[R] Simple order() data frame question.

[Matthew Dowle]

With data.table, the following is routine :

DT[order(a)]   # ascending
DT[order(-a)]  # descending, if a is numeric
DT[a>5,sum(z),by=c][order(-V1)]   # sum of z group by c, just where a>5, 
then show me the largest first
DT[order(-a,b)]  # order by a descending then by b ascending, if a and b are 
both numeric

It avoids peppering your code with $, and becomes quite natural after a 
short while; especially compound queries such as the 3rd example.

Matthew

http://datatable.r-forge.r-project.org/


"Ivan Calandra" <ivan.calandra at uni-hamburg.de> wrote in message 
news:4DCBEC8B.6040806 at uni-hamburg.de...
I was wondering whether it would be possible to make a method for
data.frame with sort().
I think it would be more intuitive than using the complex construction
of df[order(df$a),]
Is there any reason not to make it?

Ivan

Le 5/12/2011 15:40, Marc Schwartz a écrit :
> On May 12, 2011, at 8:09 AM, John Kane wrote:
>
>> Argh.  I knew it was at least partly obvious.  I never have been able to 
>> read the order() help page and understand what it is saying.
>>
>> THanks very much.
>>
>> By the way, to me it is counter-intuitive that the the command is
>>
>>> df1[order(df1[,2],decreasing=TRUE),]
>> For some reason I keep expecting it to be
>> order( , df1[,2],decreasing=TRUE)
>>
>> So clearly I don't understand what is going on but at least I a lot 
>> better off.  I may be able to get this graph to work.
>
> John,
>
> Perhaps it may be helpful to understand that order() does not actually 
> sort() the data.
>
> It returns a vector of indices into the data, where those indices are the 
> sorted ordering of the elements in the vector, or in this case, the 
> column.
>
> So you want the output of order() to be used within the brackets for the 
> row *indices*, to reflect the ordering of the column (or columns in the 
> case of a multi-level sort) that you wish to use to sort the data frame 
> rows.
>
> set.seed(1)
> x<- sample(10)
>
>> x
>   [1]  3  4  5  7  2  8  9  6 10  1
>
>
> # sort() actually returns the sorted data
>> sort(x)
>   [1]  1  2  3  4  5  6  7  8  9 10
>
>
> # order() returns the indices of 'x' in sorted order
>> order(x)
>   [1] 10  5  1  2  3  8  4  6  7  9
>
>
> # This does the same thing as sort()
>> x[order(x)]
>   [1]  1  2  3  4  5  6  7  8  9 10
>
>
> set.seed(1)
> df1<- data.frame(aa = letters[1:10], bb = rnorm(10))
>
>> df1
>     aa         bb
> 1   a -0.6264538
> 2   b  0.1836433
> 3   c -0.8356286
> 4   d  1.5952808
> 5   e  0.3295078
> 6   f -0.8204684
> 7   g  0.4874291
> 8   h  0.7383247
> 9   i  0.5757814
> 10  j -0.3053884
>
>
> # These are the indices of df1$bb in sorted order
>> order(df1$bb)
>   [1]  3  6  1 10  2  5  7  9  8  4
>
>
> # Get df1$bb in increasing order
>> df1$bb[order(df1$bb)]
>   [1] -0.8356286 -0.8204684 -0.6264538 -0.3053884  0.1836433  0.3295078
>   [7]  0.4874291  0.5757814  0.7383247  1.5952808
>
>
> # Same thing as above
>> sort(df1$bb)
>   [1] -0.8356286 -0.8204684 -0.6264538 -0.3053884  0.1836433  0.3295078
>   [7]  0.4874291  0.5757814  0.7383247  1.5952808
>
>
> You can't use the output of sort() to sort the data frame rows, so you 
> need to use order() to get the ordered indices and then use that to 
> extract the data frame rows in the sort order that you desire:
>
>> df1[order(df1$bb), ]
>     aa         bb
> 3   c -0.8356286
> 6   f -0.8204684
> 1   a -0.6264538
> 10  j -0.3053884
> 2   b  0.1836433
> 5   e  0.3295078
> 7   g  0.4874291
> 9   i  0.5757814
> 8   h  0.7383247
> 4   d  1.5952808
>
>
>> df1[order(df1$bb, decreasing = TRUE), ]
>     aa         bb
> 4   d  1.5952808
> 8   h  0.7383247
> 9   i  0.5757814
> 7   g  0.4874291
> 5   e  0.3295078
> 2   b  0.1836433
> 10  j -0.3053884
> 1   a -0.6264538
> 6   f -0.8204684
> 3   c -0.8356286
>
>
> Does that help?
>
> Regards,
>
> Marc Schwartz
>
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> PLEASE do read the posting guide 
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>

-- 
Ivan CALANDRA
PhD Student
University of Hamburg
Biozentrum Grindel und Zoologisches Museum
Abt. Säugetiere
Martin-Luther-King-Platz 3
D-20146 Hamburg, GERMANY
+49(0)40 42838 6231
ivan.calandra at uni-hamburg.de

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