# How to sort a matrix/data frame in R

### 谢益辉 / 2006-10-15

This is one of the difficulties I encountered when I was making an R script for a sampling task in a project. First I thought this would be a simple problem, and apply(x, 2, sort) might be OK. Later I found it was wrong:

```
> a
[,1] [,2]
[1,] -14 11
[2,] 1 5
[3,] 1 2
[4,] 1 3
[5,] -2 -20
> apply(a,2,sort)
[,1] [,2]
[1,] -14 -20
[2,] -2 2
[3,] 1 3
[4,] 1 5
[5,] 1 11
```

Apparently the data structure was destroyed. I want to sort the matrix by a column (or several columns), but apply(x, 2, sort) sorts all the columns **respectively**. Of course a simple function sort() will return a more “terrible” result.

The correct way is to use the function **order()**, which returns a *permutation* which rearranges its first argument into ascending or descending order, but **order()** alone still can’t solve the problem. Just compare

```
> a[order(a[, 1]), ]
[,1] [,2]
[1,] -14 11
[2,] -2 -20
[3,] 1 5
[4,] 1 2
[5,] 1 3
```

with

```
> a[order(a[ ,1], a[ ,2]), ]
[,1] [,2]
[1,] -14 11
[2,] -2 -20
[3,] 1 2
[4,] 1 3
[5,] 1 5
```

Today I looked up for this question in “R FAQ” and easily found it. It seems that I still lack the habit of searching the R site.