Game 0 Unit 7 of 15 1 hr learning time

FOR / NEXT Counts For You

Repeat a block a set number of times without copying a single line. FOR ... NEXT counts from one number to another, hands you the count as a variable you can use, and counts backwards too with STEP.

47% of Meet BASIC

When you want to do something a set number of times, you don't copy the line out by hand. FOR ... NEXT runs a block over and over, counting as it goes — and it hands you the count, so each pass can use the number it's on.

Milestone 1 — count to ten

  10 FOR i = 1 TO 10
  20 PRINT i; " ";
  30 NEXT i

FOR i = 1 TO 10 sets a box i to 1, runs everything up to NEXT i, then bumps i to 2 and runs it again — all the way to 10. The semicolon on line 20 keeps the numbers on one row.

The Spectrum screen showing 1 2 3 4 5 6 7 8 9 10 on one line, with the report 0 OK, 30:1.
One PRINT, run ten times. The loop variable i is 1, then 2, then 3 — you wrote the line once and the loop did the rest.

Milestone 2 — use the count

i isn't only a counter — it's a variable you can use. Print a times-table by multiplying it:

Step 2: use i in a calculation
+1-1
11 10 FOR i = 1 TO 10
2- 20 PRINT i; " ";
2+ 20 PRINT i; " x 7 = "; i * 7
33 30 NEXT i
44

Each pass, i * 7 works out that row of the seven-times table. The loop counts; your line does something different with the count each time round.

The Spectrum screen showing the seven-times table from 1 x 7 = 7 down to 10 x 7 = 70.
The counter, put to work: i runs 1 to 10, and i * 7 turns each pass into a row of the table.

Milestone 3 — count the other way with STEP

STEP changes how i moves each pass. STEP -1 counts down:

  10 FOR i = 10 TO 1 STEP -1
  20 PRINT i; " ";
  30 NEXT i
  40 PRINT
  50 PRINT "Lift-off!"

FOR i = 10 TO 1 STEP -1 starts at 10 and drops by one each pass until it reaches 1. After the loop, line 50 prints the finish.

The Spectrum screen showing 10 9 8 7 6 5 4 3 2 1 then Lift-off!, with the report 0 OK, 50:1.
STEP -1 runs the count backwards. STEP 2 would skip every other number; the default, with no STEP, is STEP 1.

When it doesn't work

  • NEXT without FOR. The NEXT names a variable that has no matching FOR above — often a typo, like NEXT j for a FOR i. Match the letters.
  • The loop ran once, or not at all. Check the range. FOR i = 5 TO 1 with no STEP -1 won't count down — it has nowhere to go.
  • Everything printed on top of itself. A semicolon keeps a row going; without one, each PRINT starts a new line. Choose the layout you want.

Before and after

You started repeating work by hand and finished letting FOR ... NEXT do it — counting up, using the count in a sum, and counting back down with STEP. The idea underneath: a counted loop runs a block a set number of times and hands you the count as a variable.

Try this

  • A different table. Change 7 to your own number and run the table again.
  • Skip-counting. Use STEP 2 to print 2, 4, 6, 8, 10.
  • A square of stars. Loop ten times, printing a row of stars each pass.

What you've learnt

  • FOR i = a TO b ... NEXT i repeats a block, counting i from a to b.
  • The loop variable i is a real variable — use it inside the loop.
  • STEP changes the stride: STEP -1 counts down, STEP 2 skips; the default is 1.
  • NEXT must name the same variable as its FOR.

What's next

A counted loop runs a known number of times. But a game runs until something happens — until you win, lose, or quit. In Unit 8 we build that loop with GO TO and GO SUB, and let the computer pick a secret number with RND.