Program to convert an Infix expression to Prefix form.

#include <"stdio.h">
#include <"conio.h">
#include <"string.h">
#include <"ctype.h">

#define MAX 50

struct infix
{
char target[MAX] ;
char stack[MAX] ;
char *s, *t ;
int top, l ;
} ;

void initinfix ( struct infix * ) ;
void setexpr ( struct infix *, char * ) ;
void push ( struct infix *, char ) ;
char pop ( struct infix * ) ;
void convert ( struct infix * ) ;
int priority ( char c ) ;
void show ( struct infix ) ;

void main( )
{
struct infix q ;
char expr[MAX] ;

clrscr( ) ;

initinfix ( &q ) ;

printf ( "\nEnter an expression in infix form: " ) ;
gets ( expr ) ;

setexpr ( &q, expr ) ;
convert ( &q ) ;

printf ( "The Prefix expression is: " ) ;
show ( q ) ;

getch( ) ;
}

/* initializes elements of structure variable */
void initinfix ( struct infix *pq )
{
pq -> top = -1 ;
strcpy ( pq -> target, "" ) ;
strcpy ( pq -> stack, "" ) ;
pq -> l = 0 ;
}

/* reverses the given expression */
void setexpr ( struct infix *pq, char *str )
{
pq -> s = str ;
strrev ( pq -> s ) ;
pq -> l = strlen ( pq -> s ) ;
*( pq -> target + pq -> l ) = '\0' ;
pq -> t = pq -> target + ( pq -> l - 1 ) ;
}

/* adds operator to the stack */
void push ( struct infix *pq, char c )
{
if ( pq -> top == MAX - 1 )
printf ( "\nStack is full.\n" ) ;
else
{
pq -> top++ ;
pq -> stack[pq -> top] = c ;
}
}

/* pops an operator from the stack */
char pop ( struct infix *pq )
{
if ( pq -> top == -1 )
{
printf ( "Stack is empty\n" ) ;
return -1 ;
}
else
{
char item = pq -> stack[pq -> top] ;
pq -> top-- ;
return item ;
}
}

/* converts the infix expr. to prefix form */
void convert ( struct infix *pq )
{
char opr ;

while ( *( pq -> s ) )
{
if ( *( pq -> s ) == ' ' || *( pq -> s ) == '\t' )
{
pq -> s++ ;
continue ;
}

if ( isdigit ( *( pq -> s ) ) || isalpha ( *( pq -> s ) ) )
{
while ( isdigit ( *( pq -> s ) ) || isalpha ( *( pq -> s ) ) )
{
*( pq -> t ) = *( pq -> s ) ;
pq -> s++ ;
pq -> t-- ;
}
}

if ( *( pq -> s ) == ')' )
{
push ( pq, *( pq -> s ) ) ;
pq -> s++ ;
}

if ( *( pq -> s ) == '*' || *( pq -> s ) == '+' || *( pq -> s ) == '/' ||
*( pq -> s ) == '%' || *( pq -> s ) == '-' || *( pq -> s ) == '$' )
{
if ( pq -> top != -1 )
{
opr = pop ( pq ) ;

while ( priority ( opr ) > priority ( *( pq -> s ) ) )
{
*( pq -> t ) = opr ;
pq -> t-- ;
opr = pop ( pq ) ;
}
push ( pq, opr ) ;
push ( pq, *( pq -> s ) ) ;
}
else
push ( pq, *( pq -> s ) ) ;
pq -> s++ ;
}

if ( *( pq -> s ) == '(' )
{
opr = pop ( pq ) ;
while ( opr != ')' )
{
*( pq -> t ) = opr ;
pq -> t-- ;
opr = pop ( pq ) ;
}
pq -> s++ ;
}
}

while ( pq -> top != -1 )
{
opr = pop ( pq ) ;
*( pq -> t ) = opr ;
pq -> t-- ;
}
pq -> t++ ;
}

/* returns the priotity of the operator */
int priority ( char c )
{
if ( c == '$' )
return 3 ;
if ( c == '*' || c == '/' || c == '%' )
return 2 ;
else
{
if ( c == '+' || c == '-' )
return 1 ;
else
return 0 ;
}
}

/* displays the prefix form of given expr. */
void show ( struct infix pq )
{
while ( *( pq.t ) )
{
printf ( " %c", *( pq.t ) ) ;
pq.t++ ;
}
}