100 PROGRAM "21Game.bas"
110 RANDOMIZE
120 LET SUM,ADD=0
130 LET TURN=RND(2)-1
140 CLEAR SCREEN
150 PRINT "21 is a two player game, the game is played by choosing a number (1, 2, or 3) to be added to the running total. The game is won by the player whose chosen number causes the running total to reach exactly 21."
160 PRINT "The running total starts at zero. One player will be the computer.":PRINT
170 DO
180   LET TURN=NOT TURN
190   SET #102:INK 3:PRINT "The sum is";SUM:SET #102:INK 1
200   IF TURN THEN
210     PRINT "It is your turn.":PRINT "How many would you like to add? (1-3): ";
220     LET ADD=READKEY
230     IF ADD>21-SUM THEN PRINT "You can only add";21-SUM
240   ELSE
250     LET ADD=4-MOD(SUM-1,4)
260     IF ADD=4 THEN LET ADD=RND(3)+1
270     PRINT "It is the computer's turn.":PRINT "The computer adds";ADD
280   END IF
290   PRINT :LET SUM=SUM+ADD
300 LOOP WHILE SUM<21
310 IF TURN THEN
320   PRINT "Congratulations. You win."
330 ELSE
340   PRINT "Bad luck. The computer wins."
350 END IF
360 END
370 DEF READKEY
380   DO
390     LET T$=INKEY$
400   LOOP WHILE T$>"3" OR T$<"1"
410   PRINT T$:LET READKEY=VAL(T$)
420 END DEF