程序代码：

#include <stdio.h>

int main()
{
    double sum = 0.0;    //数据的和
    double tmp;            //当前数据
    double flag;        //小数位标识
    char *p, *str = "a5.7dfg2.3.4df";        //测试字符串

    for (p = str; *p;)
    {
        while (*p && ('0' >= *p || '9' <= *p))
        {
            p++;
        }

        tmp = 0.0, flag = 0.1;
        while (*p && '.' != *p && '0' <= *p && '9' >= *p)
        {
            tmp *= 10, tmp += *p - '0';
            p++;
        }
        if (*p && '.' == *p)
        {
            p++;
            while (*p && '0' <= *p && '9' >= *p)
            {
                tmp += flag * (*p - '0');
                flag /= 10, p++;
            }
        }

        sum += tmp;
    }
    printf("%lf", sum);
    return 0;
}