Simple Google Spreadsheet For Each Loop?

Simple Google Spreadsheet For Each Loop?

It's been a long time since I've coded. I've never looked into scripts for
google spreadsheets before. I just want to make a simple effect to edit
the spreadsheet. If I understand correctly, this is doable so long as you
run it manually?
The syntax is throwing me off too much. My basic goal is to set each cell
in a fixed column range to equal itself plus the value in the adjacent
column, and then set that second value to 0.
My instinct would be to do something such as
CellRange[i][j] selected = C9:D13;
for(i=0,i<selectedrange.length,i++){
SpreadsheetApp.getActiveRange().setValue(selected[i][j]+selected[i][j+1];
SpreadsheetApp.getRange(selected[i][j+1]).setValue(0);
}
That's probably terribly wrong but I feel I ought to at least throw my
best guess out before asking for help.