Question

Thanks for all the help I got from just reading stuff.

I’m not happy with my R loops when I am only dealing within one data.frame because I have to write down the name of the dataframe over and over again which bloats up my R code.

Here is a silly example:

x<- rep(NA,10)
y <- 1:10
dat <- data.frame(x,y)

for(i in 2:nrow(dat)){
    dat$x[i] <- dat$y[i] + dat$y[i-1]
}

So what I want to get rid of is that dat$ -bit. Outside loops this can neatly be done with within(), but I am not exactly sure whether you can actually do that with R. I tried it though:

remove(x,y) # In order to avoid accidental usage of the initial vectors
within(dat,{
for(i in 2:nrow(dat)){
    x[i] <- y[i] + y[i-1]
}})

The output looks like this:

    x  y  i
1  NA  1 10
2   3  2 10
3   5  3 10
4   7  4 10
5   9  5 10
6  11  6 10
7  13  7 10
8  15  8 10
9  17  9 10
10 19 10 10

So the loop did actually work, it’s just that there is a new magical column.

Does anyone know (1) what is going on here and (2) how to elegantly deal with that kind of loops (a more complicated example wrapping within() around a loop including several if() statements and calculations failed btw?

Thanks a lot in advance!
skr

Answer 1

Ben answered your main question, by noting that i is being assigned to by the for loop. You can see that that is so by trying something like this:

for(j in 1:3) cat("hi\n") 
hi
hi
hi
> j
[1] 3

One option is just to remove the unwanted i variable by making its value NULL:

within(dat,{
for(i in 2:nrow(dat)){
    x[i] <- y[i] + y[i-1]
}
i <- NULL
})

Another is to use with() instead of within():

dat$x <- with(dat, {
    for(i in 2:nrow(dat)){
        x[i] <- y[i] + y[i-1]
    }
    x
})

Finally, though I realize yours was a toy example, the best solution will very often be to avoid for loops altogether:

d <- data.frame(y=1:10)
within(d, {x = y + c(NA, head(y, -1))})
#     y  x
# 1   1 NA
# 2   2  3
# 3   3  5
# 4   4  7
# 5   5  9
# 6   6 11
# 7   7 13
# 8   8 15
# 9   9 17
# 10 10 19