Pandas DataFrames

Last updated on 2025-10-23 | Edit this page

Overview

Questions

  • How can I do statistical analysis of tabular data?
  • What are Booleans? How can I use them?

Objectives

  • Select individual values from a Pandas dataframe.
  • Select entire rows or entire columns from a dataframe.
  • Select a subset of both rows and columns from a dataframe in a single operation.
  • Construct simple Boolean expressions with comparisons.
  • Select a subset of a dataframe by a single Boolean criterion.

Note about Pandas DataFrames/Series

A DataFrame is a collection of Series; The DataFrame is the way Pandas represents a table, and Series is the data-structure Pandas use to represent a column.

Pandas is built on top of the Numpy library, which in practice means that most of the methods defined for Numpy Arrays apply to Pandas Series/DataFrames.

What makes Pandas so attractive is the powerful interface to access individual records of the table, proper handling of missing values, and relational-databases operations between DataFrames.

Selecting values

To access a value at the position [i,j] of a DataFrame, we have two options, depending on what is the meaning of i in use. Remember that a DataFrame provides an index as a way to identify the rows of the table; a row, then, has a position inside the table as well as a label, which uniquely identifies its entry in the DataFrame.

Use DataFrame.iloc[..., ...] to select values by their (entry) position

  • Can specify location by numerical index analogously to 2D version of character selection in strings.

PYTHON 

import pandas as pd
europe = pd.read_csv('data/gapminder_gdp_europe.csv', index_col='country')
print(europe.iloc[0, 0])

OUTPUT 

1601.056136

Use DataFrame.loc[..., ...] to select values by their (entry) label.

  • Can specify location by row and/or column name.

PYTHON 

print(europe.loc["Albania", "gdpPercap_1952"])

OUTPUT 

1601.056136

Use : on its own to mean all columns or all rows.

  • Just like Python’s usual slicing notation.

PYTHON 

print(europe.loc["Albania", :])

OUTPUT 

gdpPercap_1952    1601.056136
gdpPercap_1957    1942.284244
gdpPercap_1962    2312.888958
gdpPercap_1967    2760.196931
gdpPercap_1972    3313.422188
gdpPercap_1977    3533.003910
gdpPercap_1982    3630.880722
gdpPercap_1987    3738.932735
gdpPercap_1992    2497.437901
gdpPercap_1997    3193.054604
gdpPercap_2002    4604.211737
gdpPercap_2007    5937.029526
Name: Albania, dtype: float64
  • Would get the same result printing europe.loc["Albania"] (without a second index).

PYTHON 

print(europe.loc[:, "gdpPercap_1952"])

OUTPUT 

country
Albania                    1601.056136
Austria                    6137.076492
Belgium                    8343.105127
⋮ ⋮ ⋮
Switzerland               14734.232750
Turkey                     1969.100980
United Kingdom             9979.508487
Name: gdpPercap_1952, dtype: float64
  • Would get the same result printing europe["gdpPercap_1952"]
  • Also get the same result printing europe.gdpPercap_1952 (not recommended, because easily confused with . notation for methods)

Select multiple columns or rows using DataFrame.loc and a named slice.

PYTHON 

print(europe.loc['Italy':'Poland', 'gdpPercap_1962':'gdpPercap_1972'])

OUTPUT 

             gdpPercap_1962  gdpPercap_1967  gdpPercap_1972
country
Italy           8243.582340    10022.401310    12269.273780
Montenegro      4649.593785     5907.850937     7778.414017
Netherlands    12790.849560    15363.251360    18794.745670
Norway         13450.401510    16361.876470    18965.055510
Poland          5338.752143     6557.152776     8006.506993

In the above code, we discover that slicing using loc is inclusive at both ends, which differs from slicing using iloc, where slicing indicates everything up to but not including the final index.

Result of slicing can be used in further operations.

  • Usually don’t just print a slice.
  • All the statistical operators that work on entire dataframes work the same way on slices.
  • E.g., calculate max of a slice.

