# Data Structures in `R`

The two most important data structures in `R` are the (atomic) vector and data frame.  Let's do some practice with those.

## Vectors

1. We often need to get every combination of a vector or vectors to use when fitting models. For instance if I had three vectors 
- `a` (with elements `10` and `12`)
- `b` (with elements `1` and `3`)
- `c` ( with elements `c1`, `c2`, and `c3`)
we often need to create a data frame that looks like the following:

|Var1 | Var2 | Var 3 |
|----:|-----:|------:|
|10   | 1    | c1    |
|12   | 1    | c1    |
|10   | 3    | c1    |
|12   | 3    | c1    |
|10   | 1    | c2    |
|12   | 1    | c2    |
|10   | 3    | c2    |
|12   | 3    | c2    |
|10   | 1    | c3    |
|12   | 1    | c3    |
|10   | 3    | c3    |
|12   | 3    | c3    |

<ul>
a. Create a (numeric) vector with values 10 and 12.  Save this as an R object, `a`.  Repeat to create `b`. 
    ```{r}

    ```
b. Create a (character) vector with values 'c1', 'c2', and 'c3'. Save this as an R object, `c`.
    ```{r}

    ```
c. Use the `expand.grid()` function to create the data frame above.  See the examples at the bottom of the help page (`help("expand.grid")`)
    ```{r}

    ```
</ul>

2. Another common task is to combine character strings together.  This can be done with the `paste()` function.
    a. Create a character vector (call it `adj`) of length 4 consisting of adjectives (answers will vary).
        ```{r}
    
        ```
    b. Create a second character vector of same length consisting of nouns (call it `nouns`).
        ```{r}

        ```
    c. Use the `paste()` function to combine these vectors elementwise.  Play around with the `sep = ` argument.
        ```{r}

        ```

## Data Frames

R has some built in data frames that we can use.  Run `ToothGrowth` (note the case) in your console to see a data set about tooth growth for Guinea Pigs based off of differing amounts of Vitamin C (VC vs OJ).  

1. Determine the internal structure of the `ToothGrowth` object using the `str()` function
    ```{r}

    ```
2. Have R return just the dose column using two different methods.
    ```{r}

    ```