Keep guessing

A clue needs a next guess. We’ll keep Lucky Number running until the answer is correct, then show how many guesses it took. After that, we’ll make the program enforce its own instructions.

Continue with Give a useful clue, with the secret restored to 7. Keep that program in memory. If you are joining here, enter the complete first listing into an empty 48K BASIC program; the preceding lesson explains numeric input, comparisons and entering THEN.

Return to the question

Add lines 35 and 70. Keep the other lines:

10 LET secret=7
20 PRINT "Guess a whole number 1 to 10"
30 INPUT "Your guess ";g
35 CLS
40 IF g=secret THEN PRINT "Correct!"
50 IF g<secret THEN PRINT "Too low"
60 IF g>secret THEN PRINT "Too high"
70 IF g<>secret THEN GO TO 30

For line 35, type its number and press V for CLS. The screen clears after we enter a guess, before the new clue appears. The clue stays visible while the next question waits at the bottom. This keeps repeated guesses from filling the display with old messages.

Line 70 reads: if the guess is not equal to the secret, go to line 30. <> means “not equal”. It is a single Spectrum token, so do not enter it as a separate < followed by >.

To enter line 70, type its number, press U for IF, then type g. Hold left Option/Alt and press W for <>, then release both keys. Type secret, use left Option/Alt+G for THEN, release the keys, then press G for GO TO and type 30. Press Enter to store the line. We stay in Host Keyboard mode throughout.

GO TO 30 changes which instruction runs next. Instead of falling off the end of the listing, BASIC returns to the INPUT line. This repeated route through the instructions is a loop.

Run it and guess 4, 9, then 7. The first two guesses produce clues and another question. With 7, line 40 prints Correct!; the condition in line 70 is false, so execution reaches the end and BASIC reports 0 OK.

Foundations: Until You Get It traces a loop that repeats until an answer is right. Our IF and GO TO express that idea using numbered lines, rather than pseudocode’s REPEAT … UNTIL.

Count the accepted guesses

Add lines 15, 36 and 80:

10 LET secret=7
15 LET tries=0
20 PRINT "Guess a whole number 1 to 10"
30 INPUT "Your guess ";g
35 CLS
36 LET tries=tries+1
40 IF g=secret THEN PRINT "Correct!"
50 IF g<secret THEN PRINT "Too low"
60 IF g>secret THEN PRINT "Too high"
70 IF g<>secret THEN GO TO 30
80 PRINT "Guesses: ";tries

Line 15 starts tries at zero: no guesses have been made yet. This is initialisation, giving a value its starting state. Line 36 reads the old value, adds one and stores the result back in the same variable. Type the plus sign with your usual key in Host Keyboard mode.

LET tries=tries+1 is an instruction to update a value, not a claim that two mathematical expressions are equal. With tries at 2, BASIC calculates 3, then replaces the stored 2 with 3. Foundations: A Box That Grows From Itself traces this kind of update one step at a time.

Run with 4, 9, then 7 again. The counts are 1, 2 and 3. Line 80 is reached only after a correct guess lets execution pass line 70. Its semicolon joins Guesses: to the value in tries.

Now run again and guess 7 first. Expect Guesses: 1: RUN reaches line 15 and starts a fresh count. The loop goes back to 30, after initialisation, so later guesses in the same run do not reset it.

If the count is always 1, check that line 70 returns to 30, not 10 or 15. Trace which lines execute before changing anything else.

Enforce the promised range

At present, 11 and 6.5 both count as guesses. We asked for whole numbers from 1 to 10; we should check that before increasing tries or giving a clue.

Add lines 31, 32, 33, 90, 100, 110 and 120:

10 LET secret=7
15 LET tries=0
20 PRINT "Guess a whole number 1 to 10"
30 INPUT "Your guess ";g
31 IF g<1 THEN GO TO 100
32 IF g>10 THEN GO TO 100
33 IF g<>INT g THEN GO TO 100
35 CLS
36 LET tries=tries+1
40 IF g=secret THEN PRINT "Correct!"
50 IF g<secret THEN PRINT "Too low"
60 IF g>secret THEN PRINT "Too high"
70 IF g<>secret THEN GO TO 30
80 PRINT "Guesses: ";tries
90 STOP
100 CLS
110 PRINT "Whole number from 1 to 10"
120 GO TO 30

Lines 31 and 32 reject numbers below 1 or above 10. Either failed range check sends execution to line 100. Line 33 rejects a fraction using INT:

• INT 6.5 is 6, so 6.5 is not equal to its integer result.
• INT 7 is 7, so a whole number passes this check unchanged.

An integer is a whole number. INT rounds down to an integer; it does not round to the nearest one. These positive examples are enough for our range check. Negative numbers have already been rejected by line 31.

For line 33, enter IF g<> using the same keys as line 70. To insert INT, press and release Shift+left Option/Alt together, then press R. The shift chord selects the Spectrum’s extended keyword mode; R then enters INT. Type g and continue with THEN GO TO 100 using the keyword keys already described.

For line 90, type the number, hold left Option/Alt and press A for STOP, release both keys, then press Enter. STOP ends execution here. It prevents a successful round from continuing into the rejection message below.

We now have two routes:

• An accepted guess reaches line 35, clears the screen and increases the count at line 36. The comparisons give their usual clue or success message.
• A rejected number jumps to line 100. That clears the screen, prints the rule and returns to the question. It never reaches the counter or the clues.

We keep the supplied value in g long enough to test it. Writing LET g=INT g instead would change 6.5 into 6 and silently treat it as a different guess. Here we ask the player to choose again.

Try the edges

Run the program and enter these answers in order:

The final 9 STOP statement report is BASIC confirming that it reached our STOP. It is not a failure. R enters RUN when you want to start again. Check a correct first guess too: the result should still be Guesses: 1.

Try several wrong guesses before choosing 7. Each new clue replaces the old one, rather than eventually producing a scroll? prompt. This version shows only the latest clue; remembering earlier clues is the player’s job. Keeping a visible history would be a different design choice.

These checks act on numeric values that INPUT has accepted. They are not a general text parser: a word or a malformed expression may still produce a BASIC input report. A valid expression such as 3+4 produces 7 and counts as one correct guess. Our game checks the resulting value.

Keep this version

Save with SAVE "lucky", wait for the tape prompt, press Enter and wait for 0 OK. Export the recording to lucky.tap. When reopening it, use LOAD "lucky" so an earlier recording is not mistaken for this program. Remember a name; keep the program explains the export and tape controls. After loading, try 0, 4 and 7: the finished count should be 2.

The secret is still visible in line 10. That has let us test the clues, loop and count with predictable results. Choose a new secret lets the Spectrum choose within the same range.