Discussion 4: Functions, Subgroups, Merge, and Control Flow

← return to practice.dsc10.com


The problems in this worksheet are taken from past exams. Work on them on paper, since the exams you take in this course will also be on paper.

We encourage you to complete this worksheet in a discussion section, which is held live on Monday, October 17th. Solutions will be made available after all discussion sections have concluded. You don’t need to submit your answers anywhere.


Problem 1

Included is a DataFrame named sungod that contains information on the artists who have performed at Sun God in years past. For each year that the festival was held, we have one row for each artist that performed that year. The columns are:

The rows of sungod are arranged in no particular order. The first few rows of sungod are shown below (though sungod has many more rows than pictured here).

Assume:

Fill in the blank in the code below so that chronological is a DataFrame with the same rows as sungod, but ordered chronologically by appearance on stage. That is, earlier years should come before later years, and within a single year, artists should appear in the DataFrame in the order they appeared on stage at Sun God. Note that groupby automatically sorts the index in ascending order.

chronological = sungod.groupby(___________).max().reset_index()

Answer: ['Year', 'Appearance_Order']

The fact that groupby automatically sorts the index in ascending order is important here. Since we want earlier years before later years, we could group by 'Year', however if we just group by year, all the artists who performed in a given year will be aggregated together, which is not what we want. Within each year, we want to organize the artists in ascending order of 'Appearance_Order'. In other words, we need to group by 'Year' with 'Appearance_Order' as subgroups. Therefore, the correct way to reorder the rows of sungod as desired is sungod.groupby(['Year', 'Appearance_Order']).max().reset_index(). Note that we need to reset the index so that the resulting DataFrame has 'Year' and 'Appearance_Order' as columns, like in sungod.



Difficulty: ⭐️⭐️

The average score on this problem was 85%.


Problem 2

Another DataFrame called music contains a row for every music artist that has ever released a song. The columns are:

You want to know how many musical genres have been represented at Sun God since its inception in 1983. Which of the following expressions produces a DataFrame called merged that could help determine the answer?

Answer: merged = sungod.merge(music, left_on='Artist', right_on='Name')

The question we want to answer is about Sun God music artists’ genres. In order to answer, we’ll need a DataFrame consisting of rows of artists that have performed at Sun God since its inception in 1983. If we merge the sungod DataFrame with the music DataFrame based on the artist’s name, we’ll end up with a DataFrame containing one row for each artist that has ever performed at Sun God. Since the column containing artists’ names is called 'Artist' in sungod and 'Name' in music, the correct syntax for this merge is merged = sungod.merge(music, left_on='Artist', right_on='Name'). Note that we could also interchange the left DataFrame with the right DataFrame, as swapping the roles of the two DataFrames in a merge only changes the ordering of rows and columns in the output, not the data itself. This can be written in code as merged = music.merge(sungod, left_on='Name', right_on='Artist'), but this is not one of the answer choices.



Difficulty: ⭐️⭐️

The average score on this problem was 86%.


Problem 3

Consider an artist that has only appeared once at Sun God. At the time of their Sun God performance, we’ll call the artist

Complete the function below so it outputs the appropriate description for any input artist who has appeared exactly once at Sun God.

def classify_artist(artist):
    filtered = merged[merged.get('Artist') == artist]
    year = filtered.get('Year').iloc[0]
    top_hit_year = filtered.get('Top_Hit_Year').iloc[0]
    if ___(a)___ > 0:
        return 'up-and-coming'
    elif ___(b)___:
        return 'outdated'
    else:
        return 'trending'


Problem 3.1

What goes in blank (a)?

Answer: top_hit_year - year

Before we can answer this question, we need to understand what the first three lines of the classify_artist function are doing. The first line creates a DataFrame with only one row, corresponding to the particular artist that’s passed in as input to the function. We know there is just one row because we are told that the artist being passed in as input has appeared exactly once at Sun God. The next two lines create two variables:

  • year contains the year in which the artist performed at Sun God, and
  • top_hit_year contains the year in which their top hit song was released.

Now, we can fill in blank (a). Notice that the body of the if clause is return 'up-and-coming'. Therefore we need a condition that corresponds to up-and-coming, which we are told means the top hit came out after the artist appeared at Sun God. Using the variables that have been defined for us, this condition is top_hit_year > year. However, the if statement condition is already partially set up with > 0 included. We can simply rearrange our condition top_hit_year > year by subtracting year from both sides to obtain top_hit_year - year > 0, which fits the desired format.



Difficulty: ⭐️⭐️

The average score on this problem was 89%.


