Comparison with Stata¶
For potential users coming from Stata this page is meant to demonstrate how different Stata operations would be performed in pandas.
If you’re new to pandas, you might want to first read through 10 Minutes to pandas to familiarize yourself with the library.
As is customary, we import pandas and NumPy as follows. This means that we can refer to the
libraries as pd and np, respectively, for the rest of the document.
In [1]: import pandas as pd
In [2]: import numpy as np
Note
Throughout this tutorial, the pandas DataFrame will be displayed by calling
df.head(), which displays the first N (default 5) rows of the DataFrame.
This is often used in interactive work (e.g. Jupyter notebook or terminal) – the equivalent in Stata would be:
list in 1/5
Data structures¶
General terminology translation¶
pandas |
Stata |
|---|---|
|
data set |
column |
variable |
row |
observation |
groupby |
bysort |
|
|
DataFrame / Series¶
A DataFrame in pandas is analogous to a Stata data set – a two-dimensional
data source with labeled columns that can be of different types. As will be
shown in this document, almost any operation that can be applied to a data set
in Stata can also be accomplished in pandas.
A Series is the data structure that represents one column of a
DataFrame. Stata doesn’t have a separate data structure for a single column,
but in general, working with a Series is analogous to referencing a column
of a data set in Stata.
Index¶
Every DataFrame and Series has an Index – labels on the
rows of the data. Stata does not have an exactly analogous concept. In Stata, a data set’s
rows are essentially unlabeled, other than an implicit integer index that can be
accessed with _n.
In pandas, if no index is specified, an integer index is also used by default
(first row = 0, second row = 1, and so on). While using a labeled Index or
MultiIndex can enable sophisticated analyses and is ultimately an important
part of pandas to understand, for this comparison we will essentially ignore the
Index and just treat the DataFrame as a collection of columns. Please
see the indexing documentation for much more on how to use an
Index effectively.
Data input / output¶
Constructing a DataFrame from values¶
A Stata data set can be built from specified values by
placing the data after an input statement and
specifying the column names.
input x y
1 2
3 4
5 6
end
A pandas DataFrame can be constructed in many different ways,
but for a small number of values, it is often convenient to specify it as
a Python dictionary, where the keys are the column names
and the values are the data.
In [3]: df = pd.DataFrame({"x": [1, 3, 5], "y": [2, 4, 6]})
In [4]: df
Out[4]:
x y
0 1 2
1 3 4
2 5 6
Reading external data¶
Like Stata, pandas provides utilities for reading in data from
many formats. The tips data set, found within the pandas
tests (csv)
will be used in many of the following examples.
Stata provides import delimited to read csv data into a data set in memory.
If the tips.csv file is in the current working directory, we can import it as follows.
import delimited tips.csv
The pandas method is read_csv(), which works similarly. Additionally, it will automatically download
the data set if presented with a url.
In [5]: url = (
...: "https://raw.github.com/pandas-dev"
...: "/pandas/master/pandas/tests/io/data/csv/tips.csv"
...: )
...:
In [6]: tips = pd.read_csv(url)
In [7]: tips.head()
Out[7]:
total_bill tip sex smoker day time size
0 16.99 1.01 Female No Sun Dinner 2
1 10.34 1.66 Male No Sun Dinner 3
2 21.01 3.50 Male No Sun Dinner 3
3 23.68 3.31 Male No Sun Dinner 2
4 24.59 3.61 Female No Sun Dinner 4
Like import delimited, read_csv() can take a number of parameters to specify
how the data should be parsed. For example, if the data were instead tab delimited,
did not have column names, and existed in the current working directory,
the pandas command would be:
tips = pd.read_csv("tips.csv", sep="\t", header=None)
# alternatively, read_table is an alias to read_csv with tab delimiter
tips = pd.read_table("tips.csv", header=None)
pandas can also read Stata data sets in .dta format with the read_stata() function.
df = pd.read_stata("data.dta")
In addition to text/csv and Stata files, pandas supports a variety of other data formats
such as Excel, SAS, HDF5, Parquet, and SQL databases. These are all read via a pd.read_*
function. See the IO documentation for more details.
