从R中的列聚合函数提取数据

Question

I have a large database from which I have extracted a data value (x) using the aggregate function:

library(plotrix)
aggregate(mydataNC[,c(52)],by=list(patientNC, siteNC, supNC),max)

OUTPUT:

Each (x) value has a corresponding distance value in located in a column titled (dist) in this database. What is the easiest way to extract the value dist and added to the table?

Answer 1

I'd probably start with merge() first. Here's a small reproducible example you can use to see what's going on and modify it to use your data:

# generate bogus data and view it
x1 <- rep(c("A", "B", "C"), each = 4)
x2 <- rep(c("E", "E", "F", "F"), times = 3)
y1 <- rnorm(12)
y2 <- rnorm(12)
md <- data.frame(x1, x2, y1, y2) 

> head(md)

  x1 x2         y1         y2
1  A  E -1.4603164 -0.9662473
2  A  E -0.5247227  1.7970341
3  A  F  0.8990502  1.7596285
4  A  F -0.6791145  2.2900357
5  B  E  1.2894863  0.1152571
6  B  E -0.1981511  0.6388998


# aggregate by taking maximum of each unique (x1, x2) combination
md.agg <- with(md, aggregate(y1, by = list(x1, x2), FUN = max))
names(md.agg) <- c("x1", "x2", "y1")

>  md.agg 
  x1 x2         y1
1  A  E -0.5247227
2  B  E  1.2894863
3  C  E  0.9982510
4  A  F  0.8990502
5  B  F  2.5125956
6  C  F -0.5916491


# merge y2 into the aggregated data
md.final <- merge(md, md.agg)

> md.final

  x1 x2         y1         y2
1  A  E -0.5247227  1.7970341
2  A  F  0.8990502  1.7596285
3  B  E  1.2894863  0.1152571
4  B  F  2.5125956 -0.2217510
5  C  E  0.9982510  0.6813261
6  C  F -0.5916491  1.0348518