Combine Data Frame

  1. While merge is a module function.join is an object function that lives on your DataFrame. This enables you to specify only one DataFrame, which will join the DataFrame you call.join on. Under the hood.join uses merge , but it provides a more efficient way to.
  2. We have a method called pandas.merge that merges dataframes similar to the database join operations. Follow the below steps to achieve the desired output. Merge method uses the common column for the merge operation.
  3. To combine data frames based on a common column (s), i.e., adding columns of second data frame to the first data frame with respect to a common column (s), you can use merge function. To combine data frames: with rows of second data frame added to those of.

34 Reshaping and Joining Data Frames Gathering for Tidiness. The gather function in the tidyr package makes most untidy data frames tidier. Ungathering with spread. While this organization of the data—with each row being an observation and each column being. Splitting Columns. Groupbys and split-apply-combine to answer the question. Now that you've checked out out data, it's time for the fun part. You'll first use a groupby method to split the data into groups, where each group is the set of movies released in a given year.



  • Can I work with data from multiple sources?

  • How can I combine data from different data sets?

  • Combine data from multiple files into a single DataFrame using merge and concat.

  • Combine two DataFrames using a unique ID found in both DataFrames.

  • Employ to_csv to export a DataFrame in CSV format.

  • Join DataFrames using common fields (join keys).

In many “real world” situations, the data that we want to use come in multiplefiles. We often need to combine these files into a single DataFrame to analyzethe data. The pandas package provides various methods for combiningDataFrames includingmerge and concat.

To work through the examples below, we first need to load the species andsurveys files into pandas DataFrames. In iPython:

Take note that the read_csv method we used can take some additional options whichwe didn’t use previously. Many functions in Python have a set of options thatcan be set by the user if needed. In this case, we have told pandas to assignempty values in our CSV to NaN keep_default_na=False, na_values=['].More about all of the read_csv options here.

We can use the concat function in pandas to append either columns or rows fromone DataFrame to another. Let’s grab two subsets of our data to see how thisworks.

When we concatenate DataFrames, we need to specify the axis. axis=0 tellspandas to stack the second DataFrame UNDER the first one. It will automaticallydetect whether the column names are the same and will stack accordingly.axis=1 will stack the columns in the second DataFrame to the RIGHT of thefirst DataFrame. To stack the data vertically, we need to make sure we have thesame columns and associated column format in both datasets. When we stackhorizontally, we want to make sure what we are doing makes sense (i.e. the data arerelated in some way).

Row Index Values and Concat

Have a look at the vertical_stack dataframe? Notice anything unusual?The row indexes for the two data frames survey_sub and survey_sub_last10have been repeated. We can reindex the new dataframe using the reset_index() method.

Writing Out Data to CSV

We can use the to_csv command to do export a DataFrame in CSV format. Note that the codebelow will by default save the data into the current working directory. We cansave it to a different folder by adding the foldername and a slash to the filevertical_stack.to_csv('foldername/out.csv'). We use the ‘index=False’ so thatpandas doesn’t include the index number for each line.

Check out your working directory to make sure the CSV wrote out properly, andthat you can open it! If you want, try to bring it back into Python to make sureit imports properly.

Challenge - Combine Data

In the data folder, there are two survey data files: surveys2001.csv andsurveys2002.csv. Read the data into Python and combine the files to make onenew data frame. Create a plot of average plot weight by year grouped by sex.Export your results as a CSV and make sure it reads back into Python properly.

When we concatenated our DataFrames we simply added them to each other -stacking them either vertically or side by side. Another way to combineDataFrames is to use columns in each dataset that contain common values (acommon unique id). Combining DataFrames using a common field is called“joining”. The columns containing the common values are called “join key(s)”.Joining DataFrames in this way is often useful when one DataFrame is a “lookuptable” containing additional data that we want to include in the other.


NOTE: This process of joining tables is similar to what we do with tables in anSQL database.

