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Updated quizzes
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@robjhyndman
robjhyndman committed Feb 13, 2024
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173 changes: 83 additions & 90 deletions week1/slides.qmd
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Expand Up @@ -40,14 +40,14 @@ source(here::here("course_info.R"))
names(df) <- c(1, 2)
```
1. In the following code, how much memory does `y` occupy?
2. In the following code, how much memory does `y` occupy?
```{r}
x <- runif(1e6)
y <- list(x, x, x)
```
1. On which line does `a` get copied in the following example?
3. On which line does `a` get copied in the following example?
```{r}
a <- c(1, 5, 3, 2)
Expand Down Expand Up @@ -282,17 +282,17 @@ knitr::include_graphics("../diagrams/name-value/character.png")
knitr::include_graphics("../diagrams/name-value/character-2.png")
```

## Exercises
## Quiz

3. Sketch out the relationship between the following objects:
4. Sketch out the relationship between the following objects:

```{r}
a <- 1:10
b <- list(a, a)
c <- list(b, a, 1:10)
```
4. What happens when you run this code?
5. What happens when you run this code?
```{r}
x <- list(1:10)
Expand Down Expand Up @@ -332,27 +332,27 @@ obj_size(c(1:1e6, 10))
obj_size(2 * (1:1e6))
```

## Exercises
## Quiz

Predict the output of the following code:
6. Predict the output of the following code:

```{r}
#| eval: false
a <- runif(1e6)
obj_size(a)
```{r}
#| eval: false
a <- runif(1e6)
obj_size(a)
b <- list(a, a)
obj_size(b)
obj_size(a, b)
b <- list(a, a)
obj_size(b)
obj_size(a, b)
b[[1]][[1]] <- 10
obj_size(b)
obj_size(a, b)
b[[1]][[1]] <- 10
obj_size(b)
obj_size(a, b)
b[[2]][[1]] <- 10
obj_size(b)
obj_size(a, b)
```
b[[2]][[1]] <- 10
obj_size(b)
obj_size(a, b)
```
## For loops
Expand Down Expand Up @@ -555,7 +555,7 @@ There are actually four missing values: `NA` (logical), `NA_integer_` (integer)
\fontsize{13}{14}\sf\vspace*{-0.25cm}

* For atomic vectors, all elements must be the same type.
* When you combine different types they are __coerced__ in a fixed order: character → double → integer → logical.
* When you combine different types they are __coerced__ in a fixed order: logical → integer → double → character.

\fontsize{10}{4}\sf

Expand All @@ -567,19 +567,19 @@ sum(x)
as.integer(c("1", "1.5", "a"))
```

## Exercises
## Quiz

1. Predict the output of the following:
7. Predict the output of the following:

```{r, eval=FALSE}
c(1, FALSE)
c("a", 1)
c(TRUE, 1L)
```
```{r, eval=FALSE}
c(1, FALSE)
c("a", 1)
c(TRUE, 1L)
```
1. Why is `1 == "1"` true? Why is `-1 < FALSE` true? Why is `"one" < 2` false?
8. Why is `1 == "1"` true? Why is `-1 < FALSE` true? Why is `"one" < 2` false?
1. Why is the default missing value, `NA`, a logical vector? What's special
9. Why is the default missing value, `NA`, a logical vector? What's special
about logical vectors? (Hint: think about `c(FALSE, NA_character_)`.)
## Getting and setting attributes
Expand Down Expand Up @@ -691,13 +691,11 @@ str(matrix(1:3, nrow = 1)) # row vector
str(array(1:3, 3)) # "array" vector
```

## Exercises

1. What does `dim()` return when applied to a 1-dimensional vector?
2. When might you use `NROW()` or `NCOL()`?
## Quiz

3. How would you describe the following three objects? What makes them
different from `1:5`?
10. What does `dim()` return when applied to a 1-dimensional vector?
11. When might you use `NROW()` or `NCOL()`?
12. How would you describe the following three objects? What makes them different from `1:5`?

```{r}
x1 <- array(1:5, c(1, 1, 5))
Expand Down Expand Up @@ -808,24 +806,23 @@ structure(now_ct, tzone = "Australia/Lord_Howe")
```


## Exercises
## Quiz

1. What sort of object does `table()` return? What is its type? What
13. What sort of object does `table()` return? What is its type? What
attributes does it have? How does the dimensionality change as you
tabulate more variables?

1. What happens to a factor when you modify its levels?
14. What happens to a factor when you modify its levels?

