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++ ;

}

}