ds.numNA.R

#'
#' @title Gets the number of missing values in a server-side vector
#' @description This function helps to know the number of missing values
#' in a vector that is stored on the server-side. 
#'
#' @details The number of missing entries are counted and the total for each study is returned.
#' 
#' Server function called: \code{numNaDS}
#' @param x a character string specifying the name of the vector. 
#' @param datasources a list of \code{\link{DSConnection-class}} 
#' objects obtained after login. If the \code{datasources} argument is not specified
#' the default set of connections will be used: see \code{\link{datashield.connections_default}}.
#' @return \code{ds.numNA} returns to the client-side the number of missing values
#' on a server-side vector. 
#' @author DataSHIELD Development Team
#' @export
#' @examples
#' \dontrun{
#'  ## Version 6, for version 5 see the Wiki
#'   
#'   # connecting to the Opal servers
#' 
#'   require('DSI')
#'   require('DSOpal')
#'   require('dsBaseClient')
#'
#'   builder <- DSI::newDSLoginBuilder()
#'   builder$append(server = "study1", 
#'                  url = "http://192.168.56.100:8080/", 
#'                  user = "administrator", password = "datashield_test&", 
#'                  table = "CNSIM.CNSIM1", driver = "OpalDriver")
#'   builder$append(server = "study2", 
#'                  url = "http://192.168.56.100:8080/", 
#'                  user = "administrator", password = "datashield_test&", 
#'                  table = "CNSIM.CNSIM2", driver = "OpalDriver")
#'   builder$append(server = "study3",
#'                  url = "http://192.168.56.100:8080/", 
#'                  user = "administrator", password = "datashield_test&", 
#'                  table = "CNSIM.CNSIM3", driver = "OpalDriver")
#'   logindata <- builder$build()
#'   
#'   connections <- DSI::datashield.login(logins = logindata, assign = TRUE, symbol = "D") 
#'   
#'   #Get the number of missing values on a server-side vector 
#'   
#'   ds.numNA(x = "D$LAB_TSC",
#'            datasources = connections)
#'   
#'   # clear the Datashield R sessions and logout
#'   datashield.logout(connections)
#'
#'
#' }
#'
ds.numNA <- function(x=NULL, datasources=NULL){

  # look for DS connections
  if(is.null(datasources)){
    datasources <- datashield.connections_find()
  }

  # ensure datasources is a list of DSConnection-class
  if(!(is.list(datasources) && all(unlist(lapply(datasources, function(d) {methods::is(d,"DSConnection")}))))){
    stop("The 'datasources' were expected to be a list of DSConnection-class objects", call.=FALSE)
  }

  if(is.null(x)){
    stop("Please provide the name of a vector!", call.=FALSE)
  }

  # check if the input object is defined in all the studies
  isDefined(datasources, x)
  
  # call the internal function that checks the input object is of the same class in all studies.
  typ <- checkClass(datasources, x)

  # call the server side function
  cally <- paste0("numNaDS(", x, ")")
  numNAs <- DSI::datashield.aggregate(datasources, as.symbol(cally))

  return(numNAs)
}