📄 ttt.c
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/*----------------------------------------------------*- Fundamental -*-
Facility: ttt(6)
File: ttt.c
Associated files: - [ none]
Description: Plays the game of tic-tac-toe.
Author: Leor Zolman
Editor: Anders Thulin
Rydsvagen 288
S-582 50 Linkoping
SWEDEN
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Edit history :
Vers Ed Date By Comments
---- --- ---------- ---------------- -------------------------------
1.0 0 1979-09-xx Leor Zolman Written in BDS C
1.1 1 1988-10-25 Anders Thulin Added some comments; cleaned
up generally
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/*--- Configuration: --------------------------------------------------
System configuration options:
=============================
Symbol: Conformant systems:
ANSI ANSI C
BSD BSD Unix, SunOS 3.5
SV2 AT&T UNIX System V.2, AU/X
XPG3 X/Open Portability Guide, ed. 3
If you have an ANSI C compiler, define ANSI. If not, choose one
of the other symbols.
There are no program configuration options.
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#define ANSI 1
#define BSD 0
#define SV2 0
#define XPG3 0
/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
#if ANSI
# include <ctype.h>
# include <stdio.h>
# include <stdlib.h>
#endif
#if BSD
# include <ctype.h>
# include <stdio.h>
extern int rand();
extern int srand(); /* SunOS srand returns int? */
# define EXIT_FAILURE 1
# define EXIT_SUCCESS 0
#endif
#if SV2
# include <ctype.h>
# include <stdio.h>
extern int rand();
extern void srand();
# define EXIT_FAILURE 1
# define EXIT_SUCCESS 0
#endif
#if XPG3
# include <ctype.h>
# include <stdio.h>
# include <stdlib.h>
#endif
/*--- Original comments to program: ----------------------------------
BD Software presents:
Tic Tac Toe (by exhaustive search)
written by Leor Zolman
September 1979
This program was written for use as the crux of an article in BYTE
(as yet to be published) on BDS C, structured programming and gaming.
It is also intended to get you addicted to C so you'll go out and buy
the BD Software C Compiler
only $110 on CP/M disk from:
tiny c associates
post office box 269
holmdel, new jersey 07733
The package includes the compiler, linking loader, library manager,
standard library of functions, many interesting sample programs (such as
"TELNET.C" for using your computer as a terminal and allowing I/O directly
from modem to disk and vice-versa.) Also included is a copy of the best
reference book available on the C language, written by the original
implementors at Bell Labs (the package may be purchased without the book
for $100.)
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#define TRUE 1
#define FALSE 0
#define X 1 /* The pieces on the board */
#define O 5
#define EMPTY 0
char board[9]; /* the playing board */
char BEST; /* move returned by "ttteval" */
int wins[8][3] = { /* table of winning positions */
{0,1,2},
{3,4,5},
{6,7,8},
{0,3,6},
{1,4,7},
{2,5,8},
{0,4,8},
{2,4,6}
};
/*----------------------------------------------------------------------
Local functions:
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#if __STDC__ != 0
static int ask(char *s);
static int cats(void);
static void clear(void);
static void display(void);
static int getmove(void);
static int ttteval(int me, int him);
static int win(int p);
#else
static int ask();
static int cats();
static void clear();
static void display();
static int getmove();
static int ttteval();
static int win();
#endif
/*----------------------------------------------------------------------
Routine: ask
Description: writes the string 's' to the terminal, and reads a
character in reply.
If the character is 'y' or 'Y' TRUE is returned,
otherwise FALSE is returned.
Suggestions for improvement:
1. Instead of returning FALSE if an error occurs,
we could ask again until we get a correct response.
2. Rather than examining only the first character
read, we should skip all space characters until
we get something solid. If the line is empty,
ask again.
3. Rather than testing for 'y' we should test for 'n'.
The reason is only that most languages use a
word that begins with 'n' for 'no', while very
few languages use words beginning in 'y' for 'yes'.
Internationalization is the word!
