Pandas DataFrame selection using text from another DataFrame

Question

A quick disclosure: I come from R background and am switching to pandas (running on python 3.3.3).

I would like to select rows from a dataframe by using text from a dataframe entry. It's an elementry operation but I could not get around the syntax.

For example, with this DataFrame (sorry for the line split but I want to make the example clearer):

films = pandas.DataFrame({'$title':[  "The Godfather",
                                      "Pulp Fiction",
                                      "The Godfather: Part II",
                                      "Fight Club"],

                      '$director': [  "Coppola, Francis Ford",
                                      "Tarantino, Quentin",
                                      "Coppola, Francis Ford",
                                      "Fincher, David"]})

If I want to select all the films created by the first director, which would be "Coppola, Francis Ford", the command I am using is:

In [1]: director = films.iloc[[1]]["director"]

In [2]: director

        1    Coppola, Francis Ford
        Name: director, dtype: object

In [3]: a = films[ films["director"] == director ]

        ValueError: Series lengths must match to compare

If I do this:

In [4]: a = films[ films["director"] == str(director) ]

I get an empty DataFrame. What's going on here? Seems like I'm missing something.

As kermit666 explained, there are quite errors in this question and using the dot notation made things a lot clearer. — tanvach
– tanvach, Commented Jun 24, 2014 at 6:54

metakermit · Accepted Answer · 2014-06-23 17:25:09Z

OK, first of all I see you made a couple of style/semantics mistakes which are common for R-to-Python converts:

you don't need the $ signs for your column names and it actually makes column selection nicer as you can write films.director if the name is only 'director' (it has to be a valid Python identifier for this syntactic sugar to work)
indexing in Python starts at 0, not 1, so you select the 1st director as films.director[0]

Assuming you removed the $ signs from your DataFrame definition, you can select the movies as:

In [16]: films[films['director'] == films['director'][0]]
Out[16]:
                director                   title
0  Coppola, Francis Ford           The Godfather
2  Coppola, Francis Ford  The Godfather: Part II

or even cleaner as films[films.director == films.director[0]].

Using your original DataFrame you can perform your query with:

director = films.iloc[[1]]['$director'][1]
films[films['$director'] == director]

One error was that you first defined the table with '$director' and then queried it with 'director' as the column name.

The [1] in the end is necessary because you indexed the DataFrame with a list [1], instead of a value 1, so you got back a Series, as CT Zhu already noticed. List indexing is meant more for selecting several arbitrary elements such as films.iloc[[1, 3]]. In your case it would be clearer to write

director = films.iloc[1]['$director']

Also, note that this still gets Tarantino and not Coppola.

CT Zhu · Accepted Answer · 2014-06-23 14:11:10Z

1

I think films[ films["director"] == films.ix[0, 'director' ]] will suffice.

The reason films.iloc[[1]]["director"] won't work is because it is a Series, not a string.

If you want to use iloc, do: films.iloc[1]["director"] instead of films.iloc[[1]]["director"]

Also:

In [241]:

str(films.iloc[[1]]["director"])
Out[241]:
'1    Tarantino, Quentin\nName: director, dtype: object'

so, films[ films["director"] == str(director) ] won't match anything and will return a empty dataframe.

edited Jun 23, 2014 at 14:11

answered Jun 23, 2014 at 4:42

CT Zhu

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2 Comments

tanvach Over a year ago

Thanks, I will have a look into the answer when I get back. I believe when iloc[1] won't give me the first element if I sort the dataframe.

CT Zhu Over a year ago

Also, notice that index in python starts at 0, instead of 1 in R and matlab, cheers!

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