April 22, 2019

Python List

Python Collections

There are four collection data types in python programming:

List is ordered collection. List can contain duplicate members. We can change the members of list whenever we want.
Tuple is unordered collection. Tuple can contain duplicate members. Members of tuple cannot be changed.
Set is unordered and unindexed collection. No duplicate members are allowed.
Dictionary is unordered, indexed and changeable collection. Duplicate members are not allowed.

Python lists work like arrays in C. But the main difference between an array and list is that list can contain variables of any data type and as many as desired by the programmer.

List of Contents

How to create a list?
Access list items
Add new items to list
Remove old list items
clear() method to truncate the list
Change the value of list item at the specified index
Looping through the list
How to check if an item exists in the list
Find the list length
How to copy a list
list() constructor
Examples of list

How to create a list?

Lists are created by using [] square brackets. The elements in the list are separated by using a comma (,). We can append any value at the end of list by using the append() method on the list. To access elements in the list use the usual indexing starting from 0.

Example:

theList=["computer", "science","ai"]
	print(theList)

''' Output of above code:-
['computer', 'science', 'ai']
'''

Access list items

You can access the list items by using index numbers.

print(theList[0]) # prints computer
print(theList[1]) # prints science
print(theList[2]) # prints ai

Output:

computer
science
ai

If we try to access an index that does not exist it generates an exception/error.

theList=[10,20,30]
print(theList[5]) # since there are only 3 elements it will generate an error

'''Output:-

Traceback (most recent call last):
  File "", line 2, in 
    print(mylist[10])
IndexError: list index out of range
'''

Add new items to list

Now we will append new data elements at end of list using append() method.

Example:

theList.append("awesome") # appends "awesome" at the end of list
print(theList)

'''Output:-
['computer', 'science', 'ai','awesome']
'''

If you want to add an element at the specified index then you can use the insert() method.

theList.insert(1,"python") # appends "awesome" at the end of list
print(theList)

'''Output:-
['computer', 'python', 'science', 'ai','awesome']
'''

Remove old list items

There are three ways to remove an element from the list which are pop(), remove() and del.

Removing an element from list using pop() method

The pop() method removes an element at specified index. If index is not specified by default it removes the last element of the list. Also pop() returns the element being removed.

Example:

theList=["computer", "science","ai"]
# removes and returns the last element
# as we are neither assinging or printing the returned value it will not be displayed
theList.pop() 
print(theList)

'''Output:-
['computer', 'science']
'''

Removing an element from list using remove() method

The remove() method removes the first occurrence of specified element.

Example:

theList=["computer", "science","ai"]
# removes the element "science"
theList.remove("science")
print(theList)

'''Output:-
['computer','ai']
'''

Removing an element using del

del keyword removes the element at the specified index.

Example:

theList=["computer", "science","ai"]
# removes the element at index 0
del theList[0]
print(theList)

'''Output:-
['science','ai']
'''

We can also use the del keyword to delete the list completly.

Example:

theList=["computer", "science","ai"]
# deletes the list entirely
del theList
print(theList) #if you try to print the list it will generate an error

'''Output:-

Traceback (most recent call last):
  File "main.py", line 4, in 
    print(theList)
NameError: name 'theList' is not defined
'''

clear() method to truncate the list

We can use the clear() method to delete all the elements of the list. This makes the list empty. This does not delete the list.

theList=["computer", "science","ai"]
theList.clear()
print(theList) # print only square brackets as there are no elements left

'''Output:-
['science','ai']
'''

Change the value of list item at the specified index

We can change the value of the list item using the usual indexing.

Example:

theList=["computer", "science","ai"]
theList[1] = "programming"
print(theList)

'''Output:-
['computer','programming','ai']
'''

Looping through the list

You can use loop throught the list using any loop available in python. The simplest way is to use the for loop.

Example:

theList=["computer", "science","ai"]

for x in theList:
	print(x)

'''Output:-
computer
science
ai'''

How to check if an item exists in the list

The in keyword can be used to check if an item is present in the list.

Example:

theList=["computer", "science","ai"]

for x in theList:
	if "computer" in theList:
		print("The Word 'Computer' is present in the list")

'''Output:-
The Word 'Computer' is present in the list
'''

Find the list length

To calculate how many elements are present in the list, we can use the len() method.

Example:

theList=["computer", "science","ai"]
print(len(theList))

'''Output:-
3
'''

How to copy a list

We cannot use something like list2 = list1 to copy the list. This will only save the reference to list1 in list2 variable. So any changes made to list1 will also reflect in list2. So we use the built-in copy() method to copy one list to another.

Example:

theList=["computer", "science","ai"]
theList2 = theList.copy()
print(theList2)

'''Output:-
["computer", "science","ai"]
'''

We can also use another built-in method list() to copy the list.

theList=["computer", "science","ai"]
theList2 = list(theList)
print(theList2)

'''Output:-
["computer", "science","ai"]
'''

list() constructor

We can also make a new list using the list() constrtuctor.

Example:

# note the double parenthesis
theList = list(("python", "programming","list"))
print(theList)

'''Output:-
["python", "programming","list"]
'''

Examples of Lists

List of strings

This is the example of string list.

names_of_languages=["java","python","rust"]
print(student_names[0])
print(student_names[1])
print(student_names[2])

Output of above code:

java
python
rust

List of mixed data types

According to the definition of the list, it can contain elements of different data types so we can create a mixed data type list as given below

mixedList=["Amar",120,240.01]
print(mixedList[0])
print(mixedList[1])
print(mixedList[2])

Output of above code:

Amar
120
240.01

List containing another list

Lists in python can also contain other lists possibly of different data types.

mixedList=[1,2,3,(10,20,30),120,"python"]
print(mixedList[0])
print(mixedList[3])
print(mixedList[3][1])

Output of above code:

1
(10,20,30)
20