calc.c

/*
 * calc.c : Simple command line calculator.
 * calc executes simple operations read from an input string.
 * The string can be supplied either as the first command line argument or by
 * calling "calc_parse" manually.
 *
 * EBNF for math:
 * <expression> ::= ( <expression> <operator> <expression> ) |
 *                  ( <number> <operator> <number> )
 * <operator> ::= "+" | "-" | "/" | "*"
 *
 */

#include <stdio.h>
#include <string.h>

// The macro _WIN32 must be defined when compiling for windows (gcc -D _WIN32 1).
#ifdef _WIN32
  #define SEP '\\'
#else
  #define SEP '/'
#endif


typedef enum { CALC_OK, CALC_ESYNTAX, CALC_EOPER } CalcError_t;


void calc_strerror( CalcError_t errno ) {
    switch (errno) {
        case CALC_ESYNTAX:
            return (void) fputs("CALC: Syntax Error", stderr);
        case CALC_EOPER:
            return (void) fputs("CALC: Invalid Operator", stderr);
    }
}

CalcError_t calc_parse(char *string, double *result) {
    int count, pos = -1;
    double res, a;
    char op;
    CalcError_t error = CALC_OK;


    count = sscanf(string, " %lf %c %lf%n", &res, &op, &a, &pos);

    if (count == EOF) return CALC_OK;
    if (count != 3) return CALC_ESYNTAX;

    do {
        switch (op) {
            case '+':
                res += a; break;
            case '-':
                res -= a; break;
            case '*':
                res *= a; break;
            case '/':
                res /= a; break;
            default:
                return CALC_EOPER;
        }
        string += pos;
        count = sscanf(string, " %c %lf%n", &op, &a, &pos);
    } while (count == 2);


    if (count != EOF) return CALC_ESYNTAX;
    *result = res;
    return CALC_OK;

}

int main(int argc, char *argv[]) {

    if (argc == 1) {
        char *name = strrchr(argv[0], SEP);
        name = name != NULL ? ++name : argv[0];
        fprintf(stderr, "Usage: %s expressions...\n"
                "Example: %s \"12.2 * 73 - 4.5\"\n", name, name);
        return 1;
    }

    double res = 0;
    CalcError_t error;

    error = calc_parse(argv[1], &res);

    if (error != CALC_OK) {
        calc_strerror( error );
        return 1;
    }

    printf("Result: %g", res);
    return 0;

}