The error is due to the well-known R gotcha that the : (colon operator, which calls seq()) takes higher precedence than arithmetic. Always parenthesize arguments to : if they involve arithmetic or are expressions: a[(i-3):i]
Your code a[i-3:i] doesn't do what you want it to do a[(i-3):i], it does a[i - (3:i)]). So the medians you are printing are for these slices:
4-3:1 # i.e. 1:3
8-3:1 # i.e. 5:7
12-3:1 # i.e. 9:11
16-4:1 # i.e. 13:15
PS some coding-style tips
You don't need to iterate over all possible values of i and check them modulo binsize, just do:
for(i in seq(binsize, length(a), by=binsize)) { ... }
Learn all the possible ways to use the seq() function.
But in fact you can replace even that with:
split(a, ceiling(seq_along(a)/binsize))
as per the "Split a vector into chunks in R"
To make it even clearer, you could define a helper function chunk <- function(x, binsize) { split(x, ceiling(seq_along(x)/binsize)) }
- Then you can replace the for-loop with sapply:
.
sapply(split(a, ceiling(seq_along(a)/binsize)), mean)
sapply(chunk(a,binsize), mean)
1 2 3 4
32.600 49.600 66.175 83.925
sapply(split(a, ceiling(seq_along(a)/binsize)), median)
sapply(chunk(a,binsize), median)
1 2 3 4
36.25 48.90 65.25 79.25
Much cleaner, easier to read, and prevents errors, right?