mwhich: Expanded "which"-like functionality.

In bigmemory: Manage Massive Matrices with Shared Memory and Memory-Mapped Files

Expanded “which”-like functionality.

Description

Implements which-like functionality for a big.matrix, with additional options for efficient comparisons (executed in C++); also works for regular numeric matrices without the memory overhead.

Usage

mwhich(x, cols, vals, comps, op = "AND")

Arguments

x	a big.matrix (or a numeric matrix; see below).
cols	a vector of column indices or names.
vals	a list (one component for each of cols) of vectors of length 1 or 2; length 1 is used to test equality (or inequality), while vectors of length 2 are used for checking values in the range (-Inf and Inf are allowed). If a scalar or vector of length 2 is provided instead of a list, it will be replicated length(cols) times.
comps	a list of operators (one component for each of cols), including 'eq', 'neq', 'le', 'lt', 'ge' and 'gt'. If a single operator, it will be replicated length(cols) times.
op	the comparison operator for combining the results of the individual tests, either 'AND' or 'OR'.

Details

To improve performance and avoid the creation of massive temporary vectors in R when doing comparisons, mwhich() efficiently executes column-by-column comparisons of values to the specified values or ranges, and then returns the row indices satisfying the comparison specified by the op operator. More advanced comparisons are then possible (and memory-efficient) in R by doing set operations (union and intersect, for example) on the results of multiple mwhich() calls.

Note that NA is a valid argument in conjunction with 'eq' or 'neq', replacing traditional is.na() calls. And both -Inf and Inf can be used for one-sided inequalities.

If mwhich() is used with a regular numeric R matrix, we access the data directly and thus incur no memory overhead. Interested developers might want to look at our code for this case, which uses a handy pointer trick (accessor) in C++.

Value

a vector of row indices satisfying the criteria.

Author(s)

John W. Emerson [e-mail]

See Also

big.matrix, which

Examples

x <- as.big.matrix(matrix(1:30, 10, 3))
options(bigmemory.allow.dimnames=TRUE)
colnames(x) <- c("A", "B", "C")
x[,]
x[mwhich(x, 1:2, list(c(2,3), c(11,17)),
         list(c('ge','le'), c('gt', 'lt')), 'OR'),]

x[mwhich(x, c("A","B"), list(c(2,3), c(11,17)), 
         list(c('ge','le'), c('gt', 'lt')), 'AND'),]

# These should produce the same answer with a regular matrix:
y <- matrix(1:30, 10, 3)
y[mwhich(y, 1:2, list(c(2,3), c(11,17)),
         list(c('ge','le'), c('gt', 'lt')), 'OR'),]

y[mwhich(y, -3, list(c(2,3), c(11,17)),
         list(c('ge','le'), c('gt', 'lt')), 'AND'),]


x[1,1] <- NA
mwhich(x, 1:2, NA, 'eq', 'OR')
mwhich(x, 1:2, NA, 'neq', 'AND')

# Column 1 equal to 4 and/or column 2 less than or equal to 16:
mwhich(x, 1:2, list(4, 16), list('eq', 'le'), 'OR')
mwhich(x, 1:2, list(4, 16), list('eq', 'le'), 'AND')

# Column 2 less than or equal to 15:
mwhich(x, 2, 15, 'le')

# No NAs in either column, and column 2 strictly less than 15:
mwhich(x, c(1:2,2), list(NA, NA, 15), list('neq', 'neq', 'lt'), 'AND')

x <- big.matrix(4, 2, init=1, type="double")
x[1,1] <- Inf
mwhich(x, 1, Inf, 'eq')
mwhich(x, 1, 1, 'gt')
mwhich(x, 1, 1, 'le')

bigmemory documentation built on May 29, 2024, 12:18 p.m.