In [1]:
# Run this cell to set up packages for lecture.
from lec04_imports import *

Lecture 4 – Arrays and DataFrames¶

DSC 10, Spring 2024¶

Announcements¶

  • Lab 1 is due tomorrow at 11:59PM.
    • Submit early and avoid submission errors.
  • Quiz 1 is coming up on Friday in your assigned quiz session.
    • You should have been assigned a quiz time in an email with subject "DSC 10 Quiz Time".
    • This will be a 20 minute paper-based quiz consisting of short answer and multiple choice questions.
    • No aids are allowed (no notes, no calculators, no computers, no reference sheet). Questions are designed with this in mind.
    • The quiz covers Lectures 1 through 4, or BPD 1-9 in the babypandas notes.
      • Review both of these materials to study.
      • Attend discussion section on Wednesday to work on practice problems.
      • Homework 1 is due next week, but working on the homework is excellent preparation for the quiz.
  • Come to office hours (see the schedule here) and post on Ed for help!

Agenda¶

  • Arrays.
  • Ranges.
  • DataFrames.

Note:¶

  • Remember to check the resources tab of the course website for programming resources.
  • Some key links moving forward:
    • DSC 10 reference sheet.
    • babypandas notes.
    • babypandas documentation.

Arrays¶

Recap: arrays¶

  • Arrays are provided by a module called numpy that we need to import to use.
In [2]:
import numpy as np
  • Arrays store sequences of values of the same data type.
  • To create an array, we pass a list as input to the np.array function.
In [3]:
temperature_array = np.array([68, 73, 70, 74, 76, 72, 74])
temperature_array
Out[3]:
array([68, 73, 70, 74, 76, 72, 74])
  • Use square brackets with the position number to access an array element. The first element is in position 0.
In [4]:
temperature_array
Out[4]:
array([68, 73, 70, 74, 76, 72, 74])
In [5]:
temperature_array[1]
Out[5]:
73

Array-number arithmetic¶

Arrays make it easy to perform the same operation to every element. This behavior is formally known as "broadcasting".

In [6]:
temperature_array
Out[6]:
array([68, 73, 70, 74, 76, 72, 74])
In [7]:
# Convert all temperatures to Celsius.
(5 / 9) * (temperature_array - 32)
Out[7]:
array([20.  , 22.78, 21.11, 23.33, 24.44, 22.22, 23.33])

Element-wise arithmetic¶

  • We can apply arithmetic operations to multiple arrays, provided they have the same length.
  • The result is computed element-wise, which means that the arithmetic operation is applied to one pair of elements from each array at a time.
In [8]:
a = np.array([4, 5, -1])
b = np.array([2, 3, 2])
In [9]:
a ** 2 + b ** 2
Out[9]:
array([20, 34,  5])

Array methods¶

  • Arrays work with a variety of methods, which are functions designed to operate specifically on arrays.

  • Call these methods using dot notation, e.g. array_name.method().

In [10]:
temperature_array.max()
Out[10]:
76
In [11]:
temperature_array.mean()
Out[11]:
72.42857142857143

Ranges¶

Motivation¶

We often find ourselves needing to make arrays like this:

In [12]:
day_of_month = np.array([
    1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 
    13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 
    23, 24, 25, 26, 27, 28, 29, 30, 31
])

There needs to be an easier way to do this!

Ranges¶

  • A range is an array of evenly spaced numbers. We create ranges using np.arange.
  • The most general way to create a range is np.arange(start, end, step). This returns an array such that:
    • The first number is start. By default, start is 0.
    • All subsequent numbers are spaced out by step, until (but excluding) end. By default, step is 1.
In [13]:
# Start at 0, end before 8, step by 1.
# This will be our most common use-case!
np.arange(8)
Out[13]:
array([0, 1, 2, 3, 4, 5, 6, 7])
In [14]:
# Start at 5, end before 10, step by 1.
np.arange(5, 10)
Out[14]:
array([5, 6, 7, 8, 9])
In [15]:
# Start at 3, end before 32, step by 5.
np.arange(3, 32, 5)
Out[15]:
array([ 3,  8, 13, 18, 23, 28])

Extra practice¶

The step size in np.arange can be fractional, or even negative. Predict what arrays will be produced by each line of code below. Then copy each line into a code cell and run it to see if you're right.

np.arange(-3, 2, 0.5)

np.arange(1, -10, -3)
In [16]:
...
Out[16]:
Ellipsis
In [17]:
...
Out[17]:
Ellipsis

Challenge¶

🎉 Congrats! 🎉 You won the lottery 💰. Here's how your payout works: on the first day of September, you are paid \$0.01. Every day thereafter, your pay doubles, so on the second day you're paid \\$0.02, on the third day you're paid \$0.04, on the fourth day you're paid \\$0.08, and so on.

