[R] Random sample from a data frame where ID column values don't match the values in an ID column in a second data frame

[David Winsemius]

On Mar 30, 2012, at 8:17 AM, inkhorn wrote:

> Okay, here's some sample code:
>
> ID = c(1,2,3,"A1",5,6,"A2",8,9,"A3")
> fakedata = rnorm(10, 5, .5)
> main.df = data.frame(ID,fakedata)
>
> results for my data frame:
>> main.df
>   ID     fakedata
> 1   1     5.024332
> 2   2     4.752943
> 3   3     5.408618
> 4  A1   5.362838
> 5   5    5.158660
> 6   6    4.658235
> 7  A2   5.389601
> 8   8    4.998249
> 9   9    5.248517
> 10 A3 4.159490
>
> sample1.df = main.df[sample(nrow(main.df), 4), ]
>> sample1.df
>  ID     fakedata
> 5  5     5.158660
> 9  9     5.248517
> 4 A1   5.362838
> 8  8    4.998249
>
> Here's what happens when I put a comma before the variable ID:
>
>> sample2.df = main.df[sample(nrow(main.df[! main.df[,"ID"] %in%
>> sample1.df[,"ID"]]), 5),]
> Error in `[.data.frame`(main.df, !main.df[, "ID"] %in% sample1.df[,  
> "ID"]) :
>  undefined columns selected

That was not the code I offered, which had no error:

 > sample2.df <- main.df[ ! main.df[, "ID"] %in% sample1.df[, "ID"] , ]
 > sample2.df
   ID fakedata
2  2 5.225752
4 A1 4.788752
5  5 3.973376
6  6 5.565669
8  8 5.369974
9  9 5.954552

If you want to further sub-sample from that complement which I offered  
(and that _was_ a random sample from the main dataset albeit not the  
particular sample you wanted) , then it is available for further  
sampling.

 > sample2.df[ sample(nrow(sample2.df), 3), ]
   ID fakedata
2  2 5.225752
8  8 5.369974
6  6 5.565669



>
> Here's what happens when I exclude the comma:
>
> sample2.df = main.df[sample(nrow(main.df[! main.df["ID"] %in%
> sample1.df["ID"]]), 5),]

You cannot do both steps in one line using that exact strategy. But  
you can "chain" uses of "[".  You could for instance have constructed  
indexes (indices seems to be disappearing from the English languages):

idx <- sample(nrow(main.df), 4)
subset1 <- main.df[ idx, ]
subset2 <- main.df[-idx, ][sample(nrow(main.df)-nrow(subset1), 3), ]

 > subset2
   ID fakedata
6  6 5.565669
5  5 3.973376
2  2 5.225752

-- 
David.


>> sample2.df
>   ID     fakedata
> 8   8     4.998249
> 1   1     5.024332
> 3   3     5.408618
> 5   5     5.158660
> 10 A3  4.159490
>
> As you can see, one way I get nothing other than an error, the other  
> way I
> get a sample that doesn't exclude rows that were already included in  
> the 1st
> sample.
>
> Thanks,
> Matt Dubins
>
> --
> View this message in context: http://r.789695.n4.nabble.com/Random-sample-from-a-data-frame-where-ID-column-values-don-t-match-the-values-in-an-ID-column-in-a-se-tp4516448p4518878.html
> Sent from the R help mailing list archive at Nabble.com.
>
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> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
West Hartford, CT