· r-2

# R: Ordering rows in a data frame by multiple columns

In one of the assignments of Computing for Data Analysis we needed to sort a data frame based on the values in two of the columns and then return the top value.

The initial data frame looked a bit like this:

``````
> names <- c("paul", "mark", "dave", "will", "john")
> values <- c(1,4,1,2,1)
> smallData <- data.frame(name = names, value = values)
> smallData
name value
1 paul     1
2 mark     4
3 dave     1
4 will     2
5 john     1
``````

I want to be able to sort the data frame by value and name both in ascending order so the final result should look like this:

``````
name value
3 dave     1
5 john     1
1 paul     1
4 will     2
2 mark     4
``````

To do that we can use the order function which will tell us the indices of the vector in sorted order.

e.g. in our case

``````
> order(c(1,4,1,2,1))
 1 3 5 4 2
``````

If we pass a collection of indices to the extract operation it'll reorder the rows. e.g.

``````
> smallData[c(5,4,3,2,1),]
name value
5 john     1
4 will     2
3 dave     1
2 mark     4
1 paul     1
``````

In our case we wire everything together like this to sort by the second column (value):

``````
> smallData[order(smallData[,2]),]
name value
1 paul     1
3 dave     1
5 john     1
4 will     2
2 mark     4
``````

It's a reasonably small tweak to get it to sort first by the second column and then by the first (name) which is what we want:

``````
> smallData[order(smallData[,2], smallData[,1]),]
name value
3 dave     1
5 john     1
1 paul     1
4 will     2
2 mark     4
``````

If we wanted to use the column names instead of indices we'd do the following:

``````
> smallData[order(smallData\$value, smallData\$name),]
name value
3 dave     1
5 john     1
1 paul     1
4 will     2
2 mark     4
``````

We could also rewrite it using the with function if we want to reduce the code further:

``````
> smallData[with(smallData, order(value, name)),]
name value
3 dave     1
5 john     1
1 paul     1
4 will     2
2 mark     4
``````

As I understand it, when we use with we put smallData into the environment and evaluate the second argument to with with respect to that so in this case it allows us to refer to the column names of smallData.