带有if语句的字符串 这个程序只是一个有趣的项目，我想我会用C++来练习，它只不过是计算一次公路旅行的价格。

所有这些编码都是使用CODE:：BLOCKS键入和编译的，在启动程序时没有错误，if语句的反应就像在函数中列出的第一条if语句是真的一样，无论我输入什么。我确信编码是正确的，但我觉得我可能遗漏了什么

---

main.cpp

#include <iostream>
#include "Answers.h"

using namespace std;

int main()
{
Answers roadTrip;
roadTrip.intro();
return 0;
}

#include "Answers.h"
#include <iostream>

using namespace std;



Answers::Answers(){

}

Answers::intro(){
cout << "Hello and welcome to the trip calculator, how may I be of service?" << endl;
cout << "As of now,I am only able to EXIT from this program, or calculate a simple road trip" << endl;
cout << "Which would you like to do, exit, or calculate?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Calculate"||"calculate"||"Road trip"||"road trip"){
    dataInput();
}

if(answer=="Exit"||"exit"||"Escape"||"escape"||"Quit"||"quit"){
    return 0;
}
}

void Answers::dataInput(){
cout << "How many miles to the gallon does your car get?: ";
cin >> mpg; cout << "\n";

cout << "What are the average gas prices along your trip?: ";
cin >> gasPrices; cout << "\n";

cout << "How far away in miles is your destination?: ";
cin >> distance; cout << "\n" << endl;

doubleChecking();
}

void Answers::doubleChecking(){
cout << "Alright, so if I'm not mistaken:" << endl;
cout << "Your car gets " << mpg << " miles per gallon" << endl;
cout << "The average price of gas along the trip is $" << gasPrices << endl;
cout << "And your destination is approximately " << distance << " miles away\n" << endl;

cout << "Is this information correct?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Yes"||"yes"||"Y"||"y"){
    calculations();
}

if(answer=="No"||"no"||"N"||"n"){
    incDataInput();
}
}

void Answers::incDataInput(){
cout << "Oh? Well that wont be a problem" << endl;
cout << "So what information was entered incorrectly?" << endl;
cout << "Was it MPG, gas prices, or distance?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="MPG"||"mpg"||"Miles per gallon"||"miles per gallon"){
    incDataMpg();
}

if(answer=="Gas"||"gas"||"Gas prices"||"gas prices"){
    incDataGasPrices();
}

if(answer=="Distance"||"distance"){
    incDataDistance();
}
}

void Answers::incDataMpg(){
cout << "So tell me again, how many miles to the gallon does your car get?: ";
cin >> mpg; cout << "\n";

cout << "Your car gets " << mpg << " miles per gallon, is that correct?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Yes"||"yes"||"Y"||"y"){
    doubleChecking();
}

if(answer=="No"||"no"||"N"||"n"){
    incDataMpg();
}
}

void Answers::incDataGasPrices(){
cout << "So tell me again, what's the average price of gas along your road trip?: ";
cin >> gasPrices; cout << "\n";

cout << "The average price of gas along the trip is $" << gasPrices << ", is that correct?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Yes"||"yes"||"Y"||"y"){
    doubleChecking();
}

if(answer=="No"||"no"||"N"||"n"){
    incDataGasPrices();
}
}

void Answers::incDataDistance(){
cout << "So tell me again, approximately how many miles away is your destination?: ";
cin >> distance; cout << "\n";

cout << "Your destination is approximately " << distance << " miles away, is that correct?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Yes"||"yes"||"Y"||"y"){
    doubleChecking();
}

if(answer=="No"||"no"||"N"||"n"){
    incDataDistance();
}
}

Answers::calculations(){
int extraMoney = distance*.05;
int total = (distance/mpg)*gasPrices;
cout << "So according to my calculations, in order for your car to travel the " << distance << " miles needed" << endl;
cout << "It'd be smart to have at least $" << total << "for your overall road trip, but if I were you, I'd consider" << endl;
cout << "making it $" << total+extraMoney << "Leaving you " << extraMoney << " extra dollars to spend on food an necessities for the trip" << endl;
cout << "I think it should cover " << distance << " miles"; cout << "\n" << endl;

goodbye();
}

Answers::goodbye(){
cout << "Well, hasn't this been fun? We should do it again sometime. I hope I was able to help you out" << endl;
cout << "That being said, would you like me exit this program?: ";
cin >> answer;

if("Yes"||"yes"||"Y"||"y"){
    return 0;
}

if("No"||"no"||"N"||"n"){
    finalGoodbye();
}
}

Answers::finalGoodbye(){
cout << "Listen, it's nice that you wanna keep me around and all, but literally all I can do is calculate road trips" << endl;
cout << "... and quit out of myself" << endl;

cout << "That being said, would you like me exit this program?: ";
cin >> answer;

if("Yes"||"yes"||"Y"||"y"){
    return 0;
}

if("No"||"no"||"N"||"n"){
    finalGoodbye();
}
}

#包括
#包括“答案.h”
使用名称空间std；
int main（）
{
回答路途问题；
roadTrip.intro（）；
返回0；
}

---

答案。h

#ifndef ANSWERS_H
#define ANSWERS_H
#include <iostream>

using namespace std;


class Answers
{
public:
    Answers();
    int intro();
    void dataInput();
    void doubleChecking();
    void incDataInput();

    void incDataMpg();
    void incDataGasPrices();
    void incDataDistance();
    int goodbye();
    int finalGoodbye();

    int calculations();

    int mpg;
    float gasPrices;
    int distance;

    string answer;
    string incValue;


protected:

private:
};

#endif // ANSWERS_H

#如果没有答案#
#定义答案
#包括
使用名称空间std；
课堂答案
{
公众：
答案（）；
int intro（）；
void dataInput（）；
void doubleChecking（）；
void incDataInput（）；
void incDataMpg（）；
无效价格（）；
void incDataDistance（）；
int再见（）；
int finalGoodbye（）；
int计算（）；
int mpg；
浮米；
整数距离；
字符串回答；
字符串值；
受保护的：
私人：
};
#endif//ANSWERS\u H

---

答案。cpp

#include <iostream>
#include "Answers.h"

using namespace std;

int main()
{
Answers roadTrip;
roadTrip.intro();
return 0;
}

#include "Answers.h"
#include <iostream>

using namespace std;



Answers::Answers(){

}

Answers::intro(){
cout << "Hello and welcome to the trip calculator, how may I be of