C++所有乘法 = 零

当我为我的 comp var 输入任何数字时，它会输出 0。 我最初把它当作int尝试过，四处阅读看起来double就是我想要的。 我已经尝试过价格变量只是一个double而不是const double。 我必须根据订购的课程金额申请折扣，我只是错过了一些东西，任何帮助将不胜感激：

const double price = 99.0; // price per unit
double comp;   // computer variable
double nodiscount = (price * comp);
double discount20 = (price * 0.8) * comp;   // 10-19 20%
double discount30 = (price * 0.7) * comp;   // 20-49 30%
double discount40 = (price * 0.6) * comp;   // 50-99 40%
double discount50 = (price * 0.5) * comp;   // 100 or more 50%
int main()
{
cout << "How many units sold?" << endl;
cin >> comp;
if ( comp >= 0) {
if ( comp <= 9 ) {
cout << "Total cost of purchase is " << nodiscount  << endl;
} else if ( comp >= 10 && comp <= 19) {
cout << "The total cost with 20% discount is " << discount20 << endl;
} else if ( comp >= 20 && comp <= 49) {
cout << "The total cost with 30% discount is " << discount30 << endl;
} else if ( comp >= 50 && comp <= 99) {
cout << "The total cost with 40% discount is " << discount40 << endl;
} else if ( comp >= 100 ) {
cout << "The total cost with 50% discount is " << discount50 << endl;
}
} else {
cout << "you entered in invalid charechter pleas enter a number.";
}
}