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Program to convert an Infix expression to Prefix form

Posted By: Waldemar Fischer     Category: C Programming     Views: 6328

Program to convert an Infix expression to Prefix form.

Code for Program to convert an Infix expression to Prefix form in C Programming

#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 ; elsereturn 0 ; } } /* displays the prefix form of given expr. */
void show ( struct infix pq ) { while ( *( pq.t ) ) { printf ( " %c", *( pq.t ) ) ; pq.t++ ; } }
  
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Waldemar Fischer
Waldemar Fischer author of Program to convert an Infix expression to Prefix form is from Frankfurt, Germany.
 
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