[R] using string variable as order() function argument

[Duncan Murdoch]

On 26/07/2010 2:56 PM, mirek wrote:
> Hello,
>
> In my script I would like to use a loop, which sorts the dataframe 
> according to different columns, pointed by the string variable.
>
>      id col1  col2  col3
> 1   10    0    4      8
> 2   11    1    2      2
> 3   12    0    8      3
> 4   13    0    5      5
>
> Usually the order() function can be used like this:
>
> sorted = mytable[order(column3) , ]
>
> which results in properly sorted table:
>
>      id col1  col2  col3
> 2   11    1    2      2
> 3   12    0    8      3
> 4   13    0    5      5
> 1   10    0    4      8
>
> But when trying to use a string variable instead of "column3" name:
>
> columnname = "column3"
> sorted = mytable[order(columnname) , ]
>
> this command is not properly evaluated and the effect is somewhat strange.

The argument to order() should be a vector whose sort order is to be 
returned.  So you just need to extract one column from the dataframe, e.g.

column <- mytable[, columnname]
sorted <- mytable[order(column) , ]

Duncan Murdoch