September has 30 days.

Write a one-line expression that uses the numbers 2 and 30, along with the function np.arange and the method .sum(), that computes the total amount in dollars you will be paid in September.

In [18]:
...
Out[18]:
Ellipsis

After trying the challenge problem on your own, watch this walkthrough 🎥 video.

DataFrames¶

pandas¶

  • pandas is a Python package that allows us to work with tabular data – that is, data in the form of a table that we might otherwise work with as a spreadsheet (in Excel or Google Sheets).
  • pandas is the tool for doing data science in Python.

But pandas is not so cute...¶

Enter babypandas!¶

  • We at UCSD have created a smaller, nicer version of pandas called babypandas.
  • It keeps the important stuff and has much better error messages.
  • It's easier to learn, but is still valid pandas code. You are learning pandas!
    • Think of it like learning how to build LEGOs with many, but not all, of the possible Lego blocks. You're still learning how to build LEGOs, and you can still build cool things!

DataFrames in babypandas 🐼¶

  • Tables in babypandas (and pandas) are called "DataFrames."
  • To use DataFrames, we'll need to import babypandas.
In [19]:
import babypandas as bpd

Reading data from a file 📖¶

  • We'll usually work with data stored in the CSV format. CSV stands for "comma-separated values."

  • We can read in a CSV using bpd.read_csv(...). Replace the ... with a path to the CSV file relative to your notebook; if the file is in the same folder as your notebook, this is just the name of the file.

In [20]:
# Our CSV file is stored not in the same folder as our notebook, 
# but within a folder called data.
states = bpd.read_csv('data/states.csv')
states
Out[20]:
State Region Capital City Population Land Area Party
0 Alabama South Montgomery 5024279 50645 Republican
1 Alaska West Juneau 733391 570641 Republican
2 Arizona West Phoenix 7151502 113594 Republican
3 Arkansas South Little Rock 3011524 52035 Republican
4 California West Sacramento 39538223 155779 Democratic
... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic
46 Washington West Olympia 7705281 66456 Democratic
47 West Virginia South Charleston 1793716 24038 Republican
48 Wisconsin Midwest Madison 5893718 54158 Republican
49 Wyoming West Cheyenne 576851 97093 Republican

50 rows × 6 columns

About the data 🗽¶

Most of the data is self-explanatory, but there are a few things to note:

  • 'Population' figures come from the 2020 census.
  • 'Land Area' is measured in square miles.
  • The 'Region' column places each state in one of four regions, as determined by the US Census Bureau.
  • The 'Party' column classifies each state as 'Democratic' or 'Republican' based on a political science measurement called the Cook Partisan Voter Index.
(source)

Structure of a DataFrame¶

  • DataFrames have columns and rows.
    • Think of each column as an array. Columns contain data of the same type.
  • Each column has a label, e.g. 'Capital City' and 'Land Area'.
    • Column labels are stored as strings.
  • Each row has a label too – these are shown in bold at the start of the row.
    • Right now, the row labels are 0, 1, 2, and so on.
    • Together, the row labels are called the index. The index is not a column!
In [21]:
# This DataFrame has 50 rows and 6 columns.
states
Out[21]:
State Region Capital City Population Land Area Party
0 Alabama South Montgomery 5024279 50645 Republican
1 Alaska West Juneau 733391 570641 Republican
2 Arizona West Phoenix 7151502 113594 Republican
3 Arkansas South Little Rock 3011524 52035 Republican
4 California West Sacramento 39538223 155779 Democratic
... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic
46 Washington West Olympia 7705281 66456 Democratic
47 West Virginia South Charleston 1793716 24038 Republican
48 Wisconsin Midwest Madison 5893718 54158 Republican
49 Wyoming West Cheyenne 576851 97093 Republican

50 rows × 6 columns

Example 1: Population density¶

Key concepts: Accessing columns, performing calculations with them, and adding new columns.