Problem 3.2

What goes in blank (b)?

Answer: year-top_hit_year > 5

For this part, we need a condition that corresponds to an artist being outdated which happens when their top hit came out more than five years prior to their appearance at Sun God. There are several ways to state this condition: year-top_hit_year > 5, year > top_hit_year + 5, or any equivalent condition would be considered correct.



Difficulty: ⭐️⭐️

The average score on this problem was 89%.



Problem 4

King Triton, UCSD’s mascot, is quite the traveler! For this question, we will be working with the flights DataFrame, which details several facts about each of the flights that King Triton has been on over the past few years. The first few rows of flights are shown below.

Here’s a description of the columns in flights:

Suppose we create a DataFrame called socal containing only King Triton’s flights departing from SAN, LAX, or SNA (John Wayne Airport in Orange County). socal has 10 rows; the bar chart below shows how many of these 10 flights departed from each airport.

Consider the DataFrame that results from merging socal with itself, as follows:

double_merge = socal.merge(socal, left_on='FROM', right_on='FROM')

How many rows does double_merge have?

Answer: 38

There are two flights from LAX. When we merge socal with itself on the 'FROM' column, each of these flights gets paired up with each of these flights, for a total of four rows in the output. That is, the first flight from LAX gets paired with both the first and second flights from LAX. Similarly, the second flight from LAX gets paired with both the first and second flights from LAX.

Following this logic, each of the five flights from SAN gets paired with each of the five flights from SAN, for an additional 25 rows in the output. For SNA, there will be 9 rows in the output. The total is therefore 2^2 + 5^2 + 3^2 = 4 + 25 + 9 = 38 rows.



Difficulty: ⭐️⭐️⭐️⭐️⭐️

The average score on this problem was 27%.


Problem 5

We define a “route” to be a departure and arrival airport pair. For example, all flights from 'SFO' to 'SAN' make up the “SFO to SAN route”. This is different from the “SAN to SFO route”.

Fill in the blanks below so that most_frequent.get('FROM').iloc[0] and most_frequent.get('TO').iloc[0] correspond to the departure and destination airports of the route that King Triton has spent the most time flying on.

most_frequent = flights.groupby(__(a)__).__(b)__
most_frequent = most_frequent.reset_index().sort_values(__(c)__)


Problem 5.1

What goes in blank (a)?

Answer: ['FROM', 'TO']

We want to organize flights by route. This means we need to group by both 'FROM' and 'TO' so any flights with the same pair of departure and arrival airports get grouped together. To group by multiple columns, we must use a list containing all these column names, as in flights.groupby(['FROM', 'TO']).



Difficulty: ⭐️⭐️⭐️

The average score on this problem was 72%.


Problem 5.2

What goes in blank (b)?

Answer: sum()

Every .groupby command needs an aggregation function! Since we are asked to find the route that King Triton has spent the most time flying on, we want to total the times for all flights on a given route.

Note that .count() would tell us how many flights King Triton has taken on each route. That’s meaningful information, but not what we need to address the question of which route he spent the most time flying on.



Difficulty: ⭐️⭐️⭐️

The average score on this problem was 58%.


Problem 5.3

What goes in blank (c)?

Answer: by='HOURS', ascending=False

We want to know the route that King Triton spent the most time flying on. After we group flights by route, summing flights on the same route, the 'HOURS' column contains the total amount of time spent on each route. We need most_frequent.get('FROM').iloc[0] and most_frequent.get('TO').iloc[0] to correspond with the departure and destination airports of the route that King Triton has spent the most time flying on. To do this, we need to sort in descending order of time, to bring the largest time to the top of the DataFrame. So we must sort by 'HOURS' with ascending=False.



Difficulty: ⭐️

The average score on this problem was 94%.



Problem 6

For this question we will use data from the 2021 Women’s National Basketball Association (WNBA) season for the next several problems. In basketball, players score points by shooting the ball into a hoop. The team that scores the most points wins the game.

Kelsey Plum, a WNBA player, attended La Jolla Country Day School, which is adjacent to UCSD’s campus. Her current team is the Las Vegas Aces (three-letter code 'LVA'). In 2021, the Las Vegas Aces played 31 games, and Kelsey Plum played in all 31.

The DataFrame plum contains her stats for all games the Las Vegas Aces played in 2021. The first few rows of plum are shown below (though the full DataFrame has 31 rows, not 5):

Each row in plum corresponds to a single game. For each game, we have:


Problem 6.1

Consider the function unknown, defined below.

def unknown(df):
    grouped = plum.groupby('Opp').max().get(['Date', 'PTS'])
    return np.array(grouped.reset_index().index)[df]

What does unknown(3) evaluate to?

