Fast sum over subset of vector elements
Computes the sum of all or a subset of values.
sum2(x, idxs = NULL, na.rm = FALSE, mode = typeof(x), ...) sumOver(...)
x |
|
idxs |
A |
na.rm |
If |
mode |
A |
... |
Not used. |
sum2(x, idxs) gives equivalent results as sum(x[idxs]), but
is faster and more memory efficient since it avoids the actual subsetting
which requires copying of elements and garbage collection thereof.
Furthermore, sum2(x, mode = "double") is equivalent to
sum(as.numeric(x)) and may therefore be used to avoid integer
overflow(*), but at the same time is much more memory efficient that
the regular sum() function when x is an
integer vector.
(*) In R (>= 3.5.0), sum(x) will no longer integer overflow
and return NA_integer_. Instead it will return the correct sum in
form of a double value.
Returns a scalar of the data type specified by argument mode.
If mode = "integer", then integer overflow occurs if the sum
is outside the range of defined integer values.
Note that the intermediate sum (sum(x[1:n])) is internally
represented as a floating point value and will therefore never be outside of
the range.
If mode = "integer" and typeof{x} == "double", then a warning
is generated.
Henrik Bengtsson
x <- 1:10 n <- length(x) idxs <- seq(from = 1, to = n, by = 2) s1 <- sum(x[idxs]) # 25 s2 <- sum2(x, idxs = idxs) # 25 stopifnot(identical(s1, s2)) idxs <- seq(from = n, to = 1, by = -2) s1 <- sum(x[idxs]) # 25 s2 <- sum2(x, idxs = idxs) # 25 stopifnot(identical(s1, s2)) s1 <- sum(x) # 55 s2 <- sum2(x) # 55 stopifnot(identical(s1, s2)) # Total gives integer overflow x <- c(.Machine$integer.max, 1L, -.Machine$integer.max) s1 <- sum(x[1:2]) # NA_integer_ in R (< 3.5.0) s2 <- sum2(x[1:2]) # NA_integer_ # Total gives integer overflow (coerce to numeric) s1 <- sum(as.numeric(x[1:2])) # 2147483648 s2 <- sum2(as.numeric(x[1:2])) # 2147483648 s3 <- sum2(x[1:2], mode = "double") # 2147483648 w/out copy stopifnot(identical(s1, s2)) stopifnot(identical(s1, s3)) # Cumulative sum would give integer overflow but not the total s1 <- sum(x) # 1L s2 <- sum2(x) # 1L stopifnot(identical(s1, s2))
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