Game 0 Unit 7 of 15 1 hr learning time

FOR / NEXT Counts For You

Repeat a thing a fixed number of times without retyping it. FOR sets a counter and its range; NEXT sends the loop round again; STEP changes how the counter moves — even backwards, for a countdown.

47% of Meet C64 BASIC

To do a thing ten times, you don't write ten lines. FOR / NEXT is the counted loop: FOR names a counter and the range it runs through, NEXT sends control back for the next value, and the lines between run once per step.

Milestone 1 — count from 1 to 5

10 FOR N=1 TO 5
20 PRINT "LINE";N
30 NEXT N

Line 10 sets N to 1 and remembers it must reach 5. Line 30, NEXT N, adds one to N and jumps back to line 10's partner — the line after FOR — until N passes 5. The body (line 20) runs five times, with N holding 1, 2, 3, 4, 5 in turn:

The C64 screen showing LINE 1 through LINE 5 on five lines.
One PRINT, written once, run five times — the counter N changes each time round, so each line is different.

The counter isn't just a tally you ignore — it's a value you can use, here as part of the text. That's what makes a loop more than repetition.

Milestone 2 — STEP, and counting backwards

FOR climbs by 1 unless you tell it otherwise. STEP sets the stride — and a negative STEP counts down:

Step 2: count down with STEP -1
+3-2
1-10 FOR N=1 TO 5
2-20 PRINT "LINE";N
1+10 FOR N=10 TO 1 STEP -1
2+20 PRINT N
33 30 NEXT N
4+40 PRINT "LIFTOFF!"
45

FOR N=10 TO 1 STEP -1 starts at 10 and subtracts one each time round, stopping after 1. RUN it for a countdown:

The C64 screen showing 10, 9, 8 down to 1, then LIFTOFF!
STEP -1 runs the counter backwards. When it passes 1 the loop ends and line 40 fires: LIFTOFF!

When it doesn't work

  • ?NEXT WITHOUT FOR ERROR. A NEXT ran with no matching FOR above it — usually a jump landed inside the loop, or the FOR line was deleted.
  • The loop ran once, or not at all. Check the range and STEP. FOR N=1 TO 5 STEP -1 never moves toward 5, so the body runs a single time.
  • The counter was off by one. FOR N=1 TO 5 includes both 1 and 5 — five passes. If you wanted five starting at 0, use 0 TO 4.

Before and after

You started writing one line per repetition and finished with a loop that runs the body as many times as you set — forwards or backwards. The idea underneath: FOR sets a counter and a range, NEXT advances it, STEP changes the stride. A loop with a counter you use is the workhorse of every list, every grid, every animation to come.

Try this

  • Count the evens. FOR N=2 TO 10 STEP 2 — print each. Predict the values first.
  • A row of stars. Loop five times and PRINT "*"; (with the semicolon) so they sit on one line.
  • Nest two loops. Put a FOR inside a FOR and print both counters — the start of drawing a grid.

What you've learnt

  • FOR names a counter and its range; NEXT advances it and loops back.
  • The counter is a value you can use, not just a tally.
  • STEP sets the stride; a negative STEP counts down.
  • A range like 1 TO 5 includes both ends — five passes.

What's next

FOR repeats a fixed number of times. Games need a loop that runs until something happens — and a way to package a job and call it by name. In Unit 8 we meet GOTO, GOSUB, and the endless game loop, with RND to pick a secret number.