for loop & if function in R

Question

I was writing a loop with if function in R. The table is like below:

ID  category
1   a
1   b
1   c
2   a
2   b
3   a
3   b
4   a
5   a

I want

Answer 1

There are packages and vectorized ways to do this task, but if you are practicing with loops try:

output1$rn <- 1
for (i in 2:nrow(output1)){
  if(output1[i,1] == output1[i-1,1]){
    output1[i,"rn"]<- output1[i-1,"rn"]+1
  } 

  else{
     output1[i,"rn"]<-1
   } 
}

With your original code, when you made this call output1[i-1,"rn"]+1 in the third line of your loop, you were referencing a row that didn't exist on the first pass. By first creating the row and filling it with the value 1, you give the loop something explicit to refer to.

output1
#   ID category rn
# 1  1        a  1
# 2  1        b  2
# 3  1        c  3
# 4  2        a  1
# 5  2        b  2
# 6  3        a  1
# 7  3        b  2
# 8  4        a  1
# 9  5        a  1

With the package dplyr you can accomplish it quickly with:

library(dplyr)
output1 %>% group_by(ID) %>% mutate(rn = 1:n())

Or with data.table:

library(data.table)
setDT(output1)[,rn := 1:.N, by=ID]

With base R you can also use:

output1$rn <- with(output1, ave(as.character(category), ID, FUN=seq))

There are vignettes and tutorials on the two packages mentioned, and by searching ?ave in the R console for the last approach.