R- How to move value in a specific column to a different row based on a value in a different column

Question

Let's say I have the following data frame:

Seconds = c(0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3) 
Markers= c("NA", "NA", "Start", "NA", "NA", "NA", "NA", "NA", "NA", "NA", "NA", "NA", "NA", "NA") 
df = data.frame(Seconds, Markers)

I want to move the value indicated by 'Start' in column Markers down its column (or a new column) a difference of 0.5 Seconds. Where this could be one result:

Seconds = c(0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3) 
Markers2= c("NA", "NA", "NA", "NA", "NA", "NA", "NA", "Seconds", "NA", "NA", "NA", "NA", "NA", "NA") 
df2 = data.frame(Seconds, Markers2)

Or this would work to if it's easier to accomplish:

Seconds = c(0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3) 
Markers2= c("NA", "NA", "NA", "NA", "NA", "NA", "NA", "Seconds", "NA", "NA", "NA", "NA", "NA", "NA") 
df3 = data.frame(Seconds, Markers, Markers2)

I thought maybe I could do something like this:

 df$MarkerReal <- NA
 df$MarkerReal [which(df$Markers == 'Start') + 5] <- 'Start'

This works for this example, but the real data I'm working with is time sensitive in milliseconds with thousands of rows. There isn't a specified amount of rows I need to move it, but rather base it off the value, in this example moving it by half a second.

Any ideas?

Answer 1

Here is an option:

curr.pos <- which(df$Markers == "Start")[[1]]  # taking first "Start" in case you have more than one
new.pos <- with(df, max(which(Seconds <= Seconds[[curr.pos]] + 0.5)))
df$Markers[c(curr.pos, new.pos)] <- c("NA", "Start")

Produces:

   Seconds Markers
1      0.0      NA
2      0.1      NA
3      0.2      NA
4      0.3      NA
5      0.4      NA
6      0.5      NA
7      0.6      NA
8      0.7   Start
9      0.8      NA
10     0.9      NA
11     1.0      NA
12     1.1      NA
13     1.2      NA
14     1.3      NA

This of course assumes your DF is sorted by the Seconds column.

As a side note, you should consider using real NAs (ie NA_character_ ) as opposed to strings with an "NA" value.