Doing It Again

Introduction

Printing five lines takes five PRINTs. Printing five hundred would take five hundred - unless the program can be told to do the same thing again. Repeating is what computers are for, and FOR is the simplest way to ask for it.

FOR And NEXT

10 FOR I=1 TO 5
20 PRINT I
30 NEXT I

FOR I=1 TO 5 sets I to 1. The lines down to NEXT I are done. At NEXT, I goes up by one and, if it has not passed 5, the program goes back up and does them again. So line 20 runs five times, with I being 1, 2, 3, 4 and 5.

The lines between FOR and NEXT are the loop. LIST shows them indented by two spaces, so you can see where a loop starts and ends:

   10 FOR I=1 TO 5
   20   PRINT I
   30 NEXT I

You typed them without the spaces; BBC BASIC adds them when it lists.

I is an ordinary variable, and the loop can use it - that is the point. And you can write just NEXT; the I after it is only a reminder of which loop it closes.

Steps

STEP says how much to add each time:

FOR I=1 TO 9 STEP 2

gives 1, 3, 5, 7, 9. A negative step counts down: FOR I=5 TO 1 STEP -1 gives 5, 4, 3, 2, 1.

Always At Least Once

FOR I=5 TO 1 - counting up from 5 to 1, with no STEP -1 - looks as if it should do nothing at all. It does the loop once, with I at 5. BBC BASIC only checks whether it has finished when it reaches NEXT. Remember that when a loop's limits come from a sum that might be backwards.

The Code

10 FOR I=1 TO 5
20 PRINT I,I*I
30 NEXT I
40 PRINT "DONE"

Starting from

A fresh boot, or NEW.

What you should see

>RUN
         1         1
         2         4
         3         9
         4        16
         5        25
DONE
>

The comma in line 20 puts I*I in the next ten-column space, so the numbers line up in two columns.

Change One Thing

• Change line 10 to FOR I=1 TO 9 STEP 2. Which numbers are used now?
• Make it FOR I=5 TO 1 STEP -1. What order are the lines in?
• Make it FOR I=5 TO 1, with no STEP. How many lines are printed, and why any at all?

Exercises

12.1 Print the seven times table, from 1 X 7 = 7 to 12 X 7 = 84.

12.2 Add up all the whole numbers from 1 to 100, and print the total.

12.3 Print a triangle of stars: one star, then two, then three, up to five. (S9's STRING$ helps.)

Worked solutions are in Appendix II.

When It Goes Wrong

Summary

FOR V=start TO end ... NEXT repeats the lines between, with V counting from start to end. STEP sets how much it counts by, and a negative step counts down. A loop always runs at least once.

Next

S13, Round And Round - repeating until something happens, however many times that takes.