Showing posts with label Postfix To Prefix Converter in C. Show all posts
Showing posts with label Postfix To Prefix Converter in C. Show all posts

Saturday, April 21, 2018

Postfix To Prefix Converter in C

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++ ;
}

}