For example, the species.csv file that we’ve been working with is a lookuptable. This table contains the genus, species and taxa code for 55 species. Thespecies code is unique for each line. These species are identified in our surveydata as well using the unique species code. Rather than adding 3 more columnsfor the genus, species and taxa to each of the 35,549 line Survey data table, wecan maintain the shorter table with the species information. When we want toaccess that information, we can create a query that joins the additional columnsof information to the Survey data.

Storing data in this way has many benefits including:

  1. It ensures consistency in the spelling of species attributes (genus, speciesand taxa) given each species is only entered once. Imagine the possibilitiesfor spelling errors when entering the genus and species thousands of times!
  2. It also makes it easy for us to make changes to the species information oncewithout having to find each instance of it in the larger survey data.
  3. It optimizes the size of our data.

Joining Two DataFrames

To better understand joins, let’s grab the first 10 lines of our data as asubset to work with. We’ll use the .head method to do this. We’ll also readin a subset of the species table.

In this example, species_sub is the lookup table containing genus, species, andtaxa names that we want to join with the data in survey_sub to produce a newDataFrame that contains all of the columns from both species_dfandsurvey_df.

Identifying join keys

To identify appropriate join keys we first need to know which field(s) areshared between the files (DataFrames). We might inspect both DataFrames toidentify these columns. If we are lucky, both DataFrames will have columns withthe same name that also contain the same data. If we are less lucky, we need toidentify a (differently-named) column in each DataFrame that contains the sameinformation.

In our example, the join key is the column containing the two-letter speciesidentifier, which is called species_id.

Now that we know the fields with the common species ID attributes in eachDataFrame, we are almost ready to join our data. However, since there aredifferent types of joins, wealso need to decide which type of join makes sense for our analysis.

Combine data frames

Inner joins

The most common type of join is called an inner join. An inner join combinestwo DataFrames based on a join key and returns a new DataFrame that containsonly those rows that have matching values in both of the originalDataFrames.

Combine Two Data Frames

Inner joins yield a DataFrame that contains only rows where the value beingjoined exists in BOTH tables. An example of an inner join, adapted from Jeff Atwood’s blogpost about SQL joins is below:

The pandas function for performing joins is called merge and an Inner join isthe default option:

The result of an inner join of survey_sub and species_sub is a new DataFramethat contains the combined set of columns from survey_sub and species_sub. Itonly contains rows that have two-letter species codes that are the same inboth the survey_sub and species_sub DataFrames. In other words, if a row insurvey_sub has a value of species_id that does not appear in the species_idcolumn of species, it will not be included in the DataFrame returned by aninner join. Similarly, if a row in species_sub has a value of species_idthat does not appear in the species_id column of survey_sub, that row will notbe included in the DataFrame returned by an inner join.

The two DataFrames that we want to join are passed to the merge function usingthe left and right argument. The left_on='species' argument tells mergeto use the species_id column as the join key from survey_sub (the leftDataFrame). Similarly , the right_on='species_id' argument tells merge touse the species_id column as the join key from species_sub (the rightDataFrame). For inner joins, the order of the left and right arguments doesnot matter.

The result merged_inner DataFrame contains all of the columns from survey_sub(record id, month, day, etc.) as well as all the columns from species_sub(species_id, genus, species, and taxa).

Notice that merged_inner has fewer rows than survey_sub. This is anindication that there were rows in surveys_df with value(s) for species_id thatdo not exist as value(s) for species_id in species_df.

Left joins

What if we want to add information from species_sub to survey_sub withoutlosing any of the information from survey_sub? In this case, we use a differenttype of join called a “left outer join”, or a “left join”.

Like an inner join, a left join uses join keys to combine two DataFrames. Unlikean inner join, a left join will return all of the rows from the leftDataFrame, even those rows whose join key(s) do not have values in the rightDataFrame. Rows in the left DataFrame that are missing values for the joinkey(s) in the right DataFrame will simply have null (i.e., NaN or None) valuesfor those columns in the resulting joined DataFrame.

Note: a left join will still discard rows from the right DataFrame that do nothave values for the join key(s) in the left DataFrame.

