Matrix to CSV in Scala - scala

Question

Writing an MxN matrix ( M rows, N columns ) to a CSV file:
My first attempt, using map, works, but creates N references to the stringbuffer. It also writes an unnecessary comma at the end of each row.
def matrix2csv(matrix:List[List[Double]], filename: String ) = {
val pw = new PrintWriter( filename )
val COMMA = ","
matrix.map( row => {
val sbuf = new StringBuffer
row.map( elt => sbuf.append( elt ).append( COMMA ))
pw.println(sbuf)
})
pw.flush
pw.close
}
My second attempt, using reduce, also works but looks clunky:
def matrix2csv(matrix:List[List[Double]], filename: String ) = {
val pw = new PrintWriter( filename )
val COMMA = ","
matrix.map( row => {
val sbuf = new StringBuffer
val last = row.reduce( (a,b)=> {
sbuf.append(a).append(COMMA)
b
})
sbuf.append(last)
pw.println(sbuf)
})
pw.flush
pw.close
}
Any suggestions on a more concise and idiomatic approach ? Thanks.

Answer 1

You can obtain the string representation easily:
val csvString = matrix.map{ _.mkString(", ") }.mkString("\n")
Then you just need to dump it in a file.
Pay attention to end-lines (here "\n" ), they vary according to the platform.

Answer 2

Idiomatically, you're misusing map by using it to perform side-effecting operations. You should use foreach for this instead.
This is what it could look like if you used a foreach and replaced your StringBuffer boilerplate with a call to the mkString method:
def matrix2csv(matrix:List[List[Double]], filename: String) {
val pw = new PrintWriter(filename)
val COMMA = ","
matrix.foreach { row => pw.println(row mkString COMMA) }
pw.flush
pw.close
}
Note that mkString uses a StringBuilder (a non-thread-safe StringBuffer, which is fine here).