Fast match() replacement
fmatch is a faster version of the built-in match()
function. It is slightly faster than the built-in version because it
uses more specialized code, but in addition it retains the hash table
within the table object such that it can be re-used, dramatically reducing
the look-up time especially for large tables.
Although fmatch can be used separately, in general it is also
safe to use: match <- fmatch since it is a drop-in
replacement. Any cases not directly handled by fmatch are passed
to match with a warning.
fmatch.hash is identical to fmatch but it returns the table
object with the hash table attached instead of the result, so it can be
used to create a table object in cases where direct modification is
not possible.
fmatch(x, table, nomatch = NA_integer_, incomparables = NULL) fmatch.hash(x, table, nomatch = NA_integer_, incomparables = NULL) x %fin% table
x |
values to be matched |
table |
values to be matched against |
nomatch |
the value to be returned in the case when no match is
found. It is coerced to |
incomparables |
a vector of values that cannot be matched. Any
value other than |
See match for the purpose and details of the
match function. fmatch is a drop-in replacement for
the match function with the focus on
performance. incomparables are not supported by fmatch
and will be passed down to match.
The first match against a table results in a hash table to be computed
from the table. This table is then attached as the ".match.hash"
attribute of the table so that it can be re-used on subsequent calls
to fmatch with the same table.
The hashing algorithm used is the same as the match function in
R, but it is re-implemented in a slightly different way to improve its
performance at the cost of supporting only a subset of types (integer,
real and character). For any other types fmatch falls back to
match (with a warning).
fmatch: A vector of the same length as x - see
match for details.
fmatch.hash: table, possibly coerced to match the type
of x, with the hash table attached.
%fin%: A logical vector the same length as x - see
%in% for details.
fmatch modifies the table by attaching an attribute to
it. It is expected that the values will not change unless that
attribute is dropped. Under normal circumstances this should not have
any effect from user's point of view, but there is a theoretical
chance of the cache being out of sync with the table in case the table
is modified directly (e.g. by some C code) without removing
attributes.
In cases where the table object cannot be modified (or such
modification would not survive) fmatch.hash can be used to build
the hash table and return table object including the hash
table. In that case no lookup is done and x is only used to
determine the type into which table needs to be coerced.
Also fmatch does not convert to a common encoding so strings
with different representation in two encodings don't match.
Simon Urbanek
# some random speed comparison examples:
# first use integer matching
x = as.integer(rnorm(1e6) * 1000000)
s = 1:100
# the first call to fmatch is comparable to match
system.time(fmatch(s,x))
# but the subsequent calls take no time!
system.time(fmatch(s,x))
system.time(fmatch(-50:50,x))
system.time(fmatch(-5000:5000,x))
# here is the speed of match for comparison
system.time(base::match(s, x))
# the results should be identical
identical(base::match(s, x), fmatch(s, x))
# next, match a factor against the table
# this will require both x and the factor
# to be cast to strings
s = factor(c("1","1","2","foo","3",NA))
# because the casting will have to allocate a string
# cache in R, we run a dummy conversion to take
# that out of the equation
dummy = as.character(x)
# now we can run the speed tests
system.time(fmatch(s, x))
system.time(fmatch(s, x))
# the cache is still valid for string matches as well
system.time(fmatch(c("foo","bar","1","2"),x))
# now back to match
system.time(base::match(s, x))
identical(base::match(s, x), fmatch(s, x))
# finally, some reals to match
y = rnorm(1e6)
s = c(y[sample(length(y), 100)], 123.567, NA, NaN)
system.time(fmatch(s, y))
system.time(fmatch(s, y))
system.time(fmatch(s, y))
system.time(base::match(s, y))
identical(base::match(s, y), fmatch(s, y))
# this used to fail before 0.1-2 since nomatch was ignored
identical(base::match(4L, 1:3, nomatch=0), fmatch(4L, 1:3, nomatch=0))Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.