Question

I have a collection of lines, like in this snippet:

  def insertBeforeLine(text:String,whichLine:String,what:String) = {
    val lines = text.lines
    lines.foldLeft(ListBuffer[String]())((acumulator,element) => {
       acumulator ++ { if(element == whichLine) Array(what,element) else Array(element) }
    }).mkString("\n")
  }

I am trying to prepend something before every line that’s equal to whichLine. Is there a better/cleaner way? For example if my input is:

line1
line2
line4

and I call my function like insertBeforeLine(input,"line4","line3") it will produce:

line1
line2
line3
line4

Answer 1

If you really have a string of lines (and you can include the end-of-line character, and it’s consistent), you can use replace from java.lang.String:

text.replace(which,what+which)

but if you want something more general, something like

lines.flatMap(l => if (l==which) Seq(what,l) else Seq(l))

is a compact and clear way to express it (at least to those who know how flatMap works).

If you want something maximally efficient, then it’s a long ugly answer with lots of while loops and probably some byte array intermediates.