R - Using a for loop with a test for multiple data frames

Question

In R, I'm trying to use a for loop, with a nested test, in order to append a column to multiple data frames.

I am having trouble 1) calling a data frame with a variable name and 2) using a logical test to skip.

For example, I created 3 data frames with a number, and I want to add a column that's the squared root of the value. I want to skip the data frame if it'll result in an error.

Below is what I've gotten to so far:

df1 <- data.frame(a=c(1))
df2 <- data.frame(a=c(6))
df3 <- data.frame(a=c(-3))

df_lst$b<-
for(df_lst in c("df1","df2","df3"){
    ifelse(is.na(df_lst$a) = T, skip,
           df_list$b <- sqrt(df1$a)

})

In the above example, I would ideally like to see df1 and df2 with a new column b with the squared root of column a, and then nothing happens to df3.

Any help would be GREATLY appreciated, thank you everyone!

MrFlick · Accepted Answer · 2014-05-15 16:29:57Z

1

It's generally not a good idea to just have a bunch of data.frames lying around with different names if you need to do things to all of them. You're better off storing them in a list. For example

mydfs<-list(df1, df2, df3)

Then you can use lapply and such to work with those data.frames. For example

mydfs<-lapply(mydfs, function(x) {
    if(all(x$a>0)) {
        x$b<-sqrt(x$a)
    }
    x;
})

Otherwise, changing your code to

for(df_lst in c("df1","df2","df3")) {
    df<-get(df_lst)
    if( all(df$a>=0) ) {
       df$b <- sqrt(df$a)
    }
    assign(df_lst, df)
}

should work as well, it's just generally not considered good practice.

answered May 15, 2014 at 16:29

MrFlick

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