PYTHON 

print(europe.loc['Italy':'Poland', 'gdpPercap_1962':'gdpPercap_1972'].max())

OUTPUT 

gdpPercap_1962    13450.40151
gdpPercap_1967    16361.87647
gdpPercap_1972    18965.05551
dtype: float64

PYTHON 

print(europe.loc['Italy':'Poland', 'gdpPercap_1962':'gdpPercap_1972'].min())

OUTPUT 

gdpPercap_1962    4649.593785
gdpPercap_1967    5907.850937
gdpPercap_1972    7778.414017
dtype: float64

Comparisons in Python return True or False

Let’s introduce a simple Python data type that hasn’t been explicitly introduced yet: the Boolean or bool.

  • Booleans are either True or False.
  • Capitalization matters here! true is not a valid Boolean.

PYTHON 

type(True) # Notice how True changes color when capitalized

OUTPUT 

bool

Booleans are returned when we test the truth value of a statement. To test whether two objects are equal, we use ==. (This is different from the = used for assignment.)

PYTHON 

x = 5 # Assign x the value 5
print(x)
print(x == 10) # Is the value at x equal to 10, True or False
print(x == 5) # Is the value at x equal to 5, True or False

OUTPUT 

5
False
True

Booleans are returned by statements that make numerical comparisons using operators like >, <= and !=.

  • Try !=, which can be read as “is not equal to”.

PYTHON 

5 != 3 #Is 5 equal to 3?

OUTPUT 

False
  • < is the “less than” operator.

PYTHON 

2 < 3

OUTPUT 

True

You can also do comparisons between two variables.

PYTHON 

t_celsius = 0
t_farenheit = 32
t_celsius < t_farenheit # Evaluates to True or False

OUTPUT 

True

Use comparisons to select data based on value.

  • Comparison is applied element by element.
  • Returns a similarly-shaped dataframe of True and False.

PYTHON 

# Use a subset of data to keep output readable.
subset = europe.loc['Italy':'Poland', 'gdpPercap_1962':'gdpPercap_1972']
print('Subset of europe:\n', subset)

# Which values were greater than 10000 ?
print('\nWhere are values greater than 10000?', subset > 10000)

OUTPUT 

Subset of europe:
             gdpPercap_1962  gdpPercap_1967  gdpPercap_1972
country
Italy           8243.582340    10022.401310    12269.273780
Montenegro      4649.593785     5907.850937     7778.414017
Netherlands    12790.849560    15363.251360    18794.745670
Norway         13450.401510    16361.876470    18965.055510
Poland          5338.752143     6557.152776     8006.506993

Where are values greater than 10000?
            gdpPercap_1962 gdpPercap_1967 gdpPercap_1972
country
Italy                False           True           True
Montenegro           False          False          False
Netherlands           True           True           True
Norway                True           True           True
Poland               False          False          False

Select values or NaN using a Boolean mask.

  • A frame full of Booleans is sometimes called a mask because of how it can be used.

PYTHON 

mask = subset > 10000
print(subset[mask])

OUTPUT 

             gdpPercap_1962  gdpPercap_1967  gdpPercap_1972
country
Italy                   NaN     10022.40131     12269.27378
Montenegro              NaN             NaN             NaN
Netherlands     12790.84956     15363.25136     18794.74567
Norway          13450.40151     16361.87647     18965.05551
Poland                  NaN             NaN             NaN
  • Get the value where the mask is true, and NaN (Not a Number) where it is false.
  • Useful because NaNs are ignored by operations like max, min, average, etc.

PYTHON 

print(subset[subset > 10000].describe())

OUTPUT 

       gdpPercap_1962  gdpPercap_1967  gdpPercap_1972
count        2.000000        3.000000        3.000000
mean     13120.625535    13915.843047    16676.358320
std        466.373656     3408.589070     3817.597015
min      12790.849560    10022.401310    12269.273780
25%      12955.737547    12692.826335    15532.009725
50%      13120.625535    15363.251360    18794.745670
75%      13285.513523    15862.563915    18879.900590
max      13450.401510    16361.876470    18965.055510
Challenge

Selection of Individual Values

Assume Pandas has been imported into your notebook and the Gapminder GDP data for Europe has been loaded:

PYTHON 

import pandas as pd

europe = pd.read_csv('data/gapminder_gdp_europe.csv', index_col='country')

Write an expression to find the Per Capita GDP of Serbia in 2007.