Finding population density¶

Question: What is the population density of each state, in people per square mile?

In [22]:
states
Out[22]:
State Region Capital City Population Land Area Party
0 Alabama South Montgomery 5024279 50645 Republican
1 Alaska West Juneau 733391 570641 Republican
2 Arizona West Phoenix 7151502 113594 Republican
3 Arkansas South Little Rock 3011524 52035 Republican
4 California West Sacramento 39538223 155779 Democratic
... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic
46 Washington West Olympia 7705281 66456 Democratic
47 West Virginia South Charleston 1793716 24038 Republican
48 Wisconsin Midwest Madison 5893718 54158 Republican
49 Wyoming West Cheyenne 576851 97093 Republican

50 rows × 6 columns

  • We have, separately, the population and land area of each state.
  • Steps:
    • Get the 'Population' column.
    • Get the 'Land Area' column.
    • Divide these columns element-wise.
    • Add a new column to the DataFrame with these results.

Step 1 – Getting the 'Population' column¶

  • We can get a column from a DataFrame using .get(column_name).
  • 🚨 Column names are case sensitive!
  • Column names are strings, so we need to use quotes.
  • The result looks like a 1-column DataFrame, but is actually a Series.
In [23]:
states
Out[23]:
State Region Capital City Population Land Area Party
0 Alabama South Montgomery 5024279 50645 Republican
1 Alaska West Juneau 733391 570641 Republican
2 Arizona West Phoenix 7151502 113594 Republican
3 Arkansas South Little Rock 3011524 52035 Republican
4 California West Sacramento 39538223 155779 Democratic
... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic
46 Washington West Olympia 7705281 66456 Democratic
47 West Virginia South Charleston 1793716 24038 Republican
48 Wisconsin Midwest Madison 5893718 54158 Republican
49 Wyoming West Cheyenne 576851 97093 Republican

50 rows × 6 columns

In [24]:
states.get('Population')
Out[24]:
0      5024279
1       733391
2      7151502
3      3011524
4     39538223
        ...   
45     8631393
46     7705281
47     1793716
48     5893718
49      576851
Name: Population, Length: 50, dtype: int64

Digression: Series¶

  • A Series is like an array, but with an index.
  • In particular, Series support arithmetic, just like arrays.
In [25]:
states.get('Population')
Out[25]:
0      5024279
1       733391
2      7151502
3      3011524
4     39538223
        ...   
45     8631393
46     7705281
47     1793716
48     5893718
49      576851
Name: Population, Length: 50, dtype: int64
In [26]:
type(states.get('Population'))
Out[26]:
babypandas.bpd.Series

Steps 2 and 3 – Getting the 'Land Area' column and dividing element-wise¶

In [27]:
states.get('Land Area')
Out[27]:
0      50645
1     570641
2     113594
3      52035
4     155779
       ...  
45     39490
46     66456
47     24038
48     54158
49     97093
Name: Land Area, Length: 50, dtype: int64
  • Just like with arrays, we can perform arithmetic operations with two Series, as long as they have the same length and same index.
  • Operations happen element-wise (by matching up corresponding index values), and the result is also a Series.
In [28]:
states.get('Population') / states.get('Land Area')
Out[28]:
0      99.21
1       1.29
2      62.96
3      57.87
4     253.81
       ...  
45    218.57
46    115.95
47     74.62
48    108.82
49      5.94
Length: 50, dtype: float64

Step 4 – Adding the densities to the DataFrame as a new column¶

  • Use .assign(name_of_column=data_in_series) to assign a Series (or array, or list) to a DataFrame.
  • 🚨 Don't put quotes around name_of_column.
  • This creates a new DataFrame, which we must save to a variable if we want to keep using it.
In [29]:
states.assign(
    Density=states.get('Population') / states.get('Land Area')
)
Out[29]:
State Region Capital City Population Land Area Party Density
0 Alabama South Montgomery 5024279 50645 Republican 99.21
1 Alaska West Juneau 733391 570641 Republican 1.29
2 Arizona West Phoenix 7151502 113594 Republican 62.96
3 Arkansas South Little Rock 3011524 52035 Republican 57.87
4 California West Sacramento 39538223 155779 Democratic 253.81
... ... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic 218.57
46 Washington West Olympia 7705281 66456 Democratic 115.95
47 West Virginia South Charleston 1793716 24038 Republican 74.62
48 Wisconsin Midwest Madison 5893718 54158 Republican 108.82
49 Wyoming West Cheyenne 576851 97093 Republican 5.94

