Game 0 Unit 2 of 15 1 hr learning time

The Line Number Is an Address

Edit a program by its line numbers — replace a line by reusing its number, slip a new line into a gap, delete one, and LIST them in order. Then meet the C64's full-screen editor, where you change a line in place.

13% of Meet C64 BASIC

In Unit 1 your two lines were numbered 10 and 20, and they ran in that order. The number isn't only a label for where a line sits in the running order — it's the line's address. Once you can write to an address, you can edit: replace what's there, slip something new in beside it, or take it away.

Milestone 1 — a numbered program

10 PRINT "ONE"
20 PRINT "TWO"
30 PRINT "THREE"

Three lines, numbered in tens. Type them in and RUN: they print ONE, TWO, THREE, in number order, then the C64 returns to READY..

The C64 screen showing ONE, TWO, THREE on three lines, then READY.
Three lines, run in number order. The gaps between 10, 20 and 30 are about to earn their keep.

Notice the numbering jumps in tens. Nothing forces that — 1, 2, 3 would run the same. The tens leave room between the lines, and the next milestone shows why.

Milestone 2 — slip a line into the gap

You want a line between TWO and THREE, without retyping anything. Give it a number that falls in the gap — 25, sitting between 20 and 30:

Step 2: type line 25 on its own
+1
11 10 PRINT "ONE"
22 20 PRINT "TWO"
3+25 PRINT "AND A HALF"
34 30 PRINT "THREE"
45

Type line 25 and press RETURN — on its own, with the other lines already in place. You don't move anything; the C64 files it by number. RUN, and the new line lands exactly where its number puts it: after 20, before 30.

The C64 screen showing ONE, TWO, AND A HALF, THREE — the new line sits between TWO and THREE.
Line 25 lands between 20 and 30 — slotted in by its number, with nothing else touched. This is why we number in tens.

Milestone 3 — write to an address that's taken

Use the same idea to replace a line. Type line 20 again, with new text. The number is already taken, so the new line takes the old one's place:

Step 3: retype line 20 to replace it
+1-1
11 10 PRINT "ONE"
2-20 PRINT "TWO"
2+20 PRINT "TWO, REPLACED"
33 25 PRINT "AND A HALF"
44 30 PRINT "THREE"
55
The C64 screen showing ONE, TWO, REPLACED, AND A HALF, THREE.
Same number, new text — line 20 is overwritten. Writing to an address that already holds a line replaces it.

That's the whole rule, two ways: pick a free number to add a line; reuse a number to replace one. To remove a line, type its number alone and press RETURN — line gone.

LIST, and the full-screen editor

To see the program in its true order at any time, type LIST. The C64 shows every line from lowest number to highest — the order it will run, whatever order you typed them in.

The C64 screen showing the LIST output: lines 10, 20, 25 and 30 in number order, then READY.
LIST prints the program in number order. On the C64 those listed lines are live text you can edit where they sit.

Here the C64 does something the listing makes possible. Those LISTed lines are not a frozen printout — they are live text on the screen. Move the cursor up with the CRSR keys onto line 20, type over part of it, and press RETURN while the cursor is on that line: the C64 reads the whole line back in and stores the change. This is the full-screen editor, and it is one of the C64's nicest touches — you fix a line by editing it where it sits, not by retyping it whole.

When it doesn't work

  • A new line overwrote one you wanted to keep. You reused a number that was already taken. LIST first, find a free number in a gap (like 25), and add there instead.
  • The line ran in the wrong place. The number decides the order, not where you typed it. Whatever should come first needs the lowest number.
  • You edited a LISTed line but the change didn't stick. You must press RETURN while the cursor is on that line — that's what feeds it back to BASIC. Scrolling past it changes nothing.

Before and after

You started with three lines and ended having inserted one into a gap, replaced another, and edited a line in place — without ever retyping the whole program. The idea underneath: a line number is an address. Write to a free one to add; write to a taken one to replace. Numbering in tens keeps gaps open for the lines a later step will add — a habit every program here leans on.

Try this

  • Fill another gap. Add line 15, between 10 and 20. Predict where it prints before you run it.
  • Edit in place. LIST, cursor up onto line 30, change THREE to THE END, press RETURN on the line, then RUN. The full-screen editor in action.
  • Delete and confirm. Remove line 25 (type 25, RETURN), LIST to confirm it's gone, then RUN.

What you've learnt

  • A line number is the line's address.
  • Reuse a number to replace a line; pick a free number to add one; type a number alone to delete it.
  • LIST shows the program in number order — the order it runs.
  • The C64's full-screen editor lets you change a LISTed line in place: edit it and press RETURN on the line.
  • Number in tens so there's always a gap to slip a line into later.

What's next

You can write, run, and edit a program now. Time to make it say more than a fixed word. In Unit 3 we give PRINT its punctuation — the comma and semicolon that join and space text and numbers across the line.