dimensional Array (2D Array)

A 2D array is an array of arrays that can be represented in matrix form like rows and columns. In this array, the position of data elements is defined with two indices instead of a single index.

Syntax

Array_name = [rows][columns] # declaration of 2D array Arr-name = [ [m1, m2, m3, … . mn], [n1, n2, n3, … .. nn] ]

Where m is the row and n is the column of the table.

Access Two-Dimensional Array

In Python, we can access elements of a two-dimensional array using two indices. The first index refers to the indexing of the list and the second index refers to the position of the elements. If we define only one index with an array name, it returns all the elements of 2-dimensional stored in the array.

Let's create a simple program to understand 2D (two dimensional) arrays in Python.

2dSimple.py

Output:

In the above example, we passed 1, 0, and 2 as parameters into 2D array that prints the entire row of the defined index. And we have also passed student_dt[3][4] that represents the 3rd index and 4th position of a 2-dimensional array of elements to print a particular element.

Traversing the element in 2D (two dimensional)

Program.py

Output:

Insert elements in a 2D (Two Dimensional) Array

We can insert elements into a 2 D array using the insert() function that specifies the element' index number and location to be inserted.

Insert.py

Output:

Update elements in a 2 -D (Two Dimensional) Array

In a 2D array, the existing value of the array can be updated with a new value. In this method, we can change the particular value as well as the entire index of the array. Let's understand with an example of a 2D array, as shown below.

Create a program to update the existing value of a 2D array in Python.

Update.py

Output:

Delete values from a 2D (two Dimensional) array in Python

In a 2- D array, we can remove the particular element or entire index of the array using del() function in Python. Let's understand an example to delete an element.

Delete.py

Output:

Size of a 2D array

A len() function is used to get the length of a two-dimensional array. In other words, we can say that a len() function determines the total index available in 2-dimensional arrays.

Let's understand the len() function to get the size of a 2-dimensional array in Python.

Size.py

Output:

Write a program to print the sum of the 2-dimensional arrays in Python.

Matrix.py

Output:

Next TopicPython Memory Management

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Python - 2-D Array

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Two dimensional array is an array within an array. It is an array of arrays. In this type of array the position of an data element is referred by two indices instead of one. So it represents a table with rows an dcolumns of data.

In the below example of a two dimensional array, observer that each array element itself is also an array.

Consider the example of recording temperatures 4 times a day, every day. Some times the recording instrument may be faulty and we fail to record data. Such data for 4 days can be presented as a two dimensional array as below.

The above data can be represented as a two dimensional array as below.

Graphical Representation

Graphically a 2d list can be represented in the form of a grid.

How to declare/initialize a 2d python list

There are multiple ways to initialize a 2d list in python.

1. This is the basic approach for creating a 2d list in python.

rows=[] columns=[] for i in range(3): for j in range(3): columns.append(1) rows.append(columns)

OUTPUT-

• Suppose you want to create a 2d list with coordinates as its values.

x = [[(i,j) for j in range(3)] for i in range(3)]

• If we want to create a 2d list with 0 as its value:

x = [[0 for j in range(3)] for i in range(3)]

Output-

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