50 rows × 7 columns

In [30]:
states
Out[30]:
State Region Capital City Population Land Area Party
0 Alabama South Montgomery 5024279 50645 Republican
1 Alaska West Juneau 733391 570641 Republican
2 Arizona West Phoenix 7151502 113594 Republican
3 Arkansas South Little Rock 3011524 52035 Republican
4 California West Sacramento 39538223 155779 Democratic
... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic
46 Washington West Olympia 7705281 66456 Democratic
47 West Virginia South Charleston 1793716 24038 Republican
48 Wisconsin Midwest Madison 5893718 54158 Republican
49 Wyoming West Cheyenne 576851 97093 Republican

50 rows × 6 columns

In [31]:
states = states.assign(
    Density=states.get('Population') / states.get('Land Area')
)
states
Out[31]:
State Region Capital City Population Land Area Party Density
0 Alabama South Montgomery 5024279 50645 Republican 99.21
1 Alaska West Juneau 733391 570641 Republican 1.29
2 Arizona West Phoenix 7151502 113594 Republican 62.96
3 Arkansas South Little Rock 3011524 52035 Republican 57.87
4 California West Sacramento 39538223 155779 Democratic 253.81
... ... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic 218.57
46 Washington West Olympia 7705281 66456 Democratic 115.95
47 West Virginia South Charleston 1793716 24038 Republican 74.62
48 Wisconsin Midwest Madison 5893718 54158 Republican 108.82
49 Wyoming West Cheyenne 576851 97093 Republican 5.94

50 rows × 7 columns

Example 2: Exploring population density¶

Key concept: Computing statistics of columns using Series methods.

Questions¶

  • What is the highest population density of any one state?
  • What is the average population density across all states?

Series, like arrays, have helpful methods, including .min(), .max(), and .mean().

In [32]:
states.get('Density').max()
Out[32]:
1263.1212945335872

What state does this correspond to? We'll see how to find out shortly!

Other statistics:

In [33]:
states.get('Density').min()
Out[33]:
1.2852055845969708
In [34]:
states.get('Density').mean()
Out[34]:
206.54513507096465
In [35]:
states.get('Density').median()
Out[35]:
108.31649013462203
In [36]:
# Lots of information at once!
states.get('Density').describe()
Out[36]:
count      50.00
mean      206.55
std       274.93
min         1.29
25%        47.06
50%       108.32
75%       224.57
max      1263.12
Name: Density, dtype: float64

Example 3: Which state has the highest population density?¶

Key concepts: Sorting. Accessing using integer positions.

Step 1 – Sorting the DataFrame¶

  • Use the .sort_values(by=column_name) method to sort.
    • The by= can be omitted, but helps with readability.
  • Like most DataFrame methods, this returns a new DataFrame.
In [37]:
states.sort_values(by='Density')
Out[37]:
State Region Capital City Population Land Area Party Density
1 Alaska West Juneau 733391 570641 Republican 1.29
49 Wyoming West Cheyenne 576851 97093 Republican 5.94
25 Montana West Helena 1084225 145546 Republican 7.45
33 North Dakota Midwest Bismarck 779094 69001 Republican 11.29
40 South Dakota Midwest Pierre 886667 75811 Republican 11.70
... ... ... ... ... ... ... ...
19 Maryland South Annapolis 6177224 9707 Democratic 636.37
6 Connecticut Northeast Hartford 3605944 4842 Democratic 744.72
20 Massachusetts Northeast Boston 7029917 7800 Democratic 901.27
38 Rhode Island Northeast Providence 1097379 1034 Democratic 1061.29
29 New Jersey Northeast Trenton 9288994 7354 Democratic 1263.12

50 rows × 7 columns

This sorts, but in ascending order (small to large). The opposite would be nice!