Exporting data¶
The inverse of import delimited in Stata is export delimited
export delimited tips2.csv
Similarly in pandas, the opposite of read_csv is DataFrame.to_csv().
tips.to_csv("tips2.csv")
pandas can also export to Stata file format with the DataFrame.to_stata() method.
tips.to_stata("tips2.dta")
Data operations¶
Operations on columns¶
In Stata, arbitrary math expressions can be used with the generate and
replace commands on new or existing columns. The drop command drops
the column from the data set.
replace total_bill = total_bill - 2
generate new_bill = total_bill / 2
drop new_bill
pandas provides similar vectorized operations by
specifying the individual Series in the DataFrame.
New columns can be assigned in the same way. The DataFrame.drop() method
drops a column from the DataFrame.
In [8]: tips["total_bill"] = tips["total_bill"] - 2
In [9]: tips["new_bill"] = tips["total_bill"] / 2
In [10]: tips.head()
Out[10]:
total_bill tip sex smoker day time size new_bill
0 14.99 1.01 Female No Sun Dinner 2 7.495
1 8.34 1.66 Male No Sun Dinner 3 4.170
2 19.01 3.50 Male No Sun Dinner 3 9.505
3 21.68 3.31 Male No Sun Dinner 2 10.840
4 22.59 3.61 Female No Sun Dinner 4 11.295
In [11]: tips = tips.drop("new_bill", axis=1)
Filtering¶
Filtering in Stata is done with an if clause on one or more columns.
list if total_bill > 10
DataFrames can be filtered in multiple ways; the most intuitive of which is using boolean indexing.
In [12]: tips[tips["total_bill"] > 10].head()
Out[12]:
total_bill tip sex smoker day time size
0 14.99 1.01 Female No Sun Dinner 2
2 19.01 3.50 Male No Sun Dinner 3
3 21.68 3.31 Male No Sun Dinner 2
4 22.59 3.61 Female No Sun Dinner 4
5 23.29 4.71 Male No Sun Dinner 4
If/then logic¶
In Stata, an if clause can also be used to create new columns.
generate bucket = "low" if total_bill < 10
replace bucket = "high" if total_bill >= 10
The same operation in pandas can be accomplished using
the where method from numpy.
In [13]: tips["bucket"] = np.where(tips["total_bill"] < 10, "low", "high")
In [14]: tips.head()
Out[14]:
total_bill tip sex smoker day time size bucket
0 14.99 1.01 Female No Sun Dinner 2 high
1 8.34 1.66 Male No Sun Dinner 3 low
2 19.01 3.50 Male No Sun Dinner 3 high
3 21.68 3.31 Male No Sun Dinner 2 high
4 22.59 3.61 Female No Sun Dinner 4 high
Date functionality¶
Stata provides a variety of functions to do operations on date/datetime columns.
generate date1 = mdy(1, 15, 2013)
generate date2 = date("Feb152015", "MDY")
generate date1_year = year(date1)
generate date2_month = month(date2)
* shift date to beginning of next month
generate date1_next = mdy(month(date1) + 1, 1, year(date1)) if month(date1) != 12
replace date1_next = mdy(1, 1, year(date1) + 1) if month(date1) == 12
generate months_between = mofd(date2) - mofd(date1)
list date1 date2 date1_year date2_month date1_next months_between
The equivalent pandas operations are shown below. In addition to these functions, pandas supports other Time Series features not available in Stata (such as time zone handling and custom offsets) – see the timeseries documentation for more details.
In [15]: tips["date1"] = pd.Timestamp("2013-01-15")
In [16]: tips["date2"] = pd.Timestamp("2015-02-15")
In [17]: tips["date1_year"] = tips["date1"].dt.year
In [18]: tips["date2_month"] = tips["date2"].dt.month
In [19]: tips["date1_next"] = tips["date1"] + pd.offsets.MonthBegin()
In [20]: tips["months_between"] = tips["date2"].dt.to_period("M") - tips[
....: "date1"
....: ].dt.to_period("M")
....:
In [21]: tips[
....: ["date1", "date2", "date1_year", "date2_month", "date1_next", "months_between"]
....: ].head()
....:
Out[21]:
date1 date2 date1_year date2_month date1_next months_between
0 2013-01-15 2015-02-15 2013 2 2013-02-01 <25 * MonthEnds>
1 2013-01-15 2015-02-15 2013 2 2013-02-01 <25 * MonthEnds>
2 2013-01-15 2015-02-15 2013 2 2013-02-01 <25 * MonthEnds>
3 2013-01-15 2015-02-15 2013 2 2013-02-01 <25 * MonthEnds>
4 2013-01-15 2015-02-15 2013 2 2013-02-01 <25 * MonthEnds>
Selection of columns¶
Stata provides keywords to select, drop, and rename columns.
keep sex total_bill tip
drop sex
rename total_bill total_bill_2
The same operations are expressed in pandas below. Note that in contrast to Stata, these operations do not happen in place. To make these changes persist, assign the operation back to a variable.
