How do I append rows with for loop in R?

Question

So let's say I'm trying to make a data frame(df2) that has value of twice as big as another data frame(df1). So the two data frames have exactly the same columns. Also, let's say df1 has 10 objects. Here is my code.

library(data.table)
for (i in (1:10) {
  id <- df1$ID[i]
  newAttr1 <- df1$attr1[i] * 2
  newAttr2 <- df1$attr2[i] * 2
  newAttr3 <- df1$attr3[i] * 2

  NewRow <- list(id, newAttr1, newAttr2, newAttr3)
  rbindlist(list(df2, NewRow))
}

I thought this should work, but somehow there is NO objects in df2. What is the problem?

Thanks a lot in advance :)

You don't store the object, use d2<- rbindlist(list(df2, NewRow)) — Clemsang
– Clemsang, Commented May 7, 2020 at 6:41
Why are you using for loop for this. df1$attr1 * 2 should work directly? — Ronak Shah
– Ronak Shah, Commented May 7, 2020 at 6:53
@RonakShah Oh does that do the calculation for ALL objects in the column? — Ryan Oh
– Ryan Oh, Commented May 7, 2020 at 7:25
It depends on what you have in attr1 but usually yes. See for example mtcars$mpg * 2. There could be a better way to solve your problem, we might be able to help if you could update with a reproducible example. — Ronak Shah
– Ronak Shah, Commented May 7, 2020 at 7:35

Kevin · Accepted Answer · 2020-05-07 07:35:32Z

1

Maybe this works.

library(data.table)
df2 <- NULL

for (i in (1:10) {
id <- df1$ID[i]
newAttr1 <- df1$attr1[i] * 2
newAttr2 <- df1$attr2[i] * 2
newAttr3 <- df1$attr3[i] * 2

# NewRow <- list(id, newAttr1, newAttr2, newAttr3)
# rbindlist(list(df2, NewRow))

df2 <- rbind(df2, data.frame(id, newAttr1, newAttr2, newAttr3)) 

}

However in this case I think doing it the way Ronak Shah suggests is better.

answered May 7, 2020 at 7:35

Kevin

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