Evaluate an arithmetic expression given as a string

Question

I am working on a project where I need to calculate the value of an arithmetic expression given as string.

So that\'s the way I chose to use is such that I run the strin

Answer 1

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

typedef struct exp {
    char op;
    char *term;
    struct exp *left;
    struct exp *right;
} Exp;

Exp *make_exp2(char *str){
    if(!str || !*str) return NULL;//*str == '\0' is format error.
    char *mul = strrchr(str, '*');
    char *div = strrchr(str, '/');
    Exp *node = malloc(sizeof(*node));
    if(mul == NULL && div == NULL){
        node->op = '\0';
        node->term = str;
        node->left = node->right = NULL;
        return node;
    }
    char *op;
    op = mul < div ? div : mul;
    node->op = *op;
    *op = '\0';
    node->left  = make_exp2(str );
    node->right = make_exp2(op+1);
    return node;
}

Exp *make_exp(char *str){
    if(!str || !*str) return NULL;//*str == '\0' is format error.
    char *minus = strrchr(str, '-');
    char *plus  = strrchr(str, '+');
    if(minus == NULL && plus == NULL)
        return make_exp2(str);
    char *op;
    Exp *node = malloc(sizeof(*node));
    op = minus < plus ? plus : minus;
    node->op = *op;
    *op = '\0';
    node->left  = make_exp(str );
    node->right = make_exp(op+1);
    return node;
}

#ifdef DEBUG

void print(Exp *exp, int level){
    int i;
    if(exp->op){
        for(i=0;i<level;++i)
            printf(" ");
        printf("%c\n", exp->op);
        for(i=0;i<level;++i)
            printf(" ");
        print(exp->right, level+1);
        printf("\n");
        for(i=0;i<level;++i)
            printf(" ");
        print(exp->left, level+1);
        printf("\n");
    } else {
        for(i=0;i<level;++i)
            printf(" ");
        printf("%s\n", exp->term);
    }
}

#endif

int main(void) {
    char str[] = "3+1-4*6-7";
    Exp *exp = make_exp(str);

#ifdef DEBUG
    print(exp, 0);
#endif
    //release exp
    return 0;
}