Answer: The number 3

plum.groupby('Opp').max() finds the largest value in the 'Date', 'Home', 'Won', 'PTS', 'AST', and 'TOV' columns for each unique 'Opp' (independently for each column). grouped = plum.groupby('Opp').max().get(['Date', 'PTS']) keeps only the 'Date' and 'PTS' columns. Note that in grouped, the index is 'Opp', the column we grouped on.

When grouped.reset_index() is called, the index is switched back to the default of 0, 1, 2, 3, 4, and so on. Then, grouped.reset_index().index is an Index containing the numbers [0, 1, 2, 3, 4, ...], and np.array(grouped.reset_index().index) is np.array([0, 1, 2, 3, 4, ...]). In this array, the number at position i is just i, so the number at position df is df. Here, df is the argument to unknown, and we were asked for the value of unknown(3), so the correct answer is the number at position 3 in np.array([0, 1, 2, 3, 4, ...]) which is 3.

Note that if we asked for unknown(50) (or unknown(k), where k is any integer above 30), the answer would be “An error”, since grouped could not have had 51 rows. plum has 31 rows, so grouped has at most 31 rows (but likely less, since Kelsey Plum’s team likely played the same opponent multiple times).



Difficulty: ⭐️⭐️⭐️

The average score on this problem was 72%.


Problem 6.2

For your convenience, we show the first few rows of plum again below.

Suppose that Plum’s team, the Las Vegas Aces, won at least one game in Las Vegas and lost at least one game in Las Vegas. Also, suppose they won at least one game in an opponent’s arena and lost at least one game in an opponent’s arena.

Consider the DataFrame home_won, defined below.

home_won = plum.groupby(['Home', 'Won']).mean().reset_index()
  1. How many rows does home_won have?

  2. How many columns does home_won have?

Answer: 4 rows and 5 columns.

plum.groupby(['Home', 'Won']).mean() contains one row for every unique combination of 'Home' and 'Won'. There are two values of 'Home' - True and False – and two values of 'Won'True and False – leading to 4 combinations. We can assume that there was at least one row in plum for each of these 4 combinations due to the assumption given in the problem:

Suppose that Plum’s team, the Las Vegas Aces, won at least one game in Las Vegas and lost at least one game in Las Vegas. Also, suppose they won at least one game in an opponent’s arena and lost at least one game in an opponent’s arena.

plum started with 7 columns: 'Date', 'Opp', 'Home', 'Won', 'PTS', 'AST', and 'TOV'. After grouping by ['Home', 'Won'] and using .mean(), 'Home' and 'Won' become the index. The resulting DataFrame contains all of the columns that the .mean() aggregation method can work on. We cannot take the mean of 'Date' and 'Opp', because those columns are strings, so plum.groupby(['Home', 'Won']).mean() contains a MultiIndex with 2 “columns” – 'Home' and 'Won' – and 3 regular columns – 'PTS' 'AST', and 'TOV'. Then, when using .reset_index(), 'Home' and 'Won' are restored as regular columns, meaning that plum.groupby(['Home', 'Won']).mean().reset_index() has 2 + 3 = 5 columns.



Difficulty: ⭐️⭐️

The average score on this problem was 78%.


Problem 6.3

Consider the DataFrame home_won once again.

home_won = plum.groupby(['Home', 'Won']).mean().reset_index()

Now consider the DataFrame puzzle, defined below. Note that the only difference between home_won and puzzle is the use of .count() instead of .mean().

puzzle = plum.groupby(['Home', 'Won']).count().reset_index()

How do the number of rows and columns in home_won compare to the number of rows and columns in puzzle?

Answer: home_won and puzzle have the same number of rows, but a different number of columns

All that changed between home_won and puzzle is the aggregation method. The aggregation method has no influence on the number of rows in the output DataFrame, as there is still one row for each of the 4 unique combinations of 'Home' and 'Won'.

However, puzzle has 7 columns, instead of 5. In the solution to the above subpart, we noticed that we could not use .mean() on the 'Date' and 'Opp' columns, since they contained strings. However, we can use .count() (since .count() just determines the number of non-NA values in each group), and so the 'Date' and 'Opp' columns are not “lost” when aggregating. Hence, puzzle has 2 more columns than home_won.



Difficulty: ⭐️⭐️

The average score on this problem was 85%.



👋 Feedback: Find an error? Still confused? Have a suggestion? Let us know here.