[R] Create Matrix from Loop of Vectors, Sort It and Pick Top-K
Marc Schwartz
marc_schwartz at comcast.net
Thu Jun 19 18:46:27 CEST 2008
on 06/19/2008 09:59 AM Gundala Viswanath wrote:
> Hi,
>
> I have the following dataset (simplified for example).
>
> __DATA__
> 300.35 200.25 104.30
> 22.00 31.12 89.99
> 444.50 22.10 43.00
> 22.10 200.55 66.77
>
> Now from that I wish to do the following:
>
> 1. Compute variance of each row
> 2. Pick top-2 row with highest variance
> 3. Store those selected rows for further processing
>
> To achieve this, I tried to: a) read the table and compute
> variance for each row, b) append variance with its original
> row in a vector, c) store a vector into multidimentional array (matrix),
> d) sort that array. But I am stuck at the step (b).
>
> Can anybody suggest what's the best way to achieve
> my aim above?
>
> This is the sample code I have so far (not working).
>
> __BEGIN__
>
> #data <- read.table("testdata.txt")
>
>
> # Is this a right way to initialize?
> all.arr = NULL
>
> for (gi in 1:nofrow) {
> gex <- as.vector(data.matrix(data[gi,],rownames.force=FALSE))
>
> #compute variance
> gexvar <- var(gex)
>
> # join variance with its original vector
> nvec <- c(gexvar,gex)
>
> # I'm stuck here.....This doesn't seem to work
> all.arr <- data.frame(nvec)
> }
>
> print(all.arr)
> __END__
> --
If your data is contained in a data frame 'DF':
> DF
V1 V2 V3
1 300.35 200.25 104.30
2 22.00 31.12 89.99
3 444.50 22.10 43.00
4 22.10 200.55 66.77
# Get row-wise variances and cbind() them to DF
> DF.var <- cbind(DF, var = apply(DF, 1, var, na.rm = TRUE))
> DF.var
V1 V2 V3 var
1 300.35 200.25 104.30 9610.336
2 22.00 31.12 89.99 1361.915
3 444.50 22.10 43.00 56676.803
4 22.10 200.55 66.77 8622.817
# Sort DF by 'var' using order()
> DF.var[order(DF.var$var, decreasing = TRUE), ]
V1 V2 V3 var
3 444.50 22.10 43.00 56676.803
1 300.35 200.25 104.30 9610.336
4 22.10 200.55 66.77 8622.817
2 22.00 31.12 89.99 1361.915
To get the top 2, you can take a couple of approaches:
> DF.var[order(DF.var$var, decreasing = TRUE)[1:2], ]
V1 V2 V3 var
3 444.50 22.10 43.0 56676.803
1 300.35 200.25 104.3 9610.336
or
> head(DF.var[order(DF.var$var, decreasing = TRUE), ], 2)
V1 V2 V3 var
3 444.50 22.10 43.0 56676.803
1 300.35 200.25 104.3 9610.336
See ?cbind, ?apply, ?order and ?head for more information.
HTH,
Marc Schwartz
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