Step 1 – Sorting the DataFrame in descending order¶

  • Use .sort_values(by=column_name, ascending=False) to sort in descending order.
  • ascending is an optional argument. If omitted, it will be set to True by default.
    • This is an example of a keyword argument, or a named argument.
    • If we want to specify the sorting order, we must use the keyword ascending=.
In [38]:
ordered_states = states.sort_values(by='Density', ascending=False)
ordered_states
Out[38]:
State Region Capital City Population Land Area Party Density
29 New Jersey Northeast Trenton 9288994 7354 Democratic 1263.12
38 Rhode Island Northeast Providence 1097379 1034 Democratic 1061.29
20 Massachusetts Northeast Boston 7029917 7800 Democratic 901.27
6 Connecticut Northeast Hartford 3605944 4842 Democratic 744.72
19 Maryland South Annapolis 6177224 9707 Democratic 636.37
... ... ... ... ... ... ... ...
40 South Dakota Midwest Pierre 886667 75811 Republican 11.70
33 North Dakota Midwest Bismarck 779094 69001 Republican 11.29
25 Montana West Helena 1084225 145546 Republican 7.45
49 Wyoming West Cheyenne 576851 97093 Republican 5.94
1 Alaska West Juneau 733391 570641 Republican 1.29

50 rows × 7 columns

In [39]:
# We must specify the role of False by using ascending=, 
# otherwise Python does not know how to interpret this.
states.sort_values(by='Density', False)

  File "/var/folders/28/vs8cp38n1r1520g8bhzr4v5h0000gn/T/ipykernel_17650/2563070448.py", line 3
    states.sort_values(by='Density', False)
                                          ^
SyntaxError: positional argument follows keyword argument

Step 2 – Extracting the state name¶

  • We saw that the most densely populated state is New Jersey, but how do we extract that information using code?
  • First, grab an entire column as a Series.
  • Navigate to a particular entry of the Series using .iloc[integer_position].
    • iloc stands for "integer location" and is used to count the rows, starting at 0.
In [40]:
ordered_states
Out[40]:
State Region Capital City Population Land Area Party Density
29 New Jersey Northeast Trenton 9288994 7354 Democratic 1263.12
38 Rhode Island Northeast Providence 1097379 1034 Democratic 1061.29
20 Massachusetts Northeast Boston 7029917 7800 Democratic 901.27
6 Connecticut Northeast Hartford 3605944 4842 Democratic 744.72
19 Maryland South Annapolis 6177224 9707 Democratic 636.37
... ... ... ... ... ... ... ...
40 South Dakota Midwest Pierre 886667 75811 Republican 11.70
33 North Dakota Midwest Bismarck 779094 69001 Republican 11.29
25 Montana West Helena 1084225 145546 Republican 7.45
49 Wyoming West Cheyenne 576851 97093 Republican 5.94
1 Alaska West Juneau 733391 570641 Republican 1.29

50 rows × 7 columns

In [41]:
ordered_states.get('State')
Out[41]:
29       New Jersey
38     Rhode Island
20    Massachusetts
6       Connecticut
19         Maryland
          ...      
40     South Dakota
33     North Dakota
25          Montana
49          Wyoming
1            Alaska
Name: State, Length: 50, dtype: object
In [42]:
# We want the first entry of the Series, which is at "integer location" 0.
ordered_states.get('State').iloc[0]
Out[42]:
'New Jersey'
  • The row label that goes with New Jersey is 29, because our original data was alphabetized by state and New Jersey is the 30th state alphabetically. But we don't use the row label when accessing with iloc; we use the integer position counting from the top.
  • If we try to use the row label (29) with iloc, we get the state with the 30th highest population density, which is not New Jersey.
In [43]:
ordered_states.get('State').iloc[29]
Out[43]:
'Minnesota'

Example 4: What is the population density of Pennsylvania?¶

Key concept: Accessing using row labels.

Population density of Pennsylvania¶

We know how to get the 'Density' of all states. How do we find the one that corresponds to Pennsylvania?

In [44]:
states
Out[44]:
State Region Capital City Population Land Area Party Density
0 Alabama South Montgomery 5024279 50645 Republican 99.21
1 Alaska West Juneau 733391 570641 Republican 1.29
2 Arizona West Phoenix 7151502 113594 Republican 62.96
3 Arkansas South Little Rock 3011524 52035 Republican 57.87
4 California West Sacramento 39538223 155779 Democratic 253.81
... ... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic 218.57
46 Washington West Olympia 7705281 66456 Democratic 115.95
47 West Virginia South Charleston 1793716 24038 Republican 74.62
48 Wisconsin Midwest Madison 5893718 54158 Republican 108.82
49 Wyoming West Cheyenne 576851 97093 Republican 5.94

