My first C program

rpn.c

/* rpn.c -- reverse polish notation calculator.   G.S.BURD  25-Mar-1984 */

#include <math.h>
#include <stdio.h>
#include <ctype.h>

extern char *fgets();
extern double push(), pop();

/* function codes */

#define TRUE 		1
#define FALSE 		0
#define EXITPROGRAM	1
#define GIVEHELP	2
#define CLEAR		3
#define CHANGESIGN	4
#define	ADD		5
#define SUBTRACT	6
#define MULTIPLY	7
#define DIVIDE		8
#define RECIPROCAL	9
#define SQUARE		10
#define SQUAREROOT	11
#define SINE		12
#define COSINE		13
#define TANGENT		14
#define ARCSINE		15
#define ARCOSINE	16
#define ARCTANGENT	17
#define RADIAN		18
#define DEGREE		19
#define PI		20
#define NATURALLOG	21
#define BASE10LOG	22
#define ETOTHEX		23
#define POWER		24

#define NUMFUNCTIONS	24

#define UNKNOWN		-1
#define NUMBER		0


/* function table; must be in lexical order */

struct function {
    char *text;
    int  code;
} funtable[] = {
	"*", 	MULTIPLY,
	"+",	ADD,
	"-", 	SUBTRACT,
	"/",	DIVIDE,
	"?",	GIVEHELP,
	"acos",	ARCOSINE,
	"asin", ARCSINE,
	"atan", ARCTANGENT,
	"chs",  CHANGESIGN,
	"clr",  CLEAR,
	"cos",  COSINE,
	"deg",	DEGREE,
	"exp",	ETOTHEX,
	"ln",	NATURALLOG,
	"log",	BASE10LOG,
	"pi",	PI,
	"pow",	POWER,
	"rad",	RADIAN,
	"rec",	RECIPROCAL,
	"sin",	SINE,
	"sqr",	SQUARE,
	"sqrt",	SQUAREROOT,
	"tan",	TANGENT,
	"\0",	0,
   };

#define MAXOP    40   /* max size of operand, operator */
#define MAXLINE 132   /* max size of a line */

#define RADPERDEG (3.1415926535897 / 180.0)

main(argc, argv)
int	argc;
char	*argv[];
{
    char line[MAXLINE];

    if (argc <= 1) {
      printf("\t\t\t> ");
      while ( fgets( line, MAXLINE, stdin)  != NULL ) {
        process( line );
	print_top();
        printf( "\t> ");
      }
    }
    else {
      while (--argc > 0)
         process( *++argv );
	 print_top();
    }
}


/* process -- process a string */
process( str )
char str[];
{
      char s[MAXOP];
      int i;

      i = 0;
      while ( i = getword( str, i, s ) ) 
	dofun( getfun( s ), s );
}

/* getword -- get next word from str at i */
getword( str, i, out )
char str[], out[];
int  i;
{
      int j;

      while ( isspace( str[i] ) )
            i++;
      if( str[ i ] == '\0' ) return( 0 );
      j = 0;
      while ( ( ! isspace( str[i] ) ) && ( str[i] != '\0' ) )
           out[ j++ ] = tolower( str[ i++ ] );
      out[ j ] = '\0';
      return( i );
}


/* getfun -- get function code from s */
getfun( s )
char s[];
{
   int high, low, mid, cond;

   if ( isanumber(s) )
      return( NUMBER );
   else {
     low  = 0;
     high = NUMFUNCTIONS - 1;
     while ( low <= high ) {
         mid = ( low + high ) / 2;
         if (( cond = strcmp( s, funtable[mid].text )) < 0 )
             high = mid - 1;
         else if ( cond > 0 )
             low = mid + 1;
         else
             return( funtable[mid].code );
     }
     return( UNKNOWN );
  }
}


/* isanumber -- determine if s contains a digit */
isanumber( s )
char s[];
{
       int i;

       for( i = 0; s[i] != '\0'; i++ )
           if ( isdigit( s[i] ) ) return( TRUE );
       return( FALSE );
}



