Infixtopostfix.c

#define SIZE 50
#include <ctype.h>
char s[SIZE];
int top=-1;

push(char elem)
{
    s[++top]=elem;
}

char pop()
{
    return(s[top--]);
}

int pr(char elem)
{
    switch(elem)
    {
    case '#': return 0;
    case '(': return 1;
    case '+':
    case '-': return 2;
    case '*':
    case '/': return 3;
    }
}

main()
{
    char infx[50],pofx[50],ch,elem;
    int i=0,k=0;
    printf("\n\nRead the Infix Expression ? ");
    scanf("%s",infx);
    push('#');
    while( (ch=infx[i++]) != '\0')
    {
        if( ch == '(') push(ch);
        else
            if(isalnum(ch)) pofx[k++]=ch;
            else
                if( ch == ')')
                {
                    while( s[top] != '(')
                        pofx[k++]=pop();
                    elem=pop();
                }
                else
                {
                    while( pr(s[top]) >= pr(ch) )
                        pofx[k++]=pop();
                    push(ch);
                }
    }
    while( s[top] != '#')
        pofx[k++]=pop();
    pofx[k]='\0';
    printf("\n\nGiven Infix Expn: %s  Postfix Expn: %s\n",infx,pofx);
}