This course will introduce the learner to the basics of the python programming environment, including fundamental python programming techniques such as lambdas, reading and manipulating csv files, and the numpy library. The course will introduce data manipulation and cleaning techniques using the popular python pandas data science library and introduce the abstraction of the Series and DataFrame as the central data structures for data analysis, along with tutorials on how to use functions such as groupby, merge, and pivot tables effectively. By the end of this course, students will be able to take tabular data, clean it, manipulate it, and run basic inferential statistical analyses. This course should be taken before any of the other Applied Data Science with Python courses: Applied Plotting, Charting & Data Representation in Python, Applied Machine Learning in Python, Applied Text Mining in Python, Applied Social Network Analysis in Python.
The DataFrame Data Structure
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Do curso por University of Michigan
Introduction to Data Science in Python
10051 classificações
University of Michigan
10051 classificações
Curso 1 de 5 em Specialization Applied Data Science with Python
Na lição
Week 2
In this week of the course you'll learn the fundamentals of one of the most important toolkits Python has for data cleaning and processing -- pandas. You'll learn how to read in data into DataFrame structures, how to query these structures, and the details about such structures are indexed. The module ends with a programming assignment and a discussion question.
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Christopher Brooks
The DataFrame data structure is the heart of the Panda's library.
It's a primary object that you'll be working with in data analysis
and cleaning tasks.
The DataFrame is conceptually a two-dimensional series object, where
there's an index and multiple columns of content, with each column having a label.
In fact, the distinction between a column and
a row is really only a conceptual distinction.
And you can think of the DataFrame itself as simply a two-axes labeled array.
You can create a DataFrame in many different ways,
some of which you might expect.
For instance, you can use a group of series,
where each series represents a row of data.
Or you could use a group of dictionaries,
where each dictionary represents a row of data.
Let's take a look at an example.
I'm going to create three purchase order records as series objects for
a sort of fictional store.
Each series has a name of a customer,
the string which describes the item being purchased, and the cost of the items.
I like dogs, so I'll purchase a bunch of dog food.
Kevin Cullens Thompson, the instructor for the third course in the series,
he seems more like a cat man to me, so I'll have him purchase some kitty litter.
And I think Vinod, who teaches the fourth course in this series,
is more of a bird man, so I'll add some bird seed there.
Then we'll feed this into the DataFrame as the first argument and
set index values indicating which store where each purchase was done.
You'll see that when we print out a DataFrame, the Jupiter notebook's trying
to pretty it up a bit, and print it out as a table, which is nice.
Similar to the series, we can extract data using the iLock and Lock attributes.
Because the DataFrame is two-dimensional, passing a single value to the lock
indexing operator will return series if there's only one row to return.
In this example, if we wanted to select data associated with Store 2,
we would just query the lock attribute with one parameter.
You'll note that the name of the series is returned as the row index value,
while the column name is included in the output as well.
We can check the data type of the return using the python type function.
It's important to remember that the indices and
column names along either axes, horizontal or vertical, could be non-unique.
For instance, in this example, we see two purchase records for
Store 1 as different rows.
If we use a single value with the DataFrame lock attribute, multiple rows of
the DataFrame will return, not as a new series, but as a new DataFrame.
For instance, if we query for Store 1 records,
we see that Chris and Kevin both shop at the same pets supply store.
One of the powers of the Panda's DataFrame is that you can quickly select data based
on multiple axes.
For instance, if you wanted to just list the costs for Store 1, you would supply
two parameters to .log, one being the row index and the other being the column name.
If we're only interested in Store 1 costs, we could write this as df.lock('Store 1',
'Cost').
What if we just wanted to do column selection and
just get a list of all of the costs?
Well, there's a couple of options.
First, you can get a transpose of the DataFrame,
using the capital T attribute, which swaps all of the columns and rows.
This essentially turns your column names into indices.
And we can then use the .lock method.
This works, but it's pretty ugly.
Since iloc and loc are used for row selection, the Panda's developers
reserved indexing operator directly on the DataFrame for column selection.
In a Panda's DataFrame, columns always have a name.
So this selection is always label based, not as confusing as it was when
using the square bracket operator on the series objects.
For those familiar with relational databases,
this operator is analogous to column projection.
Finally, since the result of using the indexing operators,
the DataFrame or series, you can chain operations together.
For instance, we could have rewritten the query for
all Store 1 costs as df.loc('Store 1', 'Cost').
This looks pretty reasonable and gets us the result we wanted.
But chaining can come with some costs and
is best avoided if you can use another approach.
In particular, chaining tends to cause Pandas to return a copy of the DataFrame
instead of a view on the DataFrame.
For selecting a data,
this is not a big deal, though it might be slower than necessary.
If you are changing data though, this is an important distinction and
can be a source of error.
Here's another method.
As we saw, .loc does row selection, and it can take two parameters,
the row index and the list of column names.
.loc also supports slicing.
If we wanted to select all rows,
we can use a column to indicate a full slice from beginning to end.
And then add the column name as the second parameter as a string.
In fact, if we wanted to include multiply columns, we could do so in a list.
And Pandas will bring back only the columns we have asked for.
Here's an example, where we ask for
all of the name and cost values for all stores using the .loc operator.
So that's selecting and projecting data from a DataFrame based on row and
column labels.
The key concepts to remember are that the rows and columns are really just for
our benefit.
Underneath this is just a two axes labeled array, and
transposing the columns is easy.
Also, consider the issue of chaining carefully, and
try to avoid it, it can cause unpredictable results.
Where your intent was to obtain a view of the data, but
instead Pandas returns to you a copy.
In the Panda's world, friends don't let friends chain calls.
So if you see it, point it out, and share a less ambiguous solution.
Now, before we leave the discussion of accessing data in DataFrames,
let's talk about dropping data.
It's easy to delete data in series and DataFrames, and
we can use the drop function to do so.
This function takes a single parameter, which is the index or roll label, to drop.
This is another tricky place for new users to pad this.
The drop function doesn't change the DataFrame by default.
And instead, returns to you a copy of the DataFrame with the given rows removed.
We can see that our original DataFrame is still intact.
Let's make a copy with the copy method and do a drop on it instead.
This is a very typical pattern in Pandas, where in place changes to a DataFrame
are only done if need be, usually on changes involving indices.
So it's important to be aware of.
Drop has two interesting optional parameters.
The first is called in place, and if it's set to true,
the DataFrame will be updated in place, instead of a copy being returned.
The second parameter is the axes, which should be dropped.
By default, this value is 0, indicating the row axes.
But you could change it to 1 if you want to drop a column.
There is a second way to drop a column, however.
And that's directly through the use of the indexing operator, using the del keyword.
This way of dropping data,
however, takes immediate effect on the DataFrame and does not return a view.
Finally, adding a new column to the DataFrame is
as easy as assigning it to some value.
For instance,
if we wanted to add a new location as a column with default value of none,
we could do so by using the assignment operator after the square brackets.
This broadcasts the default value to the new column immediately.
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