---
title: "PS 1"
format: html
---

### Data in Trees

@. Have you completed the Stat 133 Welcome Survey linked on Ed?

Consider the file system below where the root is `/`. The shapes following the names aren't part of the file system; they're pointers used in the questions below.

```bash
/ △
└── fall
    ├── math53 ○
    │   ├── syllabus.pdf
    │   ├── midterm.pdf
    │   └── final.pdf
    ├── stat20
    │   ├── syllabus.docx ●
    │   └── labs ▽
    │       ├── lab1.qmd
    │       ├── lab2.qmd
    │       └── lab3.qmd ▲
    ├── history7B □
        ├── syllabus.pdf
        ├── essay1.docx ■
        ├── essay2.docx
        └── essay3.docx
```

@. What is the absolute path to ■?

@. What is the absolute path to ▽?

@. What is the relative path from □ to ■?

@. What is the relative path from ○ to ■?

@. What is the relative path from ▽ to ●?

@. What is the relative path from □ to ▲?

@. What is the relative path from ▽ to △?

Fill in the blank `_______` with the full command the will generate the output that follows. Let `$` be the character that indicates where you can type at the command line. The working directory at the start of each question is `/`, the root.

@. Fill in the blank.

    ```bash
    $ _______
    syllabus.pdf essay1.docx essay2.docx essay3.docx
    ```

@. Fill in the blank.

    ```bash
    $ cd fall
    $ _______
    fall
    ```

@. In the space below, sketch the file system containing files with the following absolute paths:

```bash
/yosemite/receipts/gas.jpg
/yosemite/5400.jpg
/yosemite/reservations.pdf
/hawaii/3296.jpg
/yosemite/receipts/lunch.jpg
/hawaii/3295.jpg
/hawaii/3298.jpg
/yosemite/5399.jpg
/hawaii/3297.jpg
/yosemite/receipts/dinner.jpg
```

@. Every directory and file name that you've seen so far in this class has refrained from using special characters. Which characters should you avoid using when naming directories and files?^[You can research the answer to this question online.]

### Intro to R

@. Have you installed R on your computer? Do you have access to the command line via a shell? This will likely be Terminal on a Mac (which is built-in) or git bash on Windows. See the Tools section of the course website for guidance.

You're encouraged to work on these at your R console to try things out. The next three questions continue with the same R objects. What output will each code block produce?

@. What will the following code produce?

```r
a <- 3 + 5
b <- 4 / 2
a + b
```

@. What will the following code produce?

```r
a <- b - (a / a)
```

@. What will the following code produce?

```r
a
```

@. What will the following code produce?

```r
log(a)
```

For the following questions, consult the functions help file by using `?` (e.g. `?round`).

@. What modified version of this function would return the value 3.14 instead of 3?
   
```{r}
round(3.141592)
```

@. What will the following code produce?

```r
ceiling(3.14) + floor(3.14)
```

### R on the File System

Assume you start a new R session with your working directory at `~`. Which R functions would you call to do each of the following?

```bash
~
├── Project-1
│   ├── raw-data-1.txt
│   ├── processed-data-1.csv
│   └── data-processing-1.r
├── Project-2
│   ├── raw-data-2.txt
│   ├── processed-data-2.csv
│   └── data-processing-2.r
```

@. Change your working directory to be `Project-1`?

@. Read the data in `raw-data-1.txt` and save it to the R object named `raw_data_1`.

@. Without changing your working directory, read the data in `raw-data-2.txt` and save it to the R object named `raw_data_2`.

@. You may have noticed that when creating R objects, I avoid certain types of names. What characters are permissable when naming new objects?^[You can research the answer to this question online.]

### Atomic Vectors

If you recorded data on peoples' answers to the following questions and read each one into R as a vector, what data type could be used to store them?

@. How much do you spend on average per week, in dollars and cents, on caffeinated beverages?

@. Did you vote in the last US Election?

@. How many US states have you visited?

While each of the following vectors *could* be created by simply using `c()`, instead write a command that can generate each one more efficiently using the functions from class. Read more about them by pulling up their help files with `?`.

@. `_________________________`

```{r}
#| echo: false
seq(2, 48, 2)
```

@. `_________________________`

```{r}
#| echo: false
rep(seq(0, 15, 5), each = 3)
```

@. What single command can be run on each of these vectors

```{r}
x <- c("A", "B", "C")
x <- seq(1, 5, 1)
x <- c(TRUE, FALSE)
```

and produce this output?

```{r}
#| echo: false
x <- c("A", "B", "C")
rep(x, 1:length(x))
x <- seq(1, 5, 1)
rep(x, 1:length(x))
x <- c(TRUE, FALSE)
rep(x, 1:length(x))
```

@. Predict the output of the following code.

```{r}
#| eval: false

1:3 + 1:2 + 1:1
```

@. Predict the output of the following code.

```{r}
#| eval: false

1:1 + 1:2 + 1:3
```

@. Which vector results from running the following code? Does it throw a message, warning, or error and why?

```{r}
#| eval: false

c(5, 5, 5) * c(2, 1, NULL)
```

@. Which vector results from running the following code? Does it throw a message, warning, or error and why?

```{r}
#| eval: false

c(5, 5, 5) * c(2, 1, NA)
```

@. Predict the output and type of the following three vectors.

```{r}
#| eval: false

c(1, FALSE)
c("a", 1)
c(TRUE, 1L)
```

@. Use square bracket subsetting to create the following vector from `z <- c(apple = 2, banana = 4, cherry = 8)`.

```{r}
#| echo: false

z <- c(apple = 2, banana = 4, cherry = 8)
z[c(3, 2, 1)]
```

@. Write down *three* subsetting methods (index, name, and logical) to generate the following vector from `z`. If you're not able to do it in one subsetting operation, say why not.

```{r}
#| echo: false
z[c(3, 3)]
```

@. What happens when you subset a vector with a negative number?

### Extending Vectors

@. Provide the code that will create a matrix object of type double with 3 rows and 4 columns. The entries should be the integers between 1 and 12 arrayed *rowwise*. Name the object `mat_twelve`.

@. Subset the matrix to isolate the element with the value `5`.

@. Provide the code that will break the `mat_twelve` into two separate matrices: `mat_left` with the 6 leftmost entries, and `mat_right` with the remaining six.

@. What does `mat_left + mat_right` evaluate to?

You can load the "like" matrix into R using the following code.

```r
install.packages("lda") # install lda package
library(lda)            # load lda package
data(sampson)           # load data
samplik <- sampson[["SAMPLK1"]] # extract the samplik matrix
```

@. Provide the code to extract the vector of the ranks that Basil gave other monks from 0 (unranked) to 3 (most liked).

@. Provide the code to extract the vector of the ranks that other monks gave Basil from 0 (unranked) to 3 (most liked).

@. Matrices make it easy and fast to apply the same function across all rows or columns. Use `rowSums()` or `colSum()` and the sort function to return the three most like monks and the three least liked / ranked monks.