No Fluff Guide to Python - P5 - Lists

By -

 

“To build up a library is to create a life. It's never just a random collection of books.”

 Imagine you have a box of toys. You can put different toys in the box, like a doll, a car, and a ball. Each toy has its own place in the box. You can also take toys out of the box and play with them.

A Python array is like a box of toys, but it can only hold numbers. Each number has its own place in the array. You can also add and remove numbers from the array.

To create an array, you use the [ ] brackets. For example, the following code creates an array with the numbers 1, 2, and 3:

array = [1, 2, 3]


You can access the numbers in the array using the [ ] brackets. For example, the following code prints the first number in the array:

array = [1, 2, 3]

print(array[0])


Each item in the list is assigned an Index, starting from 0


You can access individual item using these indexes:


You can also change the numbers in the array using these indexes.

For example, the following code changes the first value in the array to "morning":

list1=["hello",1,4,8,"good"]
list1[0]="morning"   
New List
["morning",1,4,8,"good"] 


Update the 3rd item in the list, i.e. index 2

New list looks like this:


You can also add and remove numbers from the array. To add a number to the end of the array, you use the append() method. 

 Code looks like this:

list1=["apple","banana","grapes"]
list1.append("strawberry")   
# strawberry is added to the list
print(list1)


#Output
#['apple', 'banana', 'grapes', 'strawberry']


# LIST - Mutable ## Update - ```py LST[2] = "e" ``` ## Append - ```py LST.append("d") ``` - Check if it exists `"d" in LIST` ## Convert - ```py NEW_LST = list(oldlist) ```

To remove a number from the array, you use the `pop()` method. For example, the following code removes the last number from the array:
list1.pop()  
# removes the last element from the list
print(list1)


#Output
#['apple', 'banana', 'grapes']
# LIST - Add ## Extend - End of List - ```py LST.Extend(Range(3)); LST.Extend(LST2); ``` ## End of List - ```py LST.append("d") ``` ## Add at Index - ```py LST.insert(1, "V") ```


Delete vs Remove


Remove elements:

# LIST - Remove ## Empty - ```py LST.clear(); ``` ## by Value - ```py LST.remove("d") ``` ## by Index - Remove and Return - ```py LST.pop(3) ```


Sorting elements:

SORT

  • orders the values in the lists
  • sorting is done - In Place
  • updates are done to the list itself without making another copy


SORTED

  • use this if the original order is important
  • makes a copy of it before sorting -> by creating a new list


Looping through all elements of a LIST:

  • use FOR loop
  • a named variable is assigned all the values one by one
    • python creates this variable 
    • this variable holds the actual value, rather than the index
  • VAR is assigned LIST1[0] in the first run
  • VAR is assigned LIST1[1] in the next run.. and so on
  • You can access the current element using the VAR variable.

Note:

the value of the variable used in FOR loop is not cleared after the loop is complete.

So, if you run print command again, the last value will be printed:


Arrays are a very useful tool for storing and manipulating data in Python. They can be used to store anything from numbers to strings to objects.


 

 





Comments

Post a Comment

Popular posts from this blog

Index Clean-Up Scripts

By -
dropping un-used ones, adding missing ones, and fine-tuning the ones I already have. I thought I’d share some of the scripts I’ve been using to accomplish this. Un-Used Indexes Script Declare @dbid int     , @dbName varchar(100);   Select @dbid = DB_ID()     , @dbName = DB_Name();   With partitionCTE (object_id, index_id, row_count, partition_count)  As (     Select [object_id]         , index_id         , Sum([rows]) As 'row_count'         , Count(partition_id) As 'partition_count'     From sys.partitions     Group By [object_id]         , index_id )    Select Object_Name(i.[object_id]) as objectName         , i.name         , Case              When i.is_unique = 1          ...
Read more

forgot sa password and no logins are added

By -
Image
Stop the SQL services Edit the properties of the SQL Service Change the startup parameters of the SQL service by adding a  –m;  in front of the existing parameters Start the SQL services. These are now running in Single User Mode. Start CMD on tthe SQL server Start the SQLCMD command. Now we create a new user. Enter following commands CREATE LOGIN recovery WITH PASSWORD = ‘TopSecret 1′ (Remember SQL server has default strong password policy Go Now this user is created Now we grant the user a SYSADMIN roles using the same SQLCMD window. sp_addsrvrolemember ‘recovery’, ‘sysadmin’ go
Read more

The SQL Server DBA’s Guide to Teradata

By -
Being a long-time devotee of all things Microsoft SQL Server, I had quite a bit of learning to do to get up to speed on our new Teradata appliance. This guide attempts to distill some of what I have learned. I will primarily focus on what I have found most lacking: examples of how to convert SQL Server T-SQL to Teradata SQL. This guide will use the vendor-supplied sample databases:  SQL Server’s AdventureWorksDW2012 , and the  Teradata’s Express 14.0 Retail database . Also, while this is probably self explanatory, the “mufford” object referenced in the Teradata examples is simply my own personal sandbox. You’ll need to replace any “mufford” reference to whatever your development sandbox may be named. Release History: 2012-08-17 – Added UPDATE examples; reformatted 2012-08-12 – Added date examples 2012-08-08 – Initial Release SELECT Statements Simple SELECT SQL Server: Best practice is to qualify the table schema; qualifying the database is optional unless ...
Read more