Exercises – Subsetting (Introduction to R)

Author

Affiliation

Jürgen Wilbert

University of Münster

1. Vector subsetting with []

1. Create a numeric vector x <- c(10, 20, 30, 40, 50) and a named version x_named with names c("a","b","c","d","e").
2. Select the 2nd and 4th elements using numeric indexing
3. Reorder the vector x <- c(10, 20, 30, 40, 50) with c(5,1,5,3).
4. Select elements "b" and "e" from x_named using character indexing.
5. Select all elements of x greater than 25 using a logical condition. How many elements were selected? Use a function to verify.
6. Use negative indices to drop the first and last element of x.

2. Subsetting data frames: columns

1. Create a data frame dat <- data.frame(id = 1:5, group = factor(c("A","A","B","B","B")), score = c(12,18,22,19,25)).
2. Select column score using a numeric index and then by name with (single) square brackets. Confirm the result is a data frame (not a vector) and explain why.
3. Provide a vector of indices to select two columns (id and score) in one operation.
4. Rename columns to c("ID","Group","Score") using names() and show the names of the renamed data.frame.

3. Subsetting data frames: rows & conditions

1. Select all rows where Score is greater or equal 20 using a logical condition inside [].
2. Use which() to obtain the row indices for Group == "B" and subset dat with these indices.
3. Use with(dat, ...) to express a condition of your choice without repeatedly writing dat$.
4. Create a new column pass with ifelse() that has logical values for Score >= 20.

4. Using subset()

1. Recreate Task 3.1 using subset().
2. Recreate Task 2.3 by selecting only columns ID and Score via subset().
3. Create a one-line expression that returns the IDs of the top two scores in dat (ties allowed). Hint: order() can help.