Read Next Block of Rows from External File

DESCRIPTION:

Reads the next block of rows from an external file.

USAGE:

readNextDataRows(dh)

REQUIRED ARGUMENTS:

dh
a data handle object to the external data file. This is created with openData.

VALUE:

a data frame created from the next block of data read from the external data file that is referenced in the dh. The number of rows in the data frame will be the value rowsToRead specified in the call to openData that created dh (or less if the last block of rows have been read).

SIDE EFFECTS:

When the last block of rows has been read the file associated with dh is closed.

SEE ALSO:

, , , .

EXAMPLES:

# Create an external data set:
exportData(environmental,"env.sav")  # an SPSS data file
# Open the external data set for subsequent reads:
dh <- openData("env.sav", rowsToRead=10)
# Compute the column means by accumulating sums in blocks of 10:
xsum <- 0; nobs <- 0
while(T) {
        cat(nobs,"\n")
        xdf <- readNextDataRows(dh)
        # if length of xdf is 0, no more data to read
        if(!length(xdf))
                break
        xsum <- xsum + colSums(xdf)
        nobs <- nobs + nrow(xdf)
}
env.means <- xsum/nobs