22 If the computer is in command mode (in other words if the last thing on the screen is > ) then you can command it to print the program in its memory by typing LIST and pressing RETURN . The computer will then give a listing of the program on the screen for you to cheek. If you discover that you have made an error, for example that you have got something wrong in line 20, then it is easy to correct the error. There are two ways of correcting major errors: – Retype the whole line. – Use the screen editor. Using the screen editor There is a group of six keys on the right hand side of the keyboard which can be used to edit, or alter, program lines that are displayed on the screen.
Four of the keys have arrows on them and are coloured a lighter brown that most of the other keys. These keys enable you to move a flashing cursor around the screen to a line that you wish to edit. As soon as you press one of these keys the computer enters a special ‘editing mode’ where it displays two cursors. The large white block is called the write cursor and it shows you where anything that you enter will appear. The other small, flashing cursor – the read cursor – is the one that can be moved around by the arrow keys. Try moving the read cursor, by using the arrow keys, until it is under a letter at the start of a word and then press the COPY key several times.
As you will see the COPY key copies everything that the read cursor passes under into the new input line. Halfway through copying a line you can always use to move the read cursor to some new place on the screen before using COPY again to copy some other text to your new input line. The DELETE key can always be used to delete characters from the input line. You can also type new characters in at any time instead of using the COPY key. When your new input line is complete just press RETURN in the usual way.