# Fast Factor Generation, Interactions and Vector Grouping

`qF.Rd`

`qF`

, shorthand for 'quick-factor' implements very fast factor generation from atomic vectors using either radix ordering or index hashing followed by sorting.

`qG`

, shorthand for 'quick-group', generates a kind of factor-light without the levels attribute but instead an attribute providing the number of levels. Optionally the levels / groups can be attached, but without converting them to character (which can have large performance implications). Objects have a class 'qG'.

`finteraction`

generates a factor or 'qG' object by interacting multiple vectors or factors. In that process missing values are always replaced with a level and unused levels/combinations are always dropped.

*collapse* internally makes optimal use of factors and 'qG' objects when passed as grouping vectors to statistical functions (`g/by`

, or `t`

arguments) i.e. typically no further grouping or ordering is performed and objects are used directly by statistical C/C++ code.

## Usage

```
qF(x, ordered = FALSE, na.exclude = TRUE, sort = .op[["sort"]], drop = FALSE,
keep.attr = TRUE, method = "auto")
qG(x, ordered = FALSE, na.exclude = TRUE, sort = .op[["sort"]],
return.groups = FALSE, method = "auto")
is_qG(x)
as_factor_qG(x, ordered = FALSE, na.exclude = TRUE)
finteraction(..., factor = TRUE, ordered = FALSE, sort = factor && .op[["sort"]],
method = "auto", sep = ".")
itn(...) # Shorthand for finteraction
```

## Arguments

- x
a atomic vector, factor or quick-group.

- ordered
logical. Adds a class 'ordered'.

- na.exclude
logical.

`TRUE`

preserves missing values (i.e. no level is generated for`NA`

).`FALSE`

attaches an additional class`"na.included"`

which is used to skip missing value checks performed before sending objects to C/C++. See Details.- sort
logical.

`TRUE`

sorts the levels in ascending order (like`factor`

);`FALSE`

provides the levels in order of first appearance, which can be significantly faster. Note that if a factor is passed as input, only`sort = FALSE`

takes effect and unused levels will be dropped (as factors usually have sorted levels and checking sortedness can be expensive).- drop
logical. If

`x`

is a factor,`TRUE`

efficiently drops unused factor levels beforehand using`fdroplevels`

.- keep.attr
logical. If

`TRUE`

and`x`

has additional attributes apart from 'levels' and 'class', these are preserved in the conversion to factor.- method
an integer or character string specifying the method of computation:

*Int.**String**Description*1 "auto" automatic selection: `if(is.double(x) && sort) "radix" else if(sort && length(x) < 1e5) "rcpp_hash" else "hash"`

.2 "radix" use radix ordering to generate factors. Supports `sort = FALSE`

only for character vectors. See Details.3 "hash" use hashing to generate factors. Since v1.8.3 this is a fast hybrid implementation using `group`

and radix ordering applied to the unique elements. See Details.4 "rcpp_hash" the previous "hash" algorithm prior to v1.8.3: uses `Rcpp::sugar::sort_unique`

and`Rcpp::sugar::match`

. Only supports`sort = TRUE`

.Note that for

`finteraction`

,`method = "hash"`

is always unsorted and`method = "rcpp_hash"`

is not available.- return.groups
logical.

`TRUE`

returns the unique elements / groups / levels of`x`

in an attribute called`"groups"`

. Unlike`qF`

, they are not converted to character.- factor
logical.

`TRUE`

returns an factor,`FALSE`

returns a 'qG' object.- sep
character. The separator passed to

`paste`

when creating factor levels from multiple grouping variables.- ...
multiple atomic vectors or factors, or a single list of equal-length vectors or factors. See Details.

## Details

Whenever a vector is passed to a Fast Statistical Function such as `fmean(mtcars, mtcars$cyl)`

, is is grouped using `qF`

, or `qG`

if `use.g.names = FALSE`

.

`qF`

is a combination of `as.factor`

and `factor`

. Applying it to a vector i.e. `qF(x)`

gives the same result as `as.factor(x)`

. `qF(x, ordered = TRUE)`

generates an ordered factor (same as `factor(x, ordered = TRUE)`

), and `qF(x, na.exclude = FALSE)`

generates a level for missing values (same as `factor(x, exclude = NULL)`

). An important addition is that `qF(x, na.exclude = FALSE)`

also adds a class 'na.included'. This prevents *collapse* functions from checking missing values in the factor, and is thus computationally more efficient. Therefore factors used in grouped operations should preferably be generated using `qF(x, na.exclude = FALSE)`

. Setting `sort = FALSE`

gathers the levels in first-appearance order (unless `method = "radix"`

and `x`

is numeric, in which case the levels are always sorted). This often gives a noticeable speed improvement.

There are 3 internal methods of computation: radix ordering, hashing, and Rcpp sugar hashing. Radix ordering is done by combining the functions `radixorder`

and `groupid`

. It is generally faster than hashing for large numeric data and pre-sorted data (although there are exceptions). Hashing uses `group`

, followed by `radixorder`

on the unique elements if `sort = TRUE`

. It is generally fastest for character data. Rcpp hashing uses `Rcpp::sugar::sort_unique`

and `Rcpp::sugar::match`

. This is often less efficient than the former on large data, but the sorting properties (relying on `std::sort`

) may be superior in borderline cases where `radixorder`

fails to deliver exact lexicographic ordering of factor levels.

