Accumulate Vector Elements
accumarray groups elements from a data set and applies a function
to each group.
accumarray(subs, val, sz = NULL, func = sum, fillval = 0) uniq(a, first = FALSE)
subs |
vector or matrix of positive integers, used as indices for the result vector. |
val |
numerical vector. |
sz |
size of the resulting array. |
func |
function to be applied to a vector of numbers. |
fillval |
value used to fill the array when there are no indices pointing to that component. |
a |
numerical vector. |
first |
logical, shall the first or last element encountered be used. |
A <- accumarray(subs, val) creates an array A by accumulating
elements of the vector val using the lines of subs as indices
and applying func to that accumulated vector. The size of the array
can be predetermined by the size vector sz.
A = uniq(a) returns a vector b identical to unique(a)
and two other vectors of indices m and n such that
b == a[m] and a == b[n].
accumarray returns an array of size the maximum in each column of
subs, or by sz.
uniq returns a list with components
b |
vector of unique elements of a. |
m |
vector of indices such that |
n |
vector of indices such that |
The Matlab function accumarray can also handle sparse matrices.
## Examples for accumarray
val = 101:105
subs = as.matrix(c(1, 2, 4, 2, 4))
accumarray(subs, val)
# [101; 206; 0; 208]
val = 101:105
subs <- matrix(c(1,2,2,2,2, 1,1,3,1,3, 1,2,2,2,2), ncol = 3)
accumarray(subs, val)
# , , 1
# [,1] [,2] [,3]
# [1,] 101 0 0
# [2,] 0 0 0
# , , 2
# [,1] [,2] [,3]
# [1,] 0 0 0
# [2,] 206 0 208
val = 101:106
subs <- matrix(c(1, 2, 1, 2, 3, 1, 4, 1, 4, 4, 4, 1), ncol = 2, byrow = TRUE)
accumarray(subs, val, func = function(x) sum(diff(x)))
# [,1] [,2] [,3] [,4]
# [1,] 0 1 0 0
# [2,] 0 0 0 0
# [3,] 0 0 0 0
# [4,] 2 0 0 0
val = 101:105
subs = matrix(c(1, 1, 2, 1, 2, 3, 2, 1, 2, 3), ncol = 2, byrow = TRUE)
accumarray(subs, val, sz = c(3, 3), func = max, fillval = NA)
# [,1] [,2] [,3]
# [1,] 101 NA NA
# [2,] 104 NA 105
# [3,] NA NA NA
## Examples for uniq
a <- c(1, 1, 5, 6, 2, 3, 3, 9, 8, 6, 2, 4)
A <- uniq(a); A
# A$b 1 5 6 2 3 9 8 4
# A$m 2 3 10 11 7 8 9 12
# A$n 1 1 2 3 4 5 5 6 7 3 4 8
A <- uniq(a, first = TRUE); A
# A$m 1 3 4 5 6 8 9 12
## Example: Subset sum problem
# Distribution of unique sums among all combinations of a vectors.
allsums <- function(a) {
S <- c(); C <- c()
for (k in 1:length(a)) {
U <- uniq(c(S, a[k], S + a[k]))
S <- U$b
C <- accumarray(U$n, c(C, 1, C))
}
o <- order(S); S <- S[o]; C <- C[o]
return(list(S = S, C = C))
}
A <- allsums(seq(1, 9, by=2)); A
# A$S 1 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 24 25
# A$C 1 1 1 1 1 1 2 2 2 1 2 2 1 2 2 2 1 1 1 1 1 1 1Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.