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static int ask(s)
char *s;
{
char string[160]; /* input string */
char c; /* the response character */
/* 1. Print the string to standard output: */
printf(s);
/* 2. Read a string from standard input: */
if (fgets(string, sizeof(string), stdin) == 0) { /* EOF or error */
return FALSE;
}
/* 3. Examine first character in string: */
c = toupper(string[0]);
return (c == 'Y');
}
/*----------------------------------------------------------------------
Routine: cats
Description: This function returns true if all nine squares
taken (usually called after checking for a win, so
that all spaces filled indicates a cat's game)
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static int cats()
{
int i;
for (i=0; i<9; ++i) {
if (board[i] == EMPTY) {
return FALSE;
}
}
return TRUE;
}
/*----------------------------------------------------------------------
Routine: clear
Description: Empty the board
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static void clear()
{
int i;
for (i=0; i<9; ++i) {
board[i] = EMPTY;
}
}
/*----------------------------------------------------------------------
Routine: display
Description: Displays the playing board.
Note: The original version of the routine accepted
a parameter that indicated if the pieces should
be displayed (the normal case), or if the
square numbers (1-9) should be shown instead.
The square numbers were only shown at the
beginning of the game, and usually scrolled out of
sight, making it difficult to remember them.
This version prints the square number for empty
squares and the piece for occupied squares.
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static void display()
{
int r,c; /* Not row and column, but almost ... */
printf("\n\n");
for (r=0; r<9; r+=3) {
for (c=r; c< r+3; ++c) {
putchar(' ');
switch (board[c]) {
default:
fprintf(stderr, "ttt(display): unknown piece (%d) on board (%d)",
board[c], c);
exit(EXIT_FAILURE);
case EMPTY:
printf("%1d", c+1);
break;
case X:
putchar('X');
break;
case O:
putchar('O');
break;
}
putchar(' ');
if (c != r+2) {
putchar('|');
}
}
putchar('\n');
if (r != 6) {
printf("---+---+---\n");
}
}
putchar('\n');
}
/*----------------------------------------------------------------------
Routine: getmove
Description: reads a string containing a digit, and returns
the digit as result.
If no digit could be read, -1 is returned.
Suggestions for improvement:
See the description of ask() above!
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static int getmove()
{
char string[160]; /* input string */
/* 1. Read a string from standard input: */
if (fgets(string, sizeof(string), stdin) == 0) { /* EOF or error */
return -1;
}
/* 2. Return value of digit: */
return string[0] - '0';
}
/*----------------------------------------------------------------------
Routine: main
Description: main program
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int main()
{
int i;
int mefirst; /* 1: computer goes first; 0: human */
int turn; /* counts number of moves played */
int finished; /* true if game is over */
int mypiece, hispiece; /* who has x or o */
int mywins, hiswins, catwins; /* game count */
int t;
/* 1. Initialize: */
srand(0); /* initialize random generator */
mywins = hiswins = catwins = 0;
mefirst = TRUE; /* let human go first (see 2a below) */
printf("\n\nWelcome to BDS Tic Tac Toe!!\n");
printf("(In this version, X always goes first.\n");
/* 2. Outer game loop: */
do {
/* 2a. Set up for new game: */
finished = FALSE;
mefirst = !mefirst; /* reverse who goes first */
turn = 0;
clear(); /* clear the game board */
if (mefirst) {
printf("I go first...\n");
display();
} else {
printf("You go first...\n");
}
mypiece = mefirst ? X : O; /* set who has got what */
hispiece = mefirst ? O : X;
if (!mefirst) goto hismove;
/* 2b. Inner game loop: */
while (!finished) {
/* 2c. Computer's move: */
if (turn == 0) { /* Random opening move */
BEST = rand() % 9;
}
if (turn == 1) { /* Response to player's opening move */
