Choose a new secret

Knowing that the secret is 7 makes testing easy. It also makes the game rather short. We’ll let the Spectrum choose the secret while keeping the same useful clues.

Continue with the final listing from Keep guessing. If you are joining here, start Emu198x in the 48K setup and enter the complete first listing into an empty program. That preceding lesson explains the comparison, counting and validation lines retained here.

From a fraction to a secret

Replace line 10. Everything else stays as it was:

10 LET secret=INT (RND*10)+1
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

RND produces a pseudo-random number: a value drawn from a calculated sequence. It can be zero but is always less than 1. The sequence is useful for games; it is not a source of secure passwords.

Read the expression from the inside out:

The star means multiplication. Parentheses keep the multiplication inside INT; the final addition shifts the whole range up by one. For an illustrative value of 0.73, these steps give 7.3, then 7, then 8.

To enter line 10, type its number, press L for LET, then type secret=. Press and release Shift+left Option/Alt, then R for INT. Type (, use Shift+left Option/Alt, then T for RND, and type *10)+1. Host Keyboard mode supplies the punctuation from your usual keys.

Run the game. Use its clues to find the secret. Line 10 runs once at the start; the wrong-guess loop returns to line 30, so it does not change the answer midway. A fresh run chooses another value, which may happen to be the same one.

Foundations: Until You Get It uses the convenient pseudocode RANDOM 1 TO 10. Here we have built that range from the functions Sinclair BASIC actually provides.

Choose where the sequence begins

Add line 5:

5 RANDOMIZE
10 LET secret=INT (RND*10)+1
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

Press T after the line number for RANDOMIZE; the Spectrum keyboard labels it RAND. This statement chooses a starting place in the sequence using the machine’s running frame counter. RND then obtains successive values from that sequence. They have different jobs.

We seed the sequence once at the beginning, rather than repeatedly restarting it while the player guesses. Restarting a machine in the same state and following the same timings can still repeat results. Random selection also permits the same number twice: a repeated answer is not by itself a fault.

For a repeatable test, temporarily replace line 5 with 5 RANDOMIZE 1. Run twice and guess 1 first each time. Both runs should succeed in one guess. This fixed starting value, called a seed, makes a fault easier to reproduce. Restore 5 RANDOMIZE afterwards.

Test what chance can hide

Temporarily replace line 10 with 10 LET secret=1. Try 0, then 1: 0 should be rejected, and 1 should win in one counted guess. Repeat with secret 10, testing 11 before 10. These deliberate test values check both ends of the game’s range. Restore the random line from the listing when finished.

You can inspect individual generated secrets after a completed game with the unnumbered command PRINT secret. Run and play several times. Values outside 1–10 would show a bug; a short run of values inside the range does not prove that every outcome is equally likely.

To experiment with 1–100, change the multiplier in line 10, the limit in line 32, and the instructions in lines 20 and 110 together. More possible numbers mean more room for clues to help, but also a longer game. Keep 1–10 for our next lesson.

Next, Feedback and another round continues this program.