用字符串

仅供参考：

#include <stdio.h>
#include <string.h>
char s[80];
double d;
char c;
int main() {
    while (1) {
        printf("Input a double:");fflush(stdout);
        rewind(stdin);
        fgets(s,80,stdin);
        if (strlen(s)>20) {
            printf("Input too long(>20)!\n");
            continue;
        }
        if (2==sscanf(s,"%lf%c",&d,&c)) {
            if ('\n'==c) {
                printf("%lg is a double.\n",d);
                break;
            }
        }
        if ('\n'==s[strlen(s)-1]) s[strlen(s)-1]=0;
        printf("\"%s\" is NOT a double!\n",s);
    }
    return 0;
}