Data Frames

Question 1

What is a data frame?

Answer 1

Two dimensional object

Question 2

Can a single data frame hold elements of multiple data types?

Answer 2

Yes

Question 3

Which functions show the first or last observations in the dataset?

Answer 3

head() and tail()

Ex: head(my_df)

Ex: tail(my_df)

Question 4

Which function gives an overview of the data in your dataset?

Answer 4

str()

Ex: str(my_df)

Question 5

What does the str() function reveal?

Answer 5

The total number of observations (e.g. 32 car types)

The total number of variables (e.g. 11 car features)

A full list of the variables names (e.g. mpg, cyl … )

The data type of each variable (e.g. num)

The first observations

Question 6

What is the function to create a data frame?

Answer 6

data.frame()

Ex: data.frame(my_vec1, my_vec2, my_vec3, …)

Question 7

How can you select a single element from a data frame?

Answer 7

[num1, num2]

Ex: my_df[1,2]

Question 8

How can you select multiple in-sequence elements from a data frame?

Answer 8

[num1:num2, num3:num4] OR [num1:num2, var]

Ex: my_df[1:3, 2:4] OR my_df[1:3, “type”]

Question 9

How can you select an entire column or row from a data frame?

Answer 9

[,num] and [num,] and $var

Ex: my_df[,1]

Ex: my_df[1,]

Ex: my_df$type

Question 10

Which function selects a portion of the data frame based on whether or not a certain condition is true?

Answer 10

subset()

Ex: subset(my_df, subset = some_condition)

Question 11

Which function sorts the data according to a certain variable in the dataset?

Answer 11

order()

Ex: order(c(100, 10, 1000)) will print 2, 1, 3

Question 12

How can we use the output of order(a) to reshuffle a?

Answer 12

a[order(a)] will print out a in order