You have already seen that programs are much more readable if they contain

sensible variable names. Additionally, all the example programs have used

only a single statement per program line. Programs can be written to contain

more than one statement per line, provided the statements are separated by

colons:

10 MODE 135:PRINT "Type any character-- ";:chosen=GET:PRINT CHR$(ch

osen):PRINT "You typed ";CHR$(chosen)

You can imagine that a program with many multi-statement lines like this is

not easy to follow!

The next section deals with the facilities BBC BASIC offers to simplify the

development and modification of programs. So far you have only used programs

made up of a sequence of instructions. The computer can also repeat

instructions, or choose which of several instructions it will obey. All programs

are built up from a combination of the three program structures: sequence,

repetition and choice. The next few sections describe how you can use these

structures in BBC BASIC.

Planning your programming

The programs in the earlier chapters have all been fairly short, but the easiest

way to write more complex programs is to organise them differently.

Look back at the Teddy program on page Cl9. The program consists of a

sequence of instructions which the computer obeys in line-number order. A

longer program might contain several hundred lines, and is simpler to write if it

is broken into small sections known as procedures.

This program shows the use of procedures:

1 REM draw butterfly

10 MODE 130

20 PROCbody

30 PROC Left_wing

40 PROCright_wing

50 END

60 DEFPROCbody

70 GCOL0,2

80 MOVE 600,500

90 MOVE 700,500

100 PLOT 205,640,700

110 ENDPROC

120 DEFPROC Left-wing

130 GCOL 0, 1

140 MOVE 200,200

C 30