# (Python Basic Tutorial VIII) list operation in Python

Posted by smallflower on Wed, 06 May 2020 03:17:12 +0200

stay In Python , the list is:

• order
• Index (index starts at 0)
• Variable
• Heterogeneous (items in the list do not have to be of the same type)
• A comma-separated list of values written between square brackets
``````listOfSubjects = ['physics', 'chemistry', "mathematics"]
listOfIds = [0, 1, 2, 3, 4]
miscList = [0, 'one', 2, 'three']
``````

## 1. Access list items

To access the values in the list, use the slicing syntax or square brackets in the form of an array index to get a single item or item range.

The index value passed can be positive or negative.If the index is negative, the count starts at the end of the list.

List [m: n] indicates that the sublist begins with index m (including) and ends with index n (excluding).

• If m is not provided, its value is assumed to be zero.
• If n is not provided, select the range to the end of the list.
``````ids = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print( ids )			# 0
print( ids[1:5] )		# [1, 2, 3, 4]
print( ids[ : 3] )		# [0, 1, 2]
print( ids[7 : ] )		# [7, 8, 9]\
print( ids[-8:-5] )		# [2, 3, 4]
``````

## 2. Modily list

To change a specific item in the list, use its index to reference the item and assign a new value.

``````charList =  ["a", "b", "c"]
charList  = "d"
print (charList)	# ['a', 'b', 'd']
``````

## 3. Iterate a list

We can use it to iterate through the list item for loop.

``````charList =  ["a", "b", "c"]

for x in charList:
print(x)

# a
# b
# c
``````

## 4. Check if a item exists in the list

Use the'in'keyword to determine if the specified item exists in the list.

``````charList =  ["a", "b", "c"]
if "a" in charList:
print("a is present")	# a is present

if "d" in charList:
print("d is present")
else:
print("d is NOT present")	# d is NOT present
``````

## 5. Finding length of the list

Use the len() function to find the length of a given list.

``````charList =  ["a", "b", "c"]
x = len (charList)
print (x)	# 3
``````

• To add an item to the end of the list, use the append(item) method.
• To add an item at a specific index location, use the insert(index, item) method.If the index is larger than the index length, the item is added to the end of the list.
``````charList =  ["a", "b", "c"]
charList.append("d")
charList.append("e")
print (charList)		# ['a', 'b', 'c', 'd', 'e']
charList.insert(5, "f")

print (charList)		# ['a', 'b', 'c', 'd', 'e', 'f']

charList.insert(10, "h")	# No error

print (charList)	# ['a', 'b', 'c', 'd', 'e', 'f', 'h']
``````

## 7. Removing items

To delete an item from the list, use one of the four ways: remove(), pop(), clear(), or del keywords.

#### 7.1. remove()

It deletes the specified item from its value.

``````charList =  ["a", "b", "c"]
charList.remove("c")
print (charList)		# ['a', 'b']
``````

#### 7.2. pop()

It deletes the specified item through the index.If no index is provided, it will delete the last item from the list.

``````charList =  ["a", "b", "c", "d"]
charList.pop()			# removes 'd' - last item
print (charList)		# ['a', 'b', 'c']
charList.pop(1)			# removes 'b'
print (charList)		# ['a', 'c']
``````

#### 7.3. clear()

It clears the list.

``````charList =  ["a", "b", "c", "d"]
charList.clear()
print (charList)		# []
``````

#### 7.4. del keyword

It can be used to delete items from the index of a list.We can also use it to delete the entire list.

``````charList =  ["a", "b", "c", "d"]
del charList

print (charList)		# ['b', 'c', 'd']

del charList

print (charList)		# NameError: name 'charList' is not defined
``````

## 8. Join two lists

We can add two given lists to Python using the'+'operator or the extend() function.

``````charList = ["a", "b", "c"]
numList	= [1, 2, 3]

list1 = charList + numList

print (list1)	# ['a', 'b', 'c', 1, 2, 3]

charList.extend(numList)

print (charList)	# ['a', 'b', 'c', 1, 2, 3]
``````

## 9. Python list methods

#### 9.1. append()

Add an element at the end of the list.

``````charList =  ["a", "b", "c"]

charList.append("d")

print (charList)	# ["a", "b", "c", "d"]
``````

#### 9.2. clear()

Remove all elements from the list.

``````charList =  ["a", "b", "c"]

charList.clear()

print (charList)	# []
``````

#### 9.3. copy()

Returns a copy of the list.

``````charList =  ["a", "b", "c"]

newList = charList.copy()

print (newList)	# ["a", "b", "c"]
``````

#### 9.4. count()

Returns the number of elements with a specified value.

``````charList =  ["a", "b", "c"]

x = charList.count('a')

print (x)	# 1
``````

#### 9.5. extend()

Add the elements of the list to the end of the current list.

``````charList = ["a", "b", "c"]
numList	= [1, 2, 3]

charList.extend(numList)

print (charList)	# ['a', 'b', 'c', 1, 2, 3]
``````

#### 9.6. index()

Returns the index of the first element with the specified value.

``````charList =  ["a", "b", "c"]

x = charList.index('a')

print (x)	# 0
``````

#### 9.7. insert()

Adds an element at a specified location.

``````charList =  ["a", "b", "c"]

charList.insert(3, 'd')

print (charList)	# ['a', 'b', 'c', 'd']
``````

#### 9.8. pop()

Delete the element at the specified location or end of the list.

``````charList =  ["a", "b", "c", "d"]

charList.pop()			# removes 'd' - last item

print (charList)		# ['a', 'b', 'c']

charList.pop(1)			# removes 'b'

print (charList)		# ['a', 'c']
``````

#### 9.9. remove()

Deletes an item with a specified value.

``````charList =  ["a", "b", "c", "d"]

charList.remove('d')

print (charList)		# ['a', 'b', 'c']
``````

#### 9.10. reverse()

Reverse the order of items in the list.

``````charList =  ["a", "b", "c", "d"]

charList.reverse()

print (charList)		# ['d', 'c', 'b', 'a']
``````

#### 9.11. sort()

By default, the given list is sorted in ascending order.

``````charList =  ["a", "c", "b", "d"]

charList.sort()

print (charList)		# ["a", "b", "c", "d"]
``````

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