if (board[4] != EMPTY) { /* Center is occupied */
BEST = rand() % 2 * 6 + rand() % 2 * 2;
} else { /* Grab center */
BEST = 4;
}
}
if (turn > 1) { /* OK, we're into the game... */
t = ttteval(mypiece, hispiece);
if (t == 1) printf("I've got ya!\n");
}
board[BEST] = mypiece; /* make the move */
++turn;
printf("I move to %d\n", BEST+1);
if (win(mypiece)) {
++mywins;
printf("\nI win!!!\n");
finished = TRUE;
} else if (cats()) {
++catwins;
printf("\nMeee-ow!\n");
finished = TRUE;
}
hismove:
if (!finished) {
/* 2b. Player to move: */
display();
do {
printf("Your move (1-9) ? ");
i = getmove();
if (i < 1 || i > 9 || board[i-1]) {
printf("\nIllegal!\n");
i = 0; /* indicates erroneous move */
}
} while (i == 0);
board[i-1] = hispiece;
++turn;
display();
if (win(hispiece)) {
++hiswins;
printf("\nYou beat me!!\n");
finished = TRUE;
} else if (cats()) {
++catwins;
printf("\nOne for Morris.\n");
finished = TRUE;
}
}
} /* Inner game loop */
} while (ask("\nAnother game (y/n) ? "));
/* 3. Print final statistics: */
printf("\n\nOK...Final score:\n");
printf("You won %d game", hiswins);
if (hiswins != 1) putchar('s');
printf(";\nI won %d game", mywins);
if (mywins != 1) putchar('s');
printf(";\nThe Cat got %d game",catwins);
if (catwins != 1) putchar('s');
printf(".\nSee ya later!!\n");
return EXIT_SUCCESS;
}
/*----------------------------------------------------------------------
Routine: ttteval
Description: The function "ttteval" returns an evaluation of
player x's position on the tic-tac-toe board. If he's
in a winning position, 1 is returned. If the best he
can hope for is a cat's game, 0 is returned. And, if
he's sure to lose if player y plays correctly, -1 is
returned. In any case, the best move available to
player x is left in external variable BEST upon return
from ttteval.
Note that a value of -1 is often returned while
recursive searching branches down into all possible
wins and losses, but with the program in the shape
it appears here, the outermost-level call to ttteval
(from the main program) will never produce a return
value of -1, since the computer has decided not to be
able to lose (the obviously logical choice of any
self-respecting problem-solver.)
In any case, the main program still bothers to
consider the possibility of losing, so that if you
want to try inserting your own "ttteval" routine
in place of the one given, it dosn't have to be
perfect in order to work with the main program.
Tic-tac-toe is, of course, just about the only game
in which it is feasible to do an exhaustive search;
most game evaluation algorithms only go so deep and
then make a "static" evaluation, or estimate of the
player's status at the terminal position. That is how
all decent chess playing programs today work.
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static int ttteval(me, him)
int me;
int him;
{
int i,safemove;
int v,loseflag;
/* 1. Is game terminated? */
if (win(me)) return 1;
if (win(him)) return -1;
if (cats()) return 0;
/* 2. Try out all moves and see what happens: */
loseflag = 1;
for (i=0; i<9; ++i) {
if (board[i] == EMPTY) { /* a legal move */
board[i] = me; /* try the move...*/
v = ttteval(him,me); /* ...evaluate the resulting position */
board[i] = EMPTY; /* and undo the move */
if (v == -1) { /* if we force a loss, yey! */
BEST = i;
return 1;
}
if (v) continue; /* uh oh! we shouldn't get beaten! */
loseflag = 0; /* cat's game guaranteed at least */
safemove = i;
}
}
BEST = safemove;
return -loseflag; /* if loseflag is true, this returns
-1 to indicate a loss; else zero
is returned (cat's game.) */
}
/*----------------------------------------------------------------------
Routine: win
Description: returns true (non-zero) if player p has three in a row
on the board:
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static int win(p)
int p;
{
int i;
for (i=0; i<8; ++i) {
if (board[wins[i][0]] == p &&
board[wins[i][1]] == p &&
board[wins[i][2]] == p) return 1;
}
return 0;
}
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