Homework task #2

prefix_notation.c

#include <stdio.h>
#include <stdlib.h>



void report_error(char *s)
{
    printf("syntax error: expected %s\n", s);
}

void proceed(char **s, char **r)
{
    **r = **s;
    (*s)++;
    (*r)++;
}

void parse_formula(char **s, char **r)
{
    switch(**s)
    {
    case 'X':
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':
        proceed(s, r);
        break;
    case '(':
    {
        proceed(s, r);
        char *old_r = (*r)++;
        parse_formula(s, r);
        switch(**s)
        {
        case '+':
        case '-':
        case '*':
            *old_r = **s;
            (*s)++;
            break;
        default:
            report_error("sign");
        }
        parse_formula(s, r);
        if(**s != ')')
            report_error("right paren");
        proceed(s, r);
        break;
    }
    default:
        report_error("formula");
    }

}

int main()
{
    char *s = malloc(90),
         *r = malloc(90),
         *old_r = r;
    gets(s);
    parse_formula(&s, &r);
    *r = 0;
    printf("%s\n", old_r);
    return 0;
}