Loop function and add columns to dataframe in R

Question

I want to loop a function over dataframes and add information from the function to the data frame. I have read the many relevant posts on looping and applying functions to dataframes that have got me close to what I need, but not quite there, so Im hoping people can help me.

I have a number of dataframes that look like this

dat1=as.data.frame(matrix(rnorm(25),ncol=5))
dat2=as.data.frame(matrix(rnorm(25),ncol=5))
dat3=as.data.frame(matrix(rnorm(25),ncol=5))

I want to calculate the mean of some of these columns and add on to the end of the dataframe. So, I wrote a function to calculate means from a dataframe:

my_fun <-  function(dataframe){
rowMeans( dataframe[ , c("V1","V2")],na.rm=TRUE) 
}

To apply this function to one dataframe, I do this:

 dat1$V6<- my_fun(dat1)

But I want to be able to loop through all dataframes and add this mean column on to the end of each df.

After reading some helpful posts, I created a list and used sapply:

dfList<-list(dat1,dat2,dat3)#create list
sapply(dfList, my_fun) #apply function to list

Which gives me the values that I want, but I dont want them in a separate output- I'd like them to simply add a column on to the original dataframe- as happens when I apply the function to the individual dataframes. Can anyone tell me how to do this? Or point me to a post that describes how to do this (I have searched high and low, but maybe typing the wrong keywords) Im sure its very straightforward if you know how!

Answer 1

It's soo much easier and at least 100x faster if you use data.table

require(data.table)
set.seed(612)
dat1=as.data.table(matrix(rnorm(25),ncol=5))
dat2=as.data.table(matrix(rnorm(25),ncol=5))
dat3=as.data.table(matrix(rnorm(25),ncol=5))

dtList <- list(dat1, dat2, dat3)

for (dat in dtList) {
     dat[,V6:=(V1 + V2)/2]
}

This gives the following output:

> dtList

[[1]]
           V1         V2         V3         V4          V5          V6
1:  0.3903228 -1.1581608  1.0171311  0.3866628  0.02756137 -0.38391897
2: -0.6030124  0.4713771 -2.4204376 -0.2843527  0.53463600 -0.06581764
3: -0.9850333  0.3343518 -1.2329712 -1.1767533  0.56714483 -0.32534080
4: -0.1591335 -0.6729444  0.5062648 -0.3001857 -0.84896068 -0.41603897
5:  1.7127203  0.3149884  1.7633945  1.7824786 -0.90316850  1.01385434

[[2]]
            V1         V2         V3          V4         V5         V6
1: -1.22790810  0.8429506  0.4921844 -0.29686607 -0.9501956 -0.1924788
2:  0.09405923 -1.6970403  0.1280003  1.22284944  0.8667643 -0.8014905
3:  0.55298783 -0.1081849  0.4120268 -0.56411756  1.9135802  0.2224015
4: -0.82621808  0.4753731  0.4755664 -0.05885804  0.9658787 -0.1754225
5:  0.44262554  0.3036363 -1.7404580  0.88870595  1.4826431  0.3731309

[[3]]
            V1          V2         V3        V4          V5          V6
1:  0.82085834  0.07221027  1.8835042 0.2563714  0.27891033  0.44653430
2:  0.00445113  1.89450534  0.3878858 1.8385587 -1.86381524  0.94947824
3:  0.66458950 -1.31023362 -0.9403257 1.2128128  0.74922668 -0.32282206
4: -1.40169143 -1.52925147  0.8232823 0.3391147  0.33463875 -1.46547145
5:  1.10566340 -1.16512217  0.3859652 0.8123110  0.04712086 -0.02972939