我正在为算法使用双精度（浮点）变量，但它给了我这个错误：
[错误]类型为'double'和'const char [9]'的无效操作数到二进制'operator！='
那我应该使用哪个运算符？
# include <iostream>
# include <math.h>
using namespace std;
int main ()
{
double a, b, Spa, Tim, Acc, Vel1, Vel2, Velocity, Space, Acceleration;
cout << "Hello. This program may calculate velocity, space, time and acceleration.\n" << endl;
cout << "Please, insert what you want to calculate.\n" << endl;
cin >> a;
do
{
cout << "Incorrect value.\n" << endl;
cout << "Please, insert what you want to calculate.\n" << endl;
cin >> a;
}
while (a != "Velocity" | a != "Acceleration");
if (a == Velocity);
{
cout << "With space or acceleration value?\n" << endl;
cin >> b;
if (b == Space);
{
cout << "Insert space and time values.\n" << endl;
cin >> Spa >> Tim;
Vel1 = Spa / Tim;
cout << "The velocity is " << Vel1 << " km/h.\n" << endl;
}
else
{
if (b == Acceleration);
{
cout << "Insert acceleration and time values.\n" << endl;
cin >> Acc >> Tim;
Vel2 = Acc / Tim;
cout << "The velocity is " << Vel2 << " km/h.\n" << endl;
}
}
}
}
“普通老式数据”（POD）类型在C ++中，如double，int，char等不携带关于它们所代表的事物的任何语义信息; 或以另外一种方式，一些是只是一个数字。
如果您希望在数字中包含某种语义含义，例如，您想要一个数字来表示“真实”（温度，速度，方向，CPU周期等），那么您需要存储该数字在另一种变量中（astd::string会适合您的程序）
IE
std::string a, b;
double Spa, Tim, Acc, Vel1, Vel2, Velocity, Space, Acceleration;
您需要确保始终将字符串a与b字符串进行比较：
if (a == "Velocity")
{
和..
cout << "With space or acceleration value?\n" << endl;
cin >> b;
if (b == "Space")
{
另外，请注意不要;在if ()语句末尾添加分号。
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