CSC 120, Lab Exercise 4.

In this lab exercise, you will write a plot_dots function that plots points at locations specified by a list of numeric vectors, plus two functions, called triangle and checkerboard, that produce such lists of vectors that can be passed to plot_dots.

The plot_dots function takes as arguments a list giving point locations, the horizontal extent of the region where dots are plotted, and the vertical extent of the region where dots are plotted. Its definition should begin with

```
plot_dots <- function (dots, horizontal, vertical)
```

It should create an emply plot with xlim set to c(0,horizontal+1) and ylim set to c(0,vertical+1), and then add points specified by the dots argument. The dots argument should be a list of numeric vectors. The first numeric vector in this list (which can be gotten by dots[[1]]) will contain the y coordinates of points that should be plotted at x coordinate 1, the second vector in dots will contain the y coordinates of points to be plotted with x coordiante 2, etc.

The triangle function should take a single integer, n, as its argument, and return a list with n elements, the first being the vector 1:1, the second 1:2, and so forth up to the last element being 1:n.

The checkerboard function should take a single integer, n, and return a list of numeric vectors that when passed to plot_dots will plot a checkerboard pattern of dots on an n by n square (ie, one that has a dot at (i,j) if i+j is an even number).

These functions should be put in a file called dots.R, which can be read with the source function to make them defined.

This file that you are reading now is the output, produced with knitr::spin, of an R script that reads the function definions from dots.R and then uses them to make three plots.

```
> source("dots.R")
>
> # Test of plot_dots with a small list created manually.
>
> plot_dots (list (c(2,5,1), c(3), c(), c(4,2)), 4, 5)
```

```
> # Test of plot_dots with the triangle function.
>
> plot_dots (triangle(7), 7, 7)
```

```
> # Test of plot_dots with the checkerboard function.
>
> plot_dots (checkerboard(15), 15, 15)
```