Regarding speed: In general `qF`

is around 5x faster than `as.factor`

on character data and about 30x faster on numeric data. Automatic method dispatch typically does a good job delivering optimal performance.

`qG`

is in the first place a programmers function. It generates a factor-'light' class 'qG' consisting of only an integer grouping vector and an attribute providing the number of groups. It is slightly faster and more memory efficient than `GRP`

for grouping atomic vectors, and also convenient as it can be stored in a data frame column, which are the main reasons for its existence.

`finteraction`

is simply a wrapper around `as_factor_GRP(GRP.default(X))`

, where X is replaced by the arguments in '...' combined in a list (so its not really an interaction function but just a multivariate grouping converted to factor, see `GRP`

for computational details). In general: All vectors, factors, or lists of vectors / factors passed can be interacted. Interactions always create a level for missing values and always drop unused levels.

## Value

`qF`

returns an (ordered) factor. `qG`

returns an object of class 'qG': an integer grouping vector with an attribute `"N.groups"`

indicating the number of groups, and, if `return.groups = TRUE`

, an attribute `"groups"`

containing the vector of unique groups / elements in `x`

corresponding to the integer-id. `finteraction`

can return either.

## Note

An efficient alternative for character vectors with multithreading support is provided by `kit::charToFact`

.

`qG(x, sort = FALSE, na.exclude = FALSE, method = "hash")`

internally calls `group(x)`

which can also be used directly and also supports multivariate groupings where `x`

can be a list of vectors.

Neither `qF`

nor `qG`

reorder groups / factor levels. An exception was added in v1.7, when calling `qF(f, sort = FALSE)`

on a factor `f`

, the levels are recast in first appearance order. These objects can however be converted into one another using `qF/qG`

or the direct method `as_factor_qG`

(called inside `qF`

). It is also possible to add a class 'ordered' (`ordered = TRUE`

) and to create am extra level / integer for missing values (`na.exclude = FALSE`

) if factors or 'qG' objects are passed to `qF`

or `qG`

.

## Examples

```
cylF <- qF(mtcars$cyl) # Factor from atomic vector
cylG <- qG(mtcars$cyl) # Quick-group from atomic vector
cylG # See the simple structure of this object
#> [1] 2 2 1 2 3 2 3 1 1 2 2 3 3 3 3 3 3 1 1 1 1 3 3 3 3 1 1 1 3 2 3 1
#> attr(,"N.groups")
#> [1] 3
#> attr(,"class")
#> [1] "qG"
cf <- qF(wlddev$country) # Bigger data
cf2 <- qF(wlddev$country, na.exclude = FALSE) # With na.included class
dat <- num_vars(wlddev)
# cf2 is faster in grouped operations because no missing value check is performed
library(microbenchmark)
microbenchmark(fmax(dat, cf), fmax(dat, cf2))
#> Unit: microseconds
#> expr min lq mean median uq max neval
#> fmax(dat, cf) 104.741 107.9355 110.1624 108.9365 110.4055 142.678 100
#> fmax(dat, cf2) 99.466 102.9310 106.4543 103.9945 105.1785 199.270 100
finteraction(mtcars$cyl, mtcars$vs) # Interacting two variables (can be factors)
#> [1] 6.0 6.0 4.1 6.1 8.0 6.1 8.0 4.1 4.1 6.1 6.1 8.0 8.0 8.0 8.0 8.0 8.0 4.1 4.1
#> [20] 4.1 4.1 8.0 8.0 8.0 8.0 4.1 4.0 4.1 8.0 6.0 8.0 4.1
#> Levels: 4.0 4.1 6.0 6.1 8.0
head(finteraction(mtcars)) # A more crude example..
#> [1] 21.6.160.110.3.9.2.62.16.46.0.1.4.4
#> [2] 21.6.160.110.3.9.2.875.17.02.0.1.4.4
#> [3] 22.8.4.108.93.3.85.2.32.18.61.1.1.4.1
#> [4] 21.4.6.258.110.3.08.3.215.19.44.1.0.3.1
#> [5] 18.7.8.360.175.3.15.3.44.17.02.0.0.3.2
#> [6] 18.1.6.225.105.2.76.3.46.20.22.1.0.3.1
#> 32 Levels: 10.4.8.460.215.3.5.424.17.82.0.0.3.4 ...
finteraction(mtcars$cyl, mtcars$vs, factor = FALSE) # Returns 'qG', by default unsorted
#> [1] 1 1 2 3 4 3 4 2 2 3 3 4 4 4 4 4 4 2 2 2 2 4 4 4 4 2 5 2 4 1 4 2
#> attr(,"N.groups")
#> [1] 5
#> attr(,"class")
#> [1] "qG" "na.included"
group(mtcars[c("cyl", "vs")]) # Same thing. Use whatever syntax is more convenient
#> [1] 1 1 2 3 4 3 4 2 2 3 3 4 4 4 4 4 4 2 2 2 2 4 4 4 4 2 5 2 4 1 4 2
#> attr(,"N.groups")
#> [1] 5
#> attr(,"class")
#> [1] "qG" "na.included"
```