Here is a program that will convert value in infix format into it's postfix format written in C language.
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Program Listing
convert.c
#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 ;
} ;
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 ) ;
void show ( struct infix ) ;
void main( )
{
struct infix p ;
char expr[MAX] ;
initinfix ( &p ) ;
clrscr( ) ;
printf ( "\nEnter an expression in infix form: " ) ;
gets ( expr ) ;
setexpr ( &p, expr ) ;
convert ( &p ) ;
printf ( "\nThe postfix expression is: " ) ;
show ( p ) ;
getch( ) ;
}
/* initializes structure elements */
void initinfix ( struct infix *p )
{
p -> top = -1 ;
strcpy ( p -> target, "" ) ;
strcpy ( p -> stack, "" ) ;
p -> t = p -> target ;
p -> s = "" ;
}
/* sets s to point to given expr. */
void setexpr ( struct infix *p, char *str )
{
p -> s = str ;
}
/* adds an operator to the stack */
void push ( struct infix *p, char c )
{
if ( p -> top == MAX )
printf ( "\nStack is full.\n" ) ;
else
{
p -> top++ ;
p -> stack[p -> top] = c ;
}
}
/* pops an operator from the stack */
char pop ( struct infix *p )
{
if ( p -> top == -1 )
{
printf ( "\nStack is empty.\n" ) ;
return -1 ;
}
else
{
char item = p -> stack[p -> top] ;
p -> top-- ;
return item ;
}
}
/* converts the given expr. from infix to postfix form */
void convert ( struct infix *p )
{
char opr ;
while ( *( p -> s ) )
{
if ( *( p -> s ) == ' ' || *( p -> s ) == '\t' )
{
p -> s++ ;
continue ;
}
if ( isdigit ( *( p -> s ) ) || isalpha ( *( p -> s ) ) )
{
while ( isdigit ( *( p -> s ) ) || isalpha ( *( p -> s ) ) )
{
*( p -> t ) = *( p -> s ) ;
p -> s++ ;
p -> t++ ;
}
}
if ( *( p -> s ) == '(' )
{
push ( p, *( p -> s ) ) ;
p -> s++ ;
}
if ( *( p -> s ) == '*' || *( p -> s ) == '+' || *( p -> s ) == '/' || *( p -> s ) == '%' || *( p -> s ) == '-' || *( p -> s ) == '$' )
{
if ( p -> top != -1 )
{
opr = pop ( p ) ;
while ( priority ( opr ) >= priority ( *( p -> s ) ) )
{
*( p -> t ) = opr ;
p -> t++ ;
opr = pop ( p ) ;
}
push ( p, opr ) ;
push ( p, *( p -> s ) ) ;
}
else
push ( p, *( p -> s ) ) ;
p -> s++ ;
}
if ( *( p -> s ) == ')' )
{
opr = pop ( p ) ;
while ( ( opr ) != '(' )
{
*( p -> t ) = opr ;
p -> t++ ;
opr = pop ( p ) ;
}
p -> s++ ;
}
}
while ( p -> top != -1 )
{
char opr = pop ( p ) ;
*( p -> t ) = opr ;
p -> t++ ;
}
*( p -> t ) = '\0' ;
}
/* returns the priority of an operator */
int priority ( char c )
{
if ( c == '$' )
return 3 ;
if ( c == '*' || c == '/' || c == '%' )
return 2 ;
else
{
if ( c == '+' || c == '-' )
return 1 ;
else
return 0 ;
}
}
/* displays the postfix form of given expr. */
void show ( struct infix p )
{
printf ( " %s", p.target ) ;
}
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