A program that will convert a format from postfix to prefix in C language.
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Program Listing
postfix.c
#include <stdio.h>
#include <conio.h>
#include <string.h>
#define MAX 50
struct postfix
{
char stack[MAX][MAX], target[MAX] ;
char temp1[2], temp2[2] ;
char str1[MAX], str2[MAX], str3[MAX] ;
int i, top ;
} ;
void initpostfix ( struct postfix * ) ;
void setexpr ( struct postfix *, char * ) ;
void push ( struct postfix *, char * ) ;
void pop ( struct postfix *, char * ) ;
void convert ( struct postfix * ) ;
void show ( struct postfix ) ;
void main( )
{
struct postfix q ;
char expr[MAX] ;
clrscr( ) ;
initpostfix ( &q ) ;
printf ( "\nEnter an expression in postfix form: " ) ;
gets ( expr ) ;
setexpr ( &q, expr ) ;
convert ( &q ) ;
printf ( "\nThe Prefix expression is: " ) ;
show ( q ) ;
getch( ) ;
}
/* initializes the elements of the structure */
void initpostfix ( struct postfix *p )
{
p -> i = 0 ;
p -> top = -1 ;
strcpy ( p -> target, "" ) ;
}
/* copies given expr. to target string */
void setexpr ( struct postfix *p, char *c )
{
strcpy ( p -> target, c ) ;
}
/* adds an operator to the stack */
void push ( struct postfix *p, char *str )
{
if ( p -> top == MAX - 1 )
printf ( "\nStack is full." ) ;
else
{
p -> top++ ;
strcpy ( p -> stack[p -> top], str ) ;
}
}
/* pops an element from the stack */
void pop ( struct postfix *p, char *a )
{
if ( p -> top == -1 )
printf ( "\nStack is empty." ) ;
else
{
strcpy ( a, p -> stack[p -> top] ) ;
p -> top-- ;
}
}
/* converts given expr. to prefix form */
void convert ( struct postfix *p )
{
while ( p -> target[p -> i] != '\0' )
{
/* skip whitespace, if any */
if ( p -> target[p -> i] == ' ')
p -> i++ ;
if( p -> target[p -> i] == '%' || p -> target[p -> i] == '*' ||
p -> target[p -> i] == '-' || p -> target[p -> i] == '+' ||
p -> target[p -> i] == '/' || p -> target[p -> i] == '$' )
{
pop ( p, p -> str2 ) ;
pop ( p, p -> str3 ) ;
p -> temp1[0] = p -> target[ p -> i] ;
p -> temp1[1] = '\0' ;
strcpy ( p -> str1, p -> temp1 ) ;
strcat ( p -> str1, p -> str3 ) ;
strcat ( p -> str1, p -> str2 ) ;
push ( p, p -> str1 ) ;
}
else
{
p -> temp1[0] = p -> target[p -> i] ;
p -> temp1[1] = '\0' ;
strcpy ( p -> temp2, p -> temp1 ) ;
push ( p, p -> temp2 ) ;
}
p -> i++ ;
}
}
/* displays the prefix form of expr. */
void show ( struct postfix p )
{
char *temp = p.stack[0] ;
while ( *temp )
{
printf ( "%c ", *temp ) ;
temp++ ;
}
}
