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13 (Let player choose X or O.)
Add a variable HUMAN'S SYMBOL whose value will be X or O, whichever the user chooses. Another variable COMPUTER'S SYMBOL is the opponent of that one. (The second variable isn't really necessary; you could use OPPONENT (HUMAN'S SYMBOL) instead. This just makes the program a little more self-documenting.)
If the user chooses O, make the computer's first move in square 5. (Note how simple it is to do that: it's just BROADCAST 5.)
The only procedure that has to be modified is NEXT MOVE FOR COMPUTER, which has to check HUMAN'S SYMBOL instead of just assuming that the computer is O.
Most of the effort goes into the interaction in which the user chooses X or O; the actual play is hardly changed.computer's symbolX's turn?trueOIf human wants to play O, make computer's first move in center square.TIF A (beat the computer): The computer's minimal strategy turns out to be pretty good! There is one obscure sequence of moves that can beat it: Choose X, and /don't/ make the first move in the center. Instead, move in square 1. The computer will then move in square 5. Now move in square 9. (This is a really counterintuitive move! It doesn't seem to help at all, because the computer has already blocked the diagonal.) The computer will now move in another corner, let's say 3. The player must now take the fourth corner (7) in order to block the computer's win. But now there are two different winning edges! Whichever the computer doesn't block, you take to win.
But the computer's strategy is weak in a different sense: If the user chooses O, the computer starts in the center, and the user moves in an edge square, the computer should win the game, but instead will tie. This is what the suggestion in TIF B is meant to fix.
The fix for the case of the player moving in a corner and then in the opposite order is made by modifying BEST EMPTY SQUARE to take the ordered list of good moves as an input, and then modifying NEXT MOVE FOR COMPUTER so that if there is an XOX as the only filled triple. then an edge is better than a corner.0TIF B (forking):
b. If there's a winning triple in which the computer already has two of the three squares, then there's no need to think about forking -- the computer can win by playing in the third (empty) square of the triple. That's "Rule 1."
LABEL will stamp text on the stage at the given font size. The direction of the text is the direction the sprite is facing, and color will match the pen color.Hello!121datamapmany1data lists11110i121cont3catchtagcontcatchtagSpriteSprite-150-50square number0111check for wins0check for forks012X's turn?X1atomsTIF B(e)repeated1TIF B(f)Take any number of input lists, and create a new list containing the items of the input lists. So
APPEND [A B] [C D]
where the [,,,] are lists reports the list
[A B C D]
not [[A B] [C D]].11111Reports a new list containing the items of the input list, but in the opposite order.1inputresultReports a new list whose items are the same as in the input list, except that if two or more equal items appear in the input list, only the last one is kept in the result.1Reports a sorted version of the list in its first input slot, using the comparison function in the second input slot. For a list of numbers, using < as the comparison function will sort from low to high; using > will sort from high to low.1even itemsmerge11#1#2The second input is an "association list," a list of two-item lists. Each of those smaller lists has a "key" as its first item and a "value" as its second. ASSOC reports the first key-value pair in the association list whose key matches the first input.11#11datacountmapmany##1#1data listscount#1datacount1falseOXXXXOOOX0oX