r-从数据帧循环创建矩阵

Question

I have many data frames id.1, id.2, ... ,id.21 and in each of which I want to extract 2 data points: id.1[5,4] and id.1[10,6] , id.2[5,4] and id.2[10,6] , etc. The first data point is a date and the second data point is a integer.

I want to export this list to obtain something like this in an .csv file:

        V1          V2
1    5/10/2016      1654395291
2    5/11/2016      1645024703
3    5/12/2016      1763825219

I have tried

 x=c(for (i in 1:21) {
            file1 = paste("id.", i, "[5,4]", sep="")}, for (i in 1:21) {
            file1 = paste("id.", i, "[10,6]", sep="")})
 write.csv(x, "x.csv")

But this yields x being NULL . How can I go about getting this vector?

Answer 1

Your problem is that a for loop doesn't return anything in R. So you can't use it in a c statement as you did. Use an [sl]apply construct instead.

I would first make a list containing all the data frames:

dfs <- list(id.1, id.2, id.3, ...)

And iterate over it, something like:

x <- sapply(dfs, function(df) {
  return(c(df[5,4], df[10,6]))
})

Finally you need to transpose the result and convert it into a data.frame if you want to:

x <- as.data.frame(t(x))