The selection can be done by using the labels for both the row (“Serbia”) and the column (“gdpPercap_2007”):

PYTHON 

print(europe.loc['Serbia', 'gdpPercap_2007'])

The output is

OUTPUT 

9786.534714
Challenge

Extent of Slicing

  1. Do the two statements below produce the same output?
  2. Based on this, what rule governs what is included (or not) in numerical slices and named slices in Pandas?

PYTHON 

print(europe.iloc[0:2, 0:2])
print(europe.loc['Albania':'Belgium', 'gdpPercap_1952':'gdpPercap_1962'])

No, they do not produce the same output! The output of the first statement is:

OUTPUT 

        gdpPercap_1952  gdpPercap_1957
country
Albania     1601.056136     1942.284244
Austria     6137.076492     8842.598030

The second statement gives:

OUTPUT 

        gdpPercap_1952  gdpPercap_1957  gdpPercap_1962
country
Albania     1601.056136     1942.284244     2312.888958
Austria     6137.076492     8842.598030    10750.721110
Belgium     8343.105127     9714.960623    10991.206760

Clearly, the second statement produces an additional column and an additional row compared to the first statement.
What conclusion can we draw? We see that a numerical slice, 0:2, omits the final index (i.e. index 2) in the range provided, while a named slice, ‘gdpPercap_1952’:‘gdpPercap_1962’, includes the final element.

Challenge

Selecting Indices

Explain in simple terms what idxmin and idxmax do in the short program below. When would you use these methods?

PYTHON 

europe = pd.read_csv('data/gapminder_gdp_europe.csv', index_col='country')
print(europe.idxmin())
print(europe.idxmax())

For each column in europe, idxmin will return the index value corresponding to each column’s minimum; idxmax will do accordingly the same for each column’s maximum value.

You can use these functions whenever you want to get the row index of the minimum/maximum value and not the actual minimum/maximum value.

Challenge

Practice with Selection

Assume Pandas has been imported and the Gapminder GDP data for Europe has been loaded. Write an expression to select each of the following:

  1. GDP per capita for all countries in 1982.
  2. GDP per capita for Denmark for all years.
  3. GDP per capita for all countries for years after 1985.
  4. GDP per capita for each country in 2007 as a multiple of GDP per capita for that country in 1952.

1:

PYTHON 

europe['gdpPercap_1982']

2:

PYTHON 

europe.loc['Denmark',:]

3:

PYTHON 

europe.loc[:,'gdpPercap_1985':]

Pandas is smart enough to recognize the number at the end of the column label and does not give you an error, although no column named gdpPercap_1985 actually exists. This is useful if new columns are added to the CSV file later.

4:

PYTHON 

europe['gdpPercap_2007']/europe['gdpPercap_1952']
Challenge

Equals (==) is Not Equals (=)

There are many instances where == and = can both be used. What combination of == and = will make the statement in the second line print False?

Replace PICKASIGN with either an = or ==.

PYTHON 

is_the_sky_blue PICKASIGN "sky" PICKASIGN "blue"
print(is_the_sky_blue)

Remember that = is used to assign a value to a variable, while == is a comparator.

PYTHON 

is_the_sky_blue = "sky" == "blue"
print(is_the_sky_blue)

Solve the problem by comparing “sky” to “blue” with ==, which evaluates to False, then assign False to is_the_sky_blue. The second line will then print False.

Challenge

Many Ways of Access

There are at least two ways of accessing a value or slice of a DataFrame: by name or index. However, there are many others. For example, a single column or row can be accessed either as a DataFrame or a Series object.

