finding "almost" duplicates indices in a data table and calculate the delta - r

Question

i have a smallish (2k) data set that contains questionnaire answers filled out by students there were sampled twice a year. not all the students that were present for the first wave were there for the second wave and vice versa. for each student, a unique id was created that consisted of the school code, the class code, the student number and the wave as a decimal point. for example 100612.1 is a student from school 10, grade 6, 12 on the names list and this was the first wave. the idea behind the decimal point was a way to identify the same student again in the data set (the only value which differs less than abs(1) from a given id is the same student on the other wave).at least that was the idea.
i was thinking of a script that would do the following:
- find the rows who's unique id is less than abs(1) from one another
- for those rows, generate a new row (in a new table) that consists of the student id and the delta of the measured variables( i.e value in the wave 2 - value in wave 1).
i a new to R but i have a tiny bit of background in other OOP. i thought about creating a for loop that runs from 1 to length(df) and just looks for it's "brother". my gut feeling tells me that this not the way things are done in R. any ideas?
all i need is a quick way of sifting through the data looking for the second wave row. i think the rest should be straight forward from there.
thank you for helping
PS. since this is my first post here i apologize beforehand for any wrongdoings in this post... :)

Answer 1

The question alludes to data.table, so here is a way to adapt #jed's answer using that package.
ids <- c(100612.1,100612.2,100613.1,100613.2,110714.1,201802.2)
answers <- c(5,4,3,4,1,0)
Example data as before, now instead of data.frame and tapply you can do this:
library(data.table)
surveyDT <- data.table(ids, answers)
surveyDT[, `:=` (child = substr(ids, 1, 6), wave = substr(ids, 8, 8))] # split ID's
# note multiple assign-by-reference := syntax above
setkey(surveyDT, child, wave) # order data
# calculate delta on keyed data, grouping by child
surveyDT[, delta := diff(answers), by = child]
unique(surveyDT[, delta, by = child]) # list results
child delta
1: 100612 -1
2: 100613 1
3: 110714 NA
4: 201802 NA
To remove rows with NA values for delta:
unique(surveyDT[, .SD[(!is.na(delta))], by = child])
child ids answers wave delta
1: 100612 100612.1 5 1 -1
2: 100613 100613.1 3 1 1
Use .SDcols to output only specific columns (in addition to the by columns), for example,
unique(surveyDT[, .SD[(!is.na(delta))], by = child, .SDcols = 'delta'])
child delta
1: 100612 -1
2: 100613 1
It took me some time to get acquainted with data.table syntax, but now I find it more intuitive, and it's fast for big data.

Answer 2

There are two ways that come to mind. The easiest is to use the function floor(), which returns the integer For example:
floor(100612.1)
#[1] 100612
floor(9.9)
#[1] 9
Alternatively, you could write a fairly simple regex expression to get rid of the decimal place too. Then you can use unique() to find the rows that are or are not duplicated entries.

Answer 3

Lets make some fake data so we can see our problem easily:
ids <- c(100612.1,100612.2,100613.1,100613.2,110714.1,201802.2)
answers <- c(5,4,3,4,1,0)
survey <- data.frame(ids,answers)
Now lets split our ids into two different columns:
survey$child_id <- substr(survey$ids,1,6)
survey$wave_id <- substr(survey$ids,8,8)
Then we'll order by child and wave, and compute differences based on child:
survey[order(survey$child_id, survey$wave_id),]
survey$delta <- unlist(tapply(survey$answers, survey$child_id, function(x) c(NA,diff(x))))
Output:
ids answers child_id wave_id delta
1 100612.1 5 100612 1 NA
2 100612.2 4 100612 2 -1
3 100613.1 3 100613 1 NA
4 100613.2 4 100613 2 1
5 110714.1 1 110714 1 NA
6 201802.2 0 201802 2 NA