C++ if 语句中的多个条件
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Multiple conditions in C++ if statement
提问by user1200066
I am very new to the concept of programming in C++. I am wanting to have a multi condition if statement using the || (or) and the && (and) in one statement. When I ask my college professor about it. She told it was possible and then insulted my limited knowledge on the subject. All examples I have access to show a multi && statement and only one showing the ||. It does not show them being used together. I would like to learn how to get the line working. I will attach the code I have. The problem area is the last bit of coding.
我对 C++ 编程的概念很陌生。我想要一个使用 || 的多条件 if 语句 (or) 和 && (and) 在一个语句中。当我问我的大学教授这件事时。她说这是可能的，然后侮辱了我对这个主题的有限知识。我有权显示多&& 语句的所有示例，只有一个显示||。它没有显示它们一起使用。我想学习如何让线路工作。我将附上我拥有的代码。问题区域是编码的最后一点。
# include <iostream>
# include <cstring>
using namespace std;
main()
{
const int maximumHours = 774;
char customerPackage;
double hoursUsed = 0,
packageA = 9.95,
packageB = 14.95,
packageC = 19.95,
overPackageA = 2.00,
overPackageB = 1.00,
overTime = 0,
amountDue = 0,
excessCharged = 0;
cout << "Please enter the customer's package: ";
cin >> customerPackage;
switch (customerPackage)
{
case 'a' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'A' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'b' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'B' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'c' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'C' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
default: cout << "Error."
<< " Please enter the customer's purchased package: ";
cin >> customerPackage;
}
if ( customerPackage ='a' || customerPackage ='A' && hoursUsed >= 10)
amountDue = packageA;
else
overTime = packageA - hoursUsed;
excessCharged = overTime * overPackageA;
amountDue = packageA + excessCharged;
}
回答by Mark B
Your problem is that &&has higher precedence than ||so you need parens. As noted in a comment you also need to use ==instead of assignment (=):
你的问题是它的&&优先级高于||所以你需要括号。如评论中所述，您还需要使用==而不是赋值 ( =)：
if ( (customerPackage =='a' || customerPackage =='A') && hoursUsed >= 10)
if ( (customerPackage =='a' || customerPackage =='A') && hoursUsed >= 10)
回答by Jerry Coffin
Others have already helped you with the problem you've noticed. I'll start with a separate problem you apparently haven't noticed (yet):
其他人已经帮助您解决了您注意到的问题。我将从一个您显然还没有注意到（还）的单独问题开始：
else
overTime = packageA - hoursUsed;
excessCharged = overTime * overPackageA;
amountDue = packageA + excessCharged;
If you want all three of those statements controlled by the else, you need to enclose them in braces to create a compound statement:
如果您希望所有这三个语句都由 控制，则else需要将它们括在大括号中以创建复合语句：
else {
overTime = packagA - hoursUsed;
excessCharged = overTime * overPackageA;
amountDue = packageA + excessCharged;
}
As it stands right now, your code is really:
就目前而言，您的代码实际上是：
else
overTime = packageA - hoursUsed;
excessCharged = overTime * overPackageA;
amountDue = packageA + excessCharged;
I.e., the computations for excessChargedand amountDueare carried out regardlessof whether the condition in the ifstatement was true or false.
即，无论语句中的条件是真还是假excessCharged，amountDue都执行和的计算。if
I'd also note that your switchstatement doesn't really accomplish much:
我还要指出，您的switch声明并没有真正起到多大作用：
switch (customerPackage)
{
case 'a' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'A' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'b' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'B' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'c' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
case 'C' :
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
default: cout << "Error."
<< " Please enter the customer's purchased package: ";
In particular, you take exactly the same action for all the cases (except the default). You can simplify this a bit by using fall-through cases:
特别是，您对所有情况采取完全相同的操作（默认情况除外）。您可以通过使用失败案例来简化这一点：
switch (customerPackage) {
case 'a':
case 'A':
case 'b':
case 'B':
case 'c':
case 'C':
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
break;
default:
cout << "Error " /* ... */;
}
Alternatively, you might consider something like:
或者，您可以考虑以下内容：
static const char valid[] = "aAbBcC";
if (strchr(valid, userPackage)) {
cout << "Please enter the number of hours used: ";
cin >> hoursUsed;
}
else {
std::cout << "Error: Please enter the customer's purchased package";
std::cin >> userPackage;
}
Personally, however, I'd structure things a bit differently: first get one valid input, then get the next:
然而，就我个人而言，我的结构有点不同：首先获得一个有效输入，然后获得下一个：
do {
std::cout << "Please enter the customer's purchased package (a, b, or c): ";
std::cin >> userPackage;
} while (!strchr(valid, userPackage));
std::cout << "Please enter the number of hours used: ";
std::cin >> hoursUsed;
if (tolower(customerPackage == 'a') && hoursUsed >= 10)
// ...
回答by Rob?
if ( customerPackage ='a' || customerPackage ='A' && hoursUsed >= 10)
You are soclose to having the right answer. Let me give you two hints:
你是如此接近有正确的答案。让我给你两个提示：
The
=operator is not the same as the==operator.=is the assignment operator. It evaluates its right-hand-side and stores the result in the variable named on its left-hand-side. You want==, the equality operator. It tests to see if its right-hand side and its left-hand-side are equal.Use parenthesis
( ... )to enforce your order-of-evaluation intention. You clearly mean to say "If either customerPackage is 'a' or it is 'A', and also hoursUsed is sufficiently large, then ...".
该
=操作是不一样的==运营商。=是赋值运算符。它评估其右侧并将结果存储在其左侧命名的变量中。你想要==，相等运算符。它测试看它的右手边和它的左手边是否相等。使用括号
( ... )来强制执行您的评估顺序意图。您显然是想说“如果 customerPackage 是 'a' 或它是 'A'，并且 hoursUsed 足够大，那么......”。
Try this line:
试试这一行：
if ( (customerPackage == 'a' || customerPackage == 'A') && hoursUsed >= 10)
回答by Adam Liss
You can use parentheses to specify the order in which the boolean operators are executed. You probably want to evaluate the ||first, so you'd use:
您可以使用括号来指定布尔运算符的执行顺序。您可能想评估第||一个，因此您可以使用：
if ((customerPackage == 'a' || customerPackage == 'A') && hoursUsed >= 10)
The &&is normally evaluated first by default, because it has higher precedence, so your code is equivalent to this:
在&&通常由默认的第一个评价，因为它具有更高的优先级，让你的代码是相同的：
if (customerPackage == 'a' || (customerPackage == 'A' && hoursUsed >= 10))
Also, as noted in the comments, use ==for comparison and =for assignment.
此外，如评论中所述，==用于比较和=分配。
回答by Mr Lister
With the new problem you're having (in the other questionyou asked), you'll need some restructuring.
对于您遇到的新问题（在您提出的另一个问题中），您需要进行一些重组。
if ( (customerPackage == 'b' || customerPackage == 'B') && hoursUsed <= 20)
amountDue = packageB;
else
{
/* calculations */
}
is not correct, that should be
不正确，应该是
if ( customerPackage == 'b' || customerPackage == 'B')
{
if (hoursUsed <= 20)
{
amountDue = packageB;
}
else
{
/* calculations */
}
}
Otherwise the first statement will only be executed when package=B AND hour=20, otherwise the calculations will be done in all other cases, like when package is A, or C.
否则第一条语句只会在 package=B AND hour=20 时执行，否则将在所有其他情况下进行计算，例如 package 是 A 或 C 时。
Hope this helps!
希望这可以帮助！