```{r, results = FALSE}
f1 <- factor(letters)
levels(f1) <- rev(levels(f1))
```
1. What does this code do? How do `f2` and `f3` differ from `f1`?
15. What does this code do? How do `f2` and `f3` differ from `f1`?
```{r, results = FALSE}
f2 <- rev(factor(letters))
f3 <- factor(letters, levels = rev(letters))
```
Expand Down Expand Up @@ -1038,13 +1035,13 @@ str(dfm)

\placefig{10}{1.25}{width=5cm,height=20cm}{../diagrams/vectors/data-frame-matrix.png}

## Exercises
## Quiz

1. Can you have a data frame with zero rows? What about zero columns?
1. What happens if you attempt to set rownames that are not unique?
1. If `df` is a data frame, what can you say about `t(df)`, and `t(t(df))`?
16. Can you have a data frame with zero rows? What about zero columns?
17. What happens if you attempt to set rownames that are not unique?
18. If `df` is a data frame, what can you say about `t(df)`, and `t(t(df))`?
Perform some experiments, making sure to try different column types.
1. What does `as.matrix()` do when applied to a data frame with
19. What does `as.matrix()` do when applied to a data frame with
columns of different types? How does it differ from `data.matrix()`?

## `NULL`
Expand All @@ -1063,19 +1060,19 @@ is.null(x)

# Subsetting

## Subsetting
## Quiz

1. What is the result of subsetting a vector with positive integers,
20. What is the result of subsetting a vector with positive integers,
negative integers, a logical vector, or a character vector?
1. What's the difference between `[`, `[[`, and `$` when applied to a list?
1. When should you use `drop = FALSE`?
1. If `x` is a matrix, what does `x[] <- 0` do? How is it different from `x <- 0`?
1. How can you use a named vector to relabel categorical variables?
21. What's the difference between `[`, `[[`, and `$` when applied to a list?
22. When should you use `drop = FALSE`?
23. If `x` is a matrix, what does `x[] <- 0` do? How is it different from `x <- 0`?
24. How can you use a named vector to relabel categorical variables?

## Subsetting
\fontsize{13}{14}\sf

1. Fix each of the following common data frame subsetting errors:
25. Fix each of the following common data frame subsetting errors:

```{r, eval = FALSE}
mtcars[mtcars$cyl = 4, ]
Expand All @@ -1084,71 +1081,67 @@ is.null(x)
mtcars[mtcars$cyl == 4 | 6, ]
```
1. Why does `mtcars[1:20]` return an error? How does it differ from the
26. Why does `mtcars[1:20]` return an error? How does it differ from the
similar `mtcars[1:20, ]`?
1. Implement your own function that extracts the diagonal entries from a
27. Implement your own function that extracts the diagonal entries from a
matrix (it should behave like `diag(x)` where `x` is a matrix).
## Subsetting
## Quiz
```{r}
#| eval: false
mod <- lm(mpg ~ wt, data = mtcars)
```
28. Extract the residual degrees of freedom from `mod`
Extract the residual degrees of freedom.
```{r}
#| eval: false
mod <- lm(mpg ~ wt, data = mtcars)
```
Extract the R squared from the model summary (`summary(mod)`)
29. Extract the R squared from the model summary (`summary(mod)`)
## Subsetting
## Quiz
1. How would you randomly permute the columns of a data frame?
2. Can you simultaneously permute the rows and columns in one step?
3. How would you select a random sample of m rows from a data frame? What if the sample had to be contiguous (i.e., with an initial row, a final row, and every row in between)?
4. How could you put the columns in a data frame in alphabetical order?
30. How would you randomly permute the columns of a data frame?
31. Can you simultaneously permute the rows and columns in one step?
32. How would you select a random sample of m rows from a data frame? What if the sample had to be contiguous (i.e., with an initial row, a final row, and every row in between)?
33. How could you put the columns in a data frame in alphabetical order?
# Control flow
## Control flow
## Quiz
* What is the difference between `if` and `ifelse()`?
34. What is the difference between `if` and `ifelse()`?
* In the following code, what will the value of `y` be if `x` is `TRUE`?
35. In the following code, what will the value of `y` be if `x` is `TRUE`?
What if `x` is `FALSE`? What if `x` is `NA`?
```{r, eval = FALSE}
y <- if (x) 3
```
* What does `switch("x", x = , y = 2, z = 3)` return?
36. What does `switch("x", x = , y = 2, z = 3)` return?
## ifelse
## Quiz
\fontsize{13}{14}\sf
What type of vector does each of the following calls to `ifelse()`
37. What type of vector does each of the following calls to `ifelse()`
return?
```{r, eval = FALSE}
ifelse(TRUE, 1, "no")
ifelse(FALSE, 1, "no")
ifelse(NA, 1, "no")
```

Read the documentation and write down the rules in your own words.

## if else
```{r, eval = FALSE}
ifelse(TRUE, 1, "no")
ifelse(FALSE, 1, "no")
ifelse(NA, 1, "no")
```
Why does the following code work?
38. Why does the following code work?\fontsize{10}{10}\sf
```{r}
x <- 1:10
if (length(x)) "not empty" else "empty"
```{r}
x <- 1:10
if (length(x)) "not empty" else "empty"
x <- numeric()
if (length(x)) "not empty" else "empty"
```
x <- numeric()
if (length(x)) "not empty" else "empty"
```
## Don't allocate memory in a for loop
Expand Down
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