# Collections and Data Structures

## Lists

Lists in R are ordered collections of data that can be of different classes. 

### Creating Lists

| Action           | Syntax                            |
| ---------------- | --------------------------------- |
| New list (empty) | listname <- list()                |
| New list (misc)  | listname <- list(1L, "abc", 10.3) |

### Accessing List Elements

<table><thead><tr><th width="342">Action</th><th>Syntax</th></tr></thead><tbody><tr><td>Access an element</td><td>list[position]</td></tr><tr><td>Change a value</td><td>list[position] &#x3C;- newvalue</td></tr><tr><td>See number of values in a list</td><td>length(list)</td></tr><tr><td>See if item is present in a list</td><td>item %in% list</td></tr></tbody></table>

### Adding and Removing List Elements

| Action                                    | Syntax                             |
| ----------------------------------------- | ---------------------------------- |
| Add item to a list                        | append(list)                       |
| Add item to a list at a specific position | append(list, after=*index number*) |
| Remove item from list                     | newlist <- list\[-*index number]*  |

#### Inputs:

```r
#Create list
mylist <- list("apple", "peach", "plum")

#Access the second element of a list
mylist[2]

#Change the value of the first element of a list
mylist[1] <- "banana"
mylist

#See the number of values in a list
length(mylist)

#Check if item exists in list
"plum" %in% mylist

#Add an item to the list
append(mylist, "orange", after=2)
mylist

#Remove an item at index=3 from a list
mylist <- list("apple", "peach", "plum")
newlist <- mylist[-3]
newlist
```

#### Outputs:

```r
#Access the second element of a list
"peach"

#Change the value of the first element of a list
[[1]]
[1] "banana"

[[2]]
[1] "peach"

[[3]]
[1] "plum"

#See the number of values in a list
3

#Check if item exists in list
TRUE

#Add an item to the list
[[1]]
[1] "banana"

[[2]]
[1] "peach"

[[3]]
[1] "orange"

[[4]]
[1] "plum"

#Remove an item from a list
[[1]]
[1] "apple"

[[2]]
[1] "peach"
```

## Matrices

### Creating Matrices

<table><thead><tr><th width="248">Action</th><th>Syntax</th></tr></thead><tbody><tr><td>New matrix (empty)</td><td>matrixname &#x3C;- matrix()</td></tr><tr><td>New matrix (numbers)</td><td>matrixname &#x3C;- matrix(data, nrow=, ncol=)</td></tr><tr><td>New matrix (strings)</td><td>matrixname &#x3C;- matrix(data, nrow=, ncol=)</td></tr></tbody></table>

### Accessing Matrix Elements

| Action                  | Syntax                                 |
| ----------------------- | -------------------------------------- |
| Access a matrix element | matrix\[row position, column position] |
| Access an entire row    | matrix\[row position,]                 |
| Access an entire column | matrix\[,column position]              |

### Adding and Removing Matrix Elements

| Action                      | Syntax                               |
| --------------------------- | ------------------------------------ |
| Create an additional row    | rbind(matrix, values for new row)    |
| Create an additional column | cbind(matrix, values for new column) |

#### Inputs:

```r
#Creating array
heart <- matrix(c("left atrium", "left ventricle", 
    "right atrium", "right ventricle"), nrow=2, ncol=2)
heart

#Access element at row=1, column=2
heart[1,2]

#Access entire row 1
heart[1,]

#Access entire column 2
heart[,2]

#Create new row
heart1 <- rbind(heart, c("x", "x"))
heart1

#Create new column
heart2 -< cbind(heart1, c("y", "y", "z"))
heart2
```

#### Outputs:

```r
#Creating array
     [,1]           [,2]      
[1,] "left atrium"    "right atrium"   
[2,] "left ventricle" "right ventricle"

#Access element at row=1, column=2
"right atrium"

#Access entire row 1
"left atrium"  "right atrium"

#Access entire column 2
"right atrium" "right ventricle"

#Create new row
     [,1]             [,2]             
[1,] "left atrium"    "right atrium"   
[2,] "left ventricle" "right ventricle"
[3,] "x"              "x"   

#Create new column
     [,1]             [,2]              [,3]
[1,] "left atrium"    "right atrium"    "y" 
[2,] "left ventricle" "right ventricle" "y" 
[3,] "x"              "x"               "z" 
```

## Arrays

### Creating Arrays

<table><thead><tr><th width="253">Action</th><th>Syntax</th></tr></thead><tbody><tr><td>New array (empty)</td><td>arrayname &#x3C;- array()</td></tr><tr><td>New array (numbers)</td><td>arrayname &#x3C;- array(data, dim(nrow=, ncol=, ndim=)</td></tr><tr><td>New array (strings</td><td>arrayname &#x3C;- array(data, dim(nrow=, ncol=, ndim=)</td></tr></tbody></table>

### Array Elements

<table><thead><tr><th width="303">Action</th><th>Syntax</th></tr></thead><tbody><tr><td>Access an array element</td><td>array[row position, column position, dimension]</td></tr><tr><td>Check if an item exists</td><td>value %in% array</td></tr><tr><td>Sort array increasing</td><td>sort(array)</td></tr><tr><td>Sort array decreasing</td><td>sort(array, decreasing = TRUE)</td></tr></tbody></table>

#### Inputs:

```r
#Creating array
a <- array(c(1:20),dim = c(4,4,2))

#Access element at row=4, column=4, dimension=1
a[4, 4, 1]

#Check if item exists in array
2 %in% a

#Sort increasing
b <- array(c(16:1),dim = c(4,4,1))
sort(b)

#Sort decreasing
c <- array(c(1:16),dim = c(4,4,1))
sort(c, decreasing = TRUE)
```

#### Outputs:

```r
#Access element at row=4, column=4, dimension=1
16

#Check if item exists in array
TRUE

#Sort increasing
1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16

#Sort decreasing
16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1
```

## Resources

* R Documentation: [Lists](https://cran.r-project.org/doc/manuals/r-release/R-intro.html#Lists)
* R Documentation: [Matrices](https://cran.r-project.org/doc/manuals/r-release/R-intro.html#Index-matrices)
* R Documentation: [Arrays](https://cran.r-project.org/doc/manuals/r-release/R-intro.html#Arrays)