/* dofun -- do a function */
dofun( i, s )
int i;
char s[];
{
   static double degrad = 1.0;
   double op, op2;
   double atof(), pop(), push();


   switch ( i ) {
 
     case NUMBER:
                push( atof(s) );
		break;
     case ADD:
                push( pop() + pop() );
		break;
     case SUBTRACT:
                op2 = pop();
                push( pop() - op2 );
		break;
     case MULTIPLY:
                push( pop() * pop() );
 		break;
     case DIVIDE:
                op2 = pop();
                if ( op2 != 0.0 ) 
                    push( pop() / op2 );
                else {
		    push( op2 );
                    printf("? zero divisor\n");
		}
		break;
     case RECIPROCAL:
                op = pop();
		if ( op != 0.0 )
		    push( 1.0 / op );
                else {
		    push( op );
                    printf("? zero divisor\n");
		}
		break;
     case CHANGESIGN: 
               	push( pop() * -1.0 );
		break;
     case DEGREE:
                degrad = RADPERDEG;
		break;
     case RADIAN:
                degrad = 1.0;
		break;
     case PI:
		push( 3.1415926535897 );
		break;
     case ARCOSINE:
                op = pop();
                if ( fabs(op) <= 1.0 ) 
		   push( acos(op)  / degrad );
                else {
		   push( op );
                   printf("? |x| > 1 for acos\n");
		}
 		break;
     case ARCSINE:
                op = pop();
                if ( fabs(op) <= 1.0 ) 
		   push( asin( op ) / degrad );
		else {
		   push( op );
                   printf("? |x| > 1 for asin\n");
		}
 		break;
     case ARCTANGENT:
                push( atan( pop() ) / degrad );
		break;
     case COSINE:
                push( cos( pop() * degrad ) );
		break;
     case ETOTHEX:
                push( exp( pop() ) );
		break;
     case NATURALLOG:
		op = pop();
		if ( op > 0.0 ) 
                    push( log( op ) );
		else {
		   push( op );
                   printf("? x <= 0 for ln\n");
		}
		break;
     case BASE10LOG:
		op = pop();
		if ( op > 0.0 )
                   push( log10( op ) );
		else {
		   push( op );
                   printf("? x <= 0 for ln\n");
		}
		break;
     case POWER:
                op2 = pop();
                push( pow( pop(), op2 ) );
		break;
     case SINE:
                push( sin( pop() * degrad ) );
		break;
     case SQUAREROOT:
                op = pop();
                if( op  < 0.0 ) {
		   push( op );
                   printf("? x < 0 for sqrt\n");
		}
                else 
                    push( sqrt( op ) );
		break;
     case SQUARE:
		op = pop();
		push( op * op );
		break;
     case TANGENT:
                push( tan( pop() * degrad ) );
		break;
     case CLEAR:
		clear();
		break;
     case GIVEHELP:
                help();
		break;
     case UNKNOWN:
                printf("? unknown function \"%s\",\n  type \"?\" for help\n",s);
                break;
    };
    error_clear();
}



/* help -- print out help information */
help()
{
   int i;

   printf("\n\nthe available functions are:\n");
   for(i=0; i < NUMFUNCTIONS; i++)
      printf((i % 7) ? " %-5s" : "\n%-5s", funtable[i].text );
   printf("\n");
}


/*            s t a c k   r o u t i n e s       */

#define MAXVAL 127

int stack_ptr = 0;         /* index of top value on stack                    */
double stack_val[MAXVAL];  /* the value stack                                */
int stack_err = 0;	 /* non zero on stack error			  */

/* push -- pushes f on value stack */
double push( f ) 
double f;
{ 
	if ( stack_err ) return( 0 );
	if ( stack_ptr < MAXVAL ) 
 		return( stack_val[ ++stack_ptr ] = f );
	else {
		printf("? stack is full\n");
		++stack_err;
		return( 0 );
        }
}

/* pop -- pops top value from stack */
double pop()
{
	if ( stack_err ) return( 0 );
	if ( stack_ptr > 0 )               
		return( stack_val[ stack_ptr-- ] );
	else { 
		printf("? stack is empty\n"); 
		++stack_err;
		return( 0 );              
        }
}


/* clear -- clear the stack */
clear()
{
	stack_ptr = 0;
}


/* print_top -- print the top of the stack */
print_top()
{
	if ( stack_ptr <= 0 ) 
		printf("                ");   /* print blanks if it is empty */
	else
		printf( "%16g" ,stack_val[ stack_ptr ] );
}


/* error_clear -- clear errors on stack access */
error_clear()
{
	stack_err = 0;
}