xtabs.R

data <- factor(letters[1:4], levels = letters[1:3])
data

## [1] a    b    c    <NA>
## Levels: a b c

xtabs(~data)

## data
## a b c 
## 1 1 1

xtabs(~data, na.action = na.pass)

## data
## a b c 
## 1 1 1

xtabs(~data, na.action = na.pass, exclude = numeric())

## data
## a b c 
## 1 1 1

xtabs(~data, na.action = na.pass, exclude = NULL)

## data
## a b c 
## 1 1 1

sessionInfo()

## R version 3.1.2 (2014-10-31)
## Platform: x86_64-pc-linux-gnu (64-bit)
## 
## locale:
##  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
##  [3] LC_TIME=de_CH.UTF-8        LC_COLLATE=en_US.UTF-8    
##  [5] LC_MONETARY=de_CH.UTF-8    LC_MESSAGES=en_US.UTF-8   
##  [7] LC_PAPER=de_CH.UTF-8       LC_NAME=C                 
##  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
## [11] LC_MEASUREMENT=de_CH.UTF-8 LC_IDENTIFICATION=C       
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] magrittr_1.5            ProjectTemplate_0.6-1.0
## 
## loaded via a namespace (and not attached):
## [1] digest_0.6.8    evaluate_0.5.7  formatR_1.0.3   htmltools_0.2.6
## [5] knitr_1.9.2     rmarkdown_0.5.1 stringr_0.6.2   tools_3.1.2    
## [9] ulimit_0.0-2