50 rows × 7 columns

In [45]:
# Which one is Pennsylvania?
states.get('Density')
Out[45]:
0      99.21
1       1.29
2      62.96
3      57.87
4     253.81
       ...  
45    218.57
46    115.95
47     74.62
48    108.82
49      5.94
Name: Density, Length: 50, dtype: float64

Utilizing the index¶

  • When we load in a DataFrame from a CSV, columns have meaningful names, but rows do not.
In [46]:
bpd.read_csv('data/states.csv')
Out[46]:
State Region Capital City Population Land Area Party
0 Alabama South Montgomery 5024279 50645 Republican
1 Alaska West Juneau 733391 570641 Republican
2 Arizona West Phoenix 7151502 113594 Republican
3 Arkansas South Little Rock 3011524 52035 Republican
4 California West Sacramento 39538223 155779 Democratic
... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic
46 Washington West Olympia 7705281 66456 Democratic
47 West Virginia South Charleston 1793716 24038 Republican
48 Wisconsin Midwest Madison 5893718 54158 Republican
49 Wyoming West Cheyenne 576851 97093 Republican

50 rows × 6 columns

  • The row labels (or the index) are how we refer to specific rows. Instead of using numbers, let's refer to these rows by the names of the states they correspond to.
  • This way, we can easily identify, for example, which row corresponds to Pennsylvania.

Setting the index¶

  • To change the index, use .set_index(column_name).
  • Row labels should be unique identifiers.
    • Each row should have a different, descriptive name that corresponds to the contents of that row's data.
In [47]:
states
Out[47]:
State Region Capital City Population Land Area Party Density
0 Alabama South Montgomery 5024279 50645 Republican 99.21
1 Alaska West Juneau 733391 570641 Republican 1.29
2 Arizona West Phoenix 7151502 113594 Republican 62.96
3 Arkansas South Little Rock 3011524 52035 Republican 57.87
4 California West Sacramento 39538223 155779 Democratic 253.81
... ... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic 218.57
46 Washington West Olympia 7705281 66456 Democratic 115.95
47 West Virginia South Charleston 1793716 24038 Republican 74.62
48 Wisconsin Midwest Madison 5893718 54158 Republican 108.82
49 Wyoming West Cheyenne 576851 97093 Republican 5.94

50 rows × 7 columns

In [48]:
states.set_index('State')
Out[48]:
Region Capital City Population Land Area Party Density
State
Alabama South Montgomery 5024279 50645 Republican 99.21
Alaska West Juneau 733391 570641 Republican 1.29
Arizona West Phoenix 7151502 113594 Republican 62.96
Arkansas South Little Rock 3011524 52035 Republican 57.87
California West Sacramento 39538223 155779 Democratic 253.81
... ... ... ... ... ... ...
Virginia South Richmond 8631393 39490 Democratic 218.57
Washington West Olympia 7705281 66456 Democratic 115.95
West Virginia South Charleston 1793716 24038 Republican 74.62
Wisconsin Midwest Madison 5893718 54158 Republican 108.82
Wyoming West Cheyenne 576851 97093 Republican 5.94

50 rows × 6 columns

  • Now there is one fewer column. When you set the index, a column becomes the index, and the old index disappears.
  • 🚨 Like most DataFrame methods, .set_index returns a new DataFrame; it does not modify the original DataFrame.
In [49]:
states
Out[49]:
State Region Capital City Population Land Area Party Density
0 Alabama South Montgomery 5024279 50645 Republican 99.21
1 Alaska West Juneau 733391 570641 Republican 1.29
2 Arizona West Phoenix 7151502 113594 Republican 62.96
3 Arkansas South Little Rock 3011524 52035 Republican 57.87
4 California West Sacramento 39538223 155779 Democratic 253.81
... ... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic 218.57
46 Washington West Olympia 7705281 66456 Democratic 115.95
47 West Virginia South Charleston 1793716 24038 Republican 74.62
48 Wisconsin Midwest Madison 5893718 54158 Republican 108.82
49 Wyoming West Cheyenne 576851 97093 Republican 5.94

