Play a Sequence

The flash subroutine can light up any panel. Now you need to flash several panels in order — a sequence. A string stores the sequence and a FOR/NEXT loop reads it one character at a time. This is the core of the game: the computer plays a pattern, the player watches, then repeats it.

VAL and STR$

Before building the sequence, you need two commands that convert between strings and numbers. Type NEW, then:

  10 LET s$="3"
  20 LET n=VAL s$
  30 PRINT "String: ";s$
  40 PRINT "Number: ";n
  50 PRINT "n+1 = ";n+1
  60 LET s$=STR$ 7
  70 PRINT "STR$ 7 = ";s$

Type RUN. The screen shows the string “3”, the number 3, and the result of adding 1 to it (4). Then it converts the number 7 back to a string.

• VAL s$ converts a string to a number. VAL "3" gives 3.
• STR$ 7 converts a number to a string. STR$ 7 gives “7”.

These two are opposites. The sequence string stores colour numbers as text characters. VAL unpacks each character into a number you can use with BORDER and BEEP. STR$ packs a number back into text so you can add it to the string.

Playing the Sequence

Add these lines to the panel program from the previous units:

  50 LET s$="132"
  52 FOR i=1 TO LEN s$
  54 LET c=VAL s$(i TO i)
  56 GO SUB 500
  58 PAUSE 12
  60 GO SUB 400
  62 PAUSE 6
  64 NEXT i

Type RUN.

The four panels appear. After a moment, panel 1 flashes bright with a low tone — blue floods the border. It fades. Then panel 3 lights up green with a higher pitch. Fade. Then panel 2, red, mid-range. Three colours, three tones, played in order: 1, 3, 2.

The sequence string s$="132" stores those three colours as characters. The FOR loop runs from 1 to LEN s$ (the length of the string — 3). On each pass:

• s$(i TO i) reads the i-th character. When i is 1, it reads “1”. When i is 2, it reads “3”.
• VAL s$(i TO i) converts the character to a number.
• GO SUB 500 flashes that panel bright with its tone.
• GO SUB 400 dims everything back to normal.

The pauses between flashes (PAUSE 12 after the flash, PAUSE 6 after the dim) give the player time to register each colour before the next one appears.

Try This

• Change s$="132" to s$="4321". Four colours in reverse order. The loop adapts because it uses LEN s$.
• Change it to s$="1111". The same colour four times — all blue, all the same tone. Hard to count when every flash looks identical.

What You’ve Learnt

• VAL converts a string character to a number — VAL "3" gives 3
• STR$ converts a number to a string — STR$ 3 gives “3”
• LEN returns the length of a string — the loop uses it to know when to stop
• String indexing — s$(i TO i) reads the i-th character (starting at 1)
• Strings as data — the sequence string stores colour numbers as text, one character per colour