Suggest different ways of doing the following operations on a DataFrame:

  1. Access a single column
  2. Access a single row
  3. Access an individual DataFrame element
  4. Access several columns
  5. Access several rows
  6. Access a subset of specific rows and columns
  7. Access a subset of row and column ranges

1. Access a single column:

PYTHON 

# by name
data["col_name"]   # as a Series
data[["col_name"]] # as a DataFrame

# by name using .loc
data.T.loc["col_name"]  # as a Series
data.T.loc[["col_name"]].T  # as a DataFrame

# Dot notation (Series)
data.col_name

# by index (iloc)
data.iloc[:, col_index]   # as a Series
data.iloc[:, [col_index]] # as a DataFrame

# using a mask
data.T[data.T.index == "col_name"].T

2. Access a single row:

PYTHON 

# by name using .loc
data.loc["row_name"] # as a Series
data.loc[["row_name"]] # as a DataFrame

# by name
data.T["row_name"] # as a Series
data.T[["row_name"]].T # as a DataFrame

# by index
data.iloc[row_index]   # as a Series
data.iloc[[row_index]]   # as a DataFrame

# using mask
data[data.index == "row_name"]

3. Access an individual DataFrame element:

PYTHON 

# by column/row names
data["column_name"]["row_name"]         # as a Series

data[["col_name"]].loc["row_name"]  # as a Series
data[["col_name"]].loc[["row_name"]]  # as a DataFrame

data.loc["row_name"]["col_name"]  # as a value
data.loc[["row_name"]]["col_name"]  # as a Series
data.loc[["row_name"]][["col_name"]]  # as a DataFrame

data.loc["row_name", "col_name"]  # as a value
data.loc[["row_name"], "col_name"]  # as a Series. Preserves index. Column name is moved to `.name`.
data.loc["row_name", ["col_name"]]  # as a Series. Index is moved to `.name.` Sets index to column name.
data.loc[["row_name"], ["col_name"]]  # as a DataFrame (preserves original index and column name)

# by column/row names: Dot notation
data.col_name.row_name

# by column/row indices
data.iloc[row_index, col_index] # as a value
data.iloc[[row_index], col_index] # as a Series. Preserves index. Column name is moved to `.name`
data.iloc[row_index, [col_index]] # as a Series. Index is moved to `.name.` Sets index to column name.
data.iloc[[row_index], [col_index]] # as a DataFrame (preserves original index and column name)

# column name + row index
data["col_name"][row_index]
data.col_name[row_index]
data["col_name"].iloc[row_index]

# column index + row name
data.iloc[:, [col_index]].loc["row_name"]  # as a Series
data.iloc[:, [col_index]].loc[["row_name"]]  # as a DataFrame

# using masks
data[data.index == "row_name"].T[data.T.index == "col_name"].T

4. Access several columns:

PYTHON 

# by name
data[["col1", "col2", "col3"]]
data.loc[:, ["col1", "col2", "col3"]]

# by index
data.iloc[:, [col1_index, col2_index, col3_index]]

5. Access several rows

PYTHON 

# by name
data.loc[["row1", "row2", "row3"]]

# by index
data.iloc[[row1_index, row2_index, row3_index]]

6. Access a subset of specific rows and columns

PYTHON 

# by names
data.loc[["row1", "row2", "row3"], ["col1", "col2", "col3"]]

# by indices
data.iloc[[row1_index, row2_index, row3_index], [col1_index, col2_index, col3_index]]

# column names + row indices
data[["col1", "col2", "col3"]].iloc[[row1_index, row2_index, row3_index]]

# column indices + row names
data.iloc[:, [col1_index, col2_index, col3_index]].loc[["row1", "row2", "row3"]]

7. Access a subset of row and column ranges

PYTHON 

# by name
data.loc["row1":"row2", "col1":"col2"]

# by index
data.iloc[row1_index:row2_index, col1_index:col2_index]

# column names + row indices
data.loc[:, "col1_name":"col2_name"].iloc[row1_index:row2_index]

# column indices + row names
data.iloc[:, col1_index:col2_index].loc["row1":"row2"]
Key Points
  • Use DataFrame.iloc[..., ...] to select values by integer location.
  • Use : on its own to mean all columns or all rows.
  • Select multiple columns or rows using DataFrame.loc and a named slice.
  • Result of slicing can be used in further operations.
  • Use comparisons to select data based on value.
  • Booleans store truth values.
  • Select values or NaN using a Boolean mask.