for loop only runs only for last row in R - r

Question

I have a simple issue with a for loop in R - I am trying to make it run for the entire dataset and it only runs for the last row. This is done with quite complex datasets which are both shapefiles and I am testing the intersection of the geometries. That is why I can't quite make a reproducible example here.
Nevertheless, this is my code:
for(i in 1:nrow(data1)){
#get intersections between data2 and data1 for specific years
output = st_join(
x = data1[i, ],
y = data2[which(data2$year %in% data1$lag.year[i]:data1$year[i]), ],
join = st_intersects
)
#Get area of intersections
output = transform(output,
inter_area = mapply(function(x, y) {
as.numeric(sf::st_area(
sf::st_intersection(x, y)
))}, x = geometry, y = geom_2))
## obtaining the proportion of area in data1 intersected by data2
output = transform(output, prop_inter = inter_area/area)
#get cycle-specific values
output <- output%>%
group_by(code, year.x)%>%
dplyr::summarise(prop_inter = sum(prop_inter),
end_date= max(end_date),
start_date= max(start_date))%>%
ungroup()
return(output)
}
As you can see I am testing the intersections of data2 on data1 and see which percentage of data1 is intersected dependent on the values they take on year and lag.year. The issue is that when I run this it only returns the desidered outcome for the last row, instead of the entire data1 object. I've tested all the different bits of code inside the loop separately and they all do as I want, but once I try to scale all of it up to the entire dataframe it just does it for the last row.
So I assume this must be some simple stupid mistake I am making for the loop.
Thanks!

Answer 1

You keep rewriting the output object; you may want to create a vector of length nrow(data) and assign the result to its i-th element. I don't think this relates to {sf} or GIS in general, it is more about how for loops and vectors work in R - consider this example:
for (i in 1:50) {
output <- i # rewriting output object 50 times
}
print(output) # this will be a single element for last row (50)
output <- numeric(50)
for (i in 1:50) {
output[i] <- i # storing result in a new element of output for each i
}
print(output) # this will be 1:50 as expected
You may want to consider something along these lines (hard to make certain without access to your data, but it should get you started).
result <- numeric(nrow(data1)) # init the vector
for(i in 1:nrow(data1)){
#get intersections between data2 and data1 for specific years
output = st_join(
x = data1[i, ],
y = data2[which(data2$year %in% data1$lag.year[i]:data1$year[i]), ],
join = st_intersects
)
#Get area of intersections
output = transform(output,
inter_area = mapply(function(x, y) {
as.numeric(sf::st_area(
sf::st_intersection(x, y)
))}, x = geometry, y = geom_2))
## obtaining the proportion of area in data1 intersected by data2
output = transform(output, prop_inter = inter_area/area)
#get cycle-specific values
result[i] <- output%>% # store in i-th element of result instead
group_by(code, year.x)%>%
dplyr::summarise(prop_inter = sum(prop_inter),
end_date= max(end_date),
start_date= max(start_date))%>%
ungroup()
# return(output) # no need for return unless you are in a function
}