50 rows × 7 columns

In [50]:
states = states.set_index('State')
states
Out[50]:
Region Capital City Population Land Area Party Density
State
Alabama South Montgomery 5024279 50645 Republican 99.21
Alaska West Juneau 733391 570641 Republican 1.29
Arizona West Phoenix 7151502 113594 Republican 62.96
Arkansas South Little Rock 3011524 52035 Republican 57.87
California West Sacramento 39538223 155779 Democratic 253.81
... ... ... ... ... ... ...
Virginia South Richmond 8631393 39490 Democratic 218.57
Washington West Olympia 7705281 66456 Democratic 115.95
West Virginia South Charleston 1793716 24038 Republican 74.62
Wisconsin Midwest Madison 5893718 54158 Republican 108.82
Wyoming West Cheyenne 576851 97093 Republican 5.94

50 rows × 6 columns

In [51]:
# Which one is Pennsylvania? The one whose row label is "Pennsylvania"!
states.get('Density')
Out[51]:
State
Alabama           99.21
Alaska             1.29
Arizona           62.96
Arkansas          57.87
California       253.81
                  ...  
Virginia         218.57
Washington       115.95
West Virginia     74.62
Wisconsin        108.82
Wyoming            5.94
Name: Density, Length: 50, dtype: float64

Accessing using the row label¶

To pull out one particular entry of a DataFrame corresponding to a row and column with certain labels:

  1. Use .get(column_name) to extract the entire column as a Series.
  2. Use .loc[] to access the element of a Series with a particular row label.

In this class, we'll always first access a column, then a row (but row, then column is also possible).

In [52]:
states.get('Density')
Out[52]:
State
Alabama           99.21
Alaska             1.29
Arizona           62.96
Arkansas          57.87
California       253.81
                  ...  
Virginia         218.57
Washington       115.95
West Virginia     74.62
Wisconsin        108.82
Wyoming            5.94
Name: Density, Length: 50, dtype: float64
In [53]:
states.get('Density').loc['Pennsylvania']
Out[53]:
290.60858681804973

Summary: Accessing elements of a DataFrame¶

  • First, .get the appropriate column as a Series.
  • Then, use one of two ways to access an element of a Series:
    • .iloc[] uses the integer position.
    • .loc[] uses the row label.
    • Each is best for different scenarios.
In [54]:
states.get('Density')
Out[54]:
State
Alabama           99.21
Alaska             1.29
Arizona           62.96
Arkansas          57.87
California       253.81
                  ...  
Virginia         218.57
Washington       115.95
West Virginia     74.62
Wisconsin        108.82
Wyoming            5.94
Name: Density, Length: 50, dtype: float64
In [55]:
states.get('Density').iloc[4]
Out[55]:
253.80971119342146
In [56]:
states.get('Density').loc['California']
Out[56]:
253.80971119342146

Note¶

  • Sometimes the integer position and row label are the same.
  • This happens by default with bpd.read_csv.
In [57]:
bpd.read_csv('data/states.csv')
Out[57]:
State Region Capital City Population Land Area Party
0 Alabama South Montgomery 5024279 50645 Republican
1 Alaska West Juneau 733391 570641 Republican
2 Arizona West Phoenix 7151502 113594 Republican
3 Arkansas South Little Rock 3011524 52035 Republican
4 California West Sacramento 39538223 155779 Democratic
... ... ... ... ... ... ...
45 Virginia South Richmond 8631393 39490 Democratic
46 Washington West Olympia 7705281 66456 Democratic
47 West Virginia South Charleston 1793716 24038 Republican
48 Wisconsin Midwest Madison 5893718 54158 Republican
49 Wyoming West Cheyenne 576851 97093 Republican

50 rows × 6 columns

In [58]:
bpd.read_csv('data/states.csv').get('Capital City').loc[35]
Out[58]:
'Oklahoma City'
In [59]:
bpd.read_csv('data/states.csv').get('Capital City').iloc[35]
Out[59]:
'Oklahoma City'

Summary, next time¶

Summary¶

  • Arrays make it easy to perform arithmetic operations on all elements of an array and to perform element-wise operations on multiple arrays.
  • Ranges are arrays of equally-spaced numbers.
  • We learned many DataFrame methods and techniques. Don't feel the need to memorize them all right away.
  • Instead, refer to this lecture, the DSC 10 reference sheet, the babypandas notes, and the babypandas documentation when working on assignments.
  • Over time, these techniques will become more and more familiar. Lab 1 will walk you through many of them.
  • Practice! Frame your own questions using this dataset and try to answer them.

Next time¶

We'll frame more questions and learn more DataFrame manipulation techniques to answer them. In particular, we'll learn about querying and grouping.