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Sorting

Two final operations that you might find useful for analysis are sorting and ranking functions.

To sort the elements of an object, use the `sort` function:

```> w <- c(5, 4, 7, 2, 7, 1)
> sort(w)
[1] 1 2 4 5 7 7```

Add the `decreasing=TRUE` option to sort in reverse order:

```> sort(w, decreasing=TRUE)
[1] 7 7 5 4 2 1```

You can control the treatment of `NA` values by setting the `na.last` argument:

```> length(w)
[1] 6
> length(w) <- 7
> # note that by default, NA.last=NA and NA values are not shown
> sort(w)
[1] 1 2 4 5 7 7
> # set NA.last=TRUE to put NA values last
> sort(w, na.last=TRUE)
[1]  1  2  4  5  7  7 NA
> # set NA.last=FALSE to put NA values first
> sort(w, na.last=FALSE)
[1] NA  1  2  4  5  7  7```

Sorting data frames is somewhat nonintuitive. To sort a data frame, you need to create a permutation of the indices from the data frame and use these to fetch the rows of the data frame in the correct order. You can generate an appropriate permutation of the indices using the `order` function:

`order(..., na.last = , decreasing = )`

The `order` function takes a set of vectors as arguments. It sorts recursively by each vector, breaking ties by looking at successive vectors in the argument list. At the end, it returns a permutation of the indices of the vector corresponding to the sorted order. (The arguments `na.last` and `decreasing` work the same way as they do for `sort`.) To see what this means, let’s use a simple example. First, we’ll define a vector with two elements out of order:

`> v <- c(11, 12, 13, 15, 14)`

You can ...