# Exercise #1 - Hints

## Computing change in price

My data frame was called petrol. I computed the change in price as

`petrol\$diff <- c(petrol\$Sunoco[-1] - petrol\$Sunoco[-430], NA)`

which could also be written

`petrol\$diff <- c(petrol\$Sunoco[2:430] - petrol\$Sunoco[1:429], NA)`

Remember that prices change during the day, and the prices were observed early each morning. The first element of diff will be the price on day 2 (Wed) minus the price on day 1 (Tues), which is the amount by which the price changed on day 1 (Tues). This calculation computes 429 differences. There is no price change available for the last day of the series, so I used c() to put NA at the end of the column because the column of differences has to be the same length as the other columns in the data frame.

## The lag plot

You were asked to plot today's price on the Y-axis against yesterday's price on the X-axis, for each day. This is called a lag plot of lag -1. Since plot() takes x as its first argument and y as its second, the correct call will be

`plot(petrol\$Sunoco[-430], petrol\$Sunoco[-1])`

If you load the time series library, you can do the same more easily with the lag.plot() function.

```library(ts)
lag.plot(petrol\$Sunoco, lag = -1)
```

## Working with days of the week

Displays like comparative box plots will, by default, put the categories in alphabetical order, so the days of the week will come out as F M R S Su T W.

One work-around is to recode the days Su through S as numbers 1 through 7, or letters a through g. This is a useful exercise, as you should know how to recode data.

A more elegant method is to force boxplot() to take the categories in the order you want:

`boxplot(split(petrol\$diff, petrol\$day)[c("Su", "M", "T", "W", "R", "F", "S")])`

Remember that R is case-sensitive; "Su" and "SU" aren't the same, for example.