A left join is performed in pandas by calling the same merge function used forinner join, but using the how='left' argument:

The result DataFrame from a left join (merged_left) looks very much like theresult DataFrame from an inner join (merged_inner) in terms of the columns itcontains. However, unlike merged_inner, merged_left contains the samenumber of rows as the original survey_sub DataFrame. When we inspectmerged_left, we find there are rows where the information that should havecome from species_sub (i.e., species_id, genus, and taxa) ismissing (they contain NaN values):

Combine data frames r

These rows are the ones where the value of species_id from survey_sub (in thiscase, PF) does not occur in species_sub.

Other join types

The pandas merge function supports two other join types:

  • Right (outer) join: Invoked by passing how='right' as an argument. Similarto a left join, except all rows from the right DataFrame are kept, whilerows from the left DataFrame without matching join key(s) values arediscarded.
  • Full (outer) join: Invoked by passing how='outer' as an argument. This jointype returns the all pairwise combinations of rows from both DataFrames; i.e.,the result DataFrame will NaN where data is missing in one of the dataframes. This join type isvery rarely used.

Challenge - Distributions

Create a new DataFrame by joining the contents of the surveys.csv andspecies.csv tables. Then calculate and plot the distribution of:

  1. taxa by plot
  2. taxa by sex by plot

Challenge - Diversity Index

  1. In the data folder, there is a plots.csv file that contains information about thetype associated with each plot. Use that data to summarize the number ofplots by plot type.
  2. Calculate a diversity index of your choice for control vs rodent exclosureplots. The index should consider both species abundance and number ofspecies. You might choose to use the simple biodiversity index describedherewhich calculates diversity as:

    the number of species in the plot / the total number of individuals in the plot = Biodiversity index.

Key Points

Combine Data Frames In A List R

  • Pandas’ merge and concat can be used to combine subsets of a DataFrame, or even data from different files.

  • join function combines DataFrames based on index or column.

  • Joining two DataFrames can be done in multiple ways (left, right, and inner) depending on what data must be in the final DataFrame.

  • to_csv can be used to write out DataFrames in CSV format.

WIP Alert This is a work in progress. Current information is correct but more content may be added in the future.

View examples on this jupyter notebook

pd.merge example

The default is an inner join. Use 'how'='left' 'right' 'outer' to change join types.

pd.merge by index

Joins by index are much faster than join on arbitrary columns!

If the columns you want to join on are Indices, use left_index and right_index.

Use 'how'='left' 'right' 'outer' to change join types. The default join type is 'left':

Join on multiple columns

Joining by multiple columns is useful for dealing with time-stamped data.

Just pass an array of column names to left_on and right_on:

RIGHT: companies and
annual revenue

merge vs join

Joining by index (using df.join) is much faster than joins on arbtitrary columns!

The difference between dataframe.merge() and dataframe.join() is that with dataframe.merge() you can join on any columns, whereas dataframe.join() only lets you join on index columns.

pd.merge() vs dataframe.join() vs dataframe.merge()

TL;DR: pd.merge() is the most generic. df.merge() is the same as pd.merge() with an implicit left dataframe. Use df.join() for merging on index columns exclusively. df.join is much faster because it joins by index

These are three different ways to do merging/joining dataframes on pandas: Combine data frames pandas
How to callPandas global methodDataframe methodDataframe method
Join onJoin on any columnJoin on index columns onlyJoin on any column
PerformanceSlow unless using indicesFast!Slow unless using indices
Examplepd.merge(left_df, right_df)left_df.join(right_df)left_df.merge(right_df)
Equivalent to
(i.e. how to write it
using pd.merge)

Rename duplicate columns

Pass suffix=(<suffix_for_left>,<suffix_for_right>) to pd.merge():

RIGHT: companies and
annual revenue
MERGED: Newly named columns are left (orange) and right (blue)
Note that column year was not duplicated,
pandas correctly identified it
was the same in both sides.

merge_asof example

Combine Data Frames Python



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