Lesson 18 · Delete forward - Build a Text Editor

Delete forward

The Delete key removes the character at the cursor rather than behind it - and at the end of a line, it pulls the next line up. Today you add forward deletion, backspace's mirror image.

The goal

Delete the character at the cursor; at the end of a line, delete the newline so the next line joins this one.

Start here - the target

TO DO

Scenario: Forward delete within a line and across a line

Givenan editor over "abc" with the cursor at Row 0, Col 1

WhenDeleteForward is called

ThenText() is "ac" and the cursor stays at Row 0, Col 1

Andover "ab\ncd" at Row 0, Col 2 (end of the first line), DeleteForward gives Text() "abcd" with the cursor unchanged, and at the very end of the buffer it does nothing

Background

Forward delete is backspace reflected. Where backspace removes what is behind the cursor, DeleteForward removes what is at it - the character the cursor sits on

and the cursor does not move, because the text to its right simply slides left to fill the gap. Inside a line that is a one-character delete at Offset().

The line-boundary case flips too: at the end of a line the character “at” the cursor is really the newline that separates this line from the next, so deleting it joins the following line onto this one, again with the cursor staying put. And at the very end of the buffer there is nothing ahead to delete, so it is a no-op. Notice both edits reuse the exact same Buf.Delete and line-join behavior from chapter one - backspace and forward delete differ only in which offset they target and whether the cursor moves. With these two, plus typing and Enter, the editor can make any change to the text.

Make it work

func (e *Editor) DeleteForward() {
  if e.Col < e.Buf.LineLen(e.Row) {
    e.Buf.Delete(e.Offset(), 1)          // the char under the cursor
  } else if e.Row < e.Buf.LineCount()-1 {
    e.Buf.Delete(e.Offset(), 1)          // the newline ahead -> join
  } // at the buffer's end: nothing to delete
}

CheckpointDONE

Forward delete works within and across lines. Commit and stop here.