# keep
In [22]: tips[["sex", "total_bill", "tip"]].head()
Out[22]:
sex total_bill tip
0 Female 14.99 1.01
1 Male 8.34 1.66
2 Male 19.01 3.50
3 Male 21.68 3.31
4 Female 22.59 3.61
# drop
In [23]: tips.drop("sex", axis=1).head()
Out[23]:
total_bill tip smoker day time size
0 14.99 1.01 No Sun Dinner 2
1 8.34 1.66 No Sun Dinner 3
2 19.01 3.50 No Sun Dinner 3
3 21.68 3.31 No Sun Dinner 2
4 22.59 3.61 No Sun Dinner 4
# rename
In [24]: tips.rename(columns={"total_bill": "total_bill_2"}).head()
Out[24]:
total_bill_2 tip sex smoker day time size
0 14.99 1.01 Female No Sun Dinner 2
1 8.34 1.66 Male No Sun Dinner 3
2 19.01 3.50 Male No Sun Dinner 3
3 21.68 3.31 Male No Sun Dinner 2
4 22.59 3.61 Female No Sun Dinner 4
Sorting by values¶
Sorting in Stata is accomplished via sort
sort sex total_bill
pandas objects have a DataFrame.sort_values() method, which
takes a list of columns to sort by.
In [25]: tips = tips.sort_values(["sex", "total_bill"])
In [26]: tips.head()
Out[26]:
total_bill tip sex smoker day time size
67 1.07 1.00 Female Yes Sat Dinner 1
92 3.75 1.00 Female Yes Fri Dinner 2
111 5.25 1.00 Female No Sat Dinner 1
145 6.35 1.50 Female No Thur Lunch 2
135 6.51 1.25 Female No Thur Lunch 2
String processing¶
Finding length of string¶
Stata determines the length of a character string with the strlen() and
ustrlen() functions for ASCII and Unicode strings, respectively.
generate strlen_time = strlen(time)
generate ustrlen_time = ustrlen(time)
Python determines the length of a character string with the len function.
In Python 3, all strings are Unicode strings. len includes trailing blanks.
Use len and rstrip to exclude trailing blanks.
In [27]: tips["time"].str.len().head()
Out[27]:
67 6
92 6
111 6
145 5
135 5
Name: time, dtype: int64
In [28]: tips["time"].str.rstrip().str.len().head()
Out[28]:
67 6
92 6
111 6
145 5
135 5
Name: time, dtype: int64
Finding position of substring¶
Stata determines the position of a character in a string with the strpos() function.
This takes the string defined by the first argument and searches for the
first position of the substring you supply as the second argument.
generate str_position = strpos(sex, "ale")
Python determines the position of a character in a string with the
find() function. find searches for the first position of the
substring. If the substring is found, the function returns its
position. Keep in mind that Python indexes are zero-based and
the function will return -1 if it fails to find the substring.
In [29]: tips["sex"].str.find("ale").head()
Out[29]:
67 3
92 3
111 3
145 3
135 3
Name: sex, dtype: int64
Extracting substring by position¶
Stata extracts a substring from a string based on its position with the substr() function.
generate short_sex = substr(sex, 1, 1)
With pandas you can use [] notation to extract a substring
from a string by position locations. Keep in mind that Python
indexes are zero-based.
In [30]: tips["sex"].str[0:1].head()
Out[30]:
67 F
92 F
111 F
145 F
135 F
Name: sex, dtype: object
Extracting nth word¶
The Stata word() function returns the nth word from a string.
The first argument is the string you want to parse and the
second argument specifies which word you want to extract.
clear
input str20 string
"John Smith"
"Jane Cook"
end
generate first_name = word(name, 1)
generate last_name = word(name, -1)
Python extracts a substring from a string based on its text by using regular expressions. There are much more powerful approaches, but this just shows a simple approach.
In [31]: firstlast = pd.DataFrame({"string": ["John Smith", "Jane Cook"]})
In [32]: firstlast["First_Name"] = firstlast["string"].str.split(" ", expand=True)[0]
In [33]: firstlast["Last_Name"] = firstlast["string"].str.rsplit(" ", expand=True)[0]
In [34]: firstlast
Out[34]:
string First_Name Last_Name
0 John Smith John John
1 Jane Cook Jane Jane
Changing case¶
The Stata strupper(), strlower(), strproper(),
ustrupper(), ustrlower(), and ustrtitle() functions
change the case of ASCII and Unicode strings, respectively.
clear
input str20 string
"John Smith"
"Jane Cook"
end
generate upper = strupper(string)
generate lower = strlower(string)
generate title = strproper(string)
list
The equivalent Python functions are upper, lower, and title.
In [35]: firstlast = pd.DataFrame({"string": ["John Smith", "Jane Cook"]})
In [36]: firstlast["upper"] = firstlast["string"].str.upper()
In [37]: firstlast["lower"] = firstlast["string"].str.lower()
In [38]: firstlast["title"] = firstlast["string"].str.title()
In [39]: firstlast
Out[39]:
string upper lower title
0 John Smith JOHN SMITH john smith John Smith
1 Jane Cook JANE COOK jane cook Jane Cook
Merging¶
The following tables will be used in the merge examples
In [40]: df1 = pd.DataFrame({"key": ["A", "B", "C", "D"], "value": np.random.randn(4)})
In [41]: df1
Out[41]:
key value
0 A 0.469112
1 B -0.282863
2 C -1.509059
3 D -1.135632
In [42]: df2 = pd.DataFrame({"key": ["B", "D", "D", "E"], "value": np.random.randn(4)})
In [43]: df2
Out[43]:
key value
0 B 1.212112
1 D -0.173215
2 D 0.119209
3 E -1.044236
In Stata, to perform a merge, one data set must be in memory
and the other must be referenced as a file name on disk. In
contrast, Python must have both DataFrames already in memory.
By default, Stata performs an outer join, where all observations
from both data sets are left in memory after the merge. One can
keep only observations from the initial data set, the merged data set,
or the intersection of the two by using the values created in the
_merge variable.
* First create df2 and save to disk
clear
input str1 key
B
D
D
E
end
generate value = rnormal()
save df2.dta
* Now create df1 in memory
clear
input str1 key
A
B
C
D
end
generate value = rnormal()
preserve
* Left join
merge 1:n key using df2.dta
keep if _merge == 1
* Right join
restore, preserve
merge 1:n key using df2.dta
keep if _merge == 2
* Inner join
restore, preserve
merge 1:n key using df2.dta
keep if _merge == 3
* Outer join
restore
merge 1:n key using df2.dta
pandas DataFrames have a DataFrame.merge() method, which provides
similar functionality. Note that different join
types are accomplished via the how keyword.
In [44]: inner_join = df1.merge(df2, on=["key"], how="inner")
In [45]: inner_join
Out[45]:
key value_x value_y
0 B -0.282863 1.212112
1 D -1.135632 -0.173215
2 D -1.135632 0.119209
In [46]: left_join = df1.merge(df2, on=["key"], how="left")
In [47]: left_join
Out[47]:
key value_x value_y
0 A 0.469112 NaN
1 B -0.282863 1.212112
2 C -1.509059 NaN
3 D -1.135632 -0.173215
4 D -1.135632 0.119209
In [48]: right_join = df1.merge(df2, on=["key"], how="right")
In [49]: right_join
Out[49]:
key value_x value_y
0 B -0.282863 1.212112
1 D -1.135632 -0.173215
2 D -1.135632 0.119209
3 E NaN -1.044236
In [50]: outer_join = df1.merge(df2, on=["key"], how="outer")
In [51]: outer_join
Out[51]:
key value_x value_y
0 A 0.469112 NaN
1 B -0.282863 1.212112
2 C -1.509059 NaN
3 D -1.135632 -0.173215
4 D -1.135632 0.119209
5 E NaN -1.044236
Missing data¶
Like Stata, pandas has a representation for missing data – the
special float value NaN (not a number). Many of the semantics
are the same; for example missing data propagates through numeric
operations, and is ignored by default for aggregations.
In [52]: outer_join
Out[52]:
key value_x value_y
0 A 0.469112 NaN
1 B -0.282863 1.212112
2 C -1.509059 NaN
3 D -1.135632 -0.173215
4 D -1.135632 0.119209
5 E NaN -1.044236
In [53]: outer_join["value_x"] + outer_join["value_y"]
Out[53]:
0 NaN
1 0.929249
2 NaN
3 -1.308847
4 -1.016424
5 NaN
dtype: float64
In [54]: outer_join["value_x"].sum()
Out[54]: -3.5940742896293765
One difference is that missing data cannot be compared to its sentinel value. For example, in Stata you could do this to filter missing values.
* Keep missing values
list if value_x == .
* Keep non-missing values
list if value_x != .
This doesn’t work in pandas. Instead, the pd.isna() or pd.notna() functions
should be used for comparisons.
In [55]: outer_join[pd.isna(outer_join["value_x"])]
Out[55]:
key value_x value_y
5 E NaN -1.044236
In [56]: outer_join[pd.notna(outer_join["value_x"])]
Out[56]:
key value_x value_y
0 A 0.469112 NaN
1 B -0.282863 1.212112
2 C -1.509059 NaN
3 D -1.135632 -0.173215
4 D -1.135632 0.119209
pandas also provides a variety of methods to work with missing data – some of which would be challenging to express in Stata. For example, there are methods to drop all rows with any missing values, replacing missing values with a specified value, like the mean, or forward filling from previous rows. See the missing data documentation for more.
# Drop rows with any missing value
In [57]: outer_join.dropna()
Out[57]:
key value_x value_y
1 B -0.282863 1.212112
3 D -1.135632 -0.173215
4 D -1.135632 0.119209
# Fill forwards
In [58]: outer_join.fillna(method="ffill")
Out[58]:
key value_x value_y
0 A 0.469112 NaN
1 B -0.282863 1.212112
2 C -1.509059 1.212112
3 D -1.135632 -0.173215
4 D -1.135632 0.119209
5 E -1.135632 -1.044236
# Impute missing values with the mean
In [59]: outer_join["value_x"].fillna(outer_join["value_x"].mean())
Out[59]:
0 0.469112
1 -0.282863
2 -1.509059
3 -1.135632
4 -1.135632
5 -0.718815
Name: value_x, dtype: float64
GroupBy¶
Aggregation¶
Stata’s collapse can be used to group by one or
more key variables and compute aggregations on
numeric columns.
collapse (sum) total_bill tip, by(sex smoker)
pandas provides a flexible groupby mechanism that
allows similar aggregations. See the groupby documentation
for more details and examples.
In [60]: tips_summed = tips.groupby(["sex", "smoker"])[["total_bill", "tip"]].sum()
In [61]: tips_summed.head()
Out[61]:
total_bill tip
sex smoker
Female No 869.68 149.77
Yes 527.27 96.74
Male No 1725.75 302.00
Yes 1217.07 183.07
Transformation¶
In Stata, if the group aggregations need to be used with the
original data set, one would usually use bysort with egen().
For example, to subtract the mean for each observation by smoker group.
bysort sex smoker: egen group_bill = mean(total_bill)
generate adj_total_bill = total_bill - group_bill
pandas groupby provides a transform mechanism that allows
these type of operations to be succinctly expressed in one
operation.
In [62]: gb = tips.groupby("smoker")["total_bill"]
In [63]: tips["adj_total_bill"] = tips["total_bill"] - gb.transform("mean")
In [64]: tips.head()
Out[64]:
total_bill tip sex smoker day time size adj_total_bill
67 1.07 1.00 Female Yes Sat Dinner 1 -17.686344
92 3.75 1.00 Female Yes Fri Dinner 2 -15.006344
111 5.25 1.00 Female No Sat Dinner 1 -11.938278
145 6.35 1.50 Female No Thur Lunch 2 -10.838278
135 6.51 1.25 Female No Thur Lunch 2 -10.678278
By group processing¶
In addition to aggregation, pandas groupby can be used to
replicate most other bysort processing from Stata. For example,
the following example lists the first observation in the current
sort order by sex/smoker group.
bysort sex smoker: list if _n == 1
In pandas this would be written as:
In [65]: tips.groupby(["sex", "smoker"]).first()
Out[65]:
total_bill tip day time size adj_total_bill
sex smoker
Female No 5.25 1.00 Sat Dinner 1 -11.938278
Yes 1.07 1.00 Sat Dinner 1 -17.686344
Male No 5.51 2.00 Thur Lunch 2 -11.678278
Yes 5.25 5.15 Sun Dinner 2 -13.506344
Other considerations¶
Disk vs memory¶
pandas and Stata both operate exclusively in memory. This means that the size of
data able to be loaded in pandas is limited by your machine’s memory.
If out of core processing is needed, one possibility is the
dask.dataframe
library, which provides a subset of pandas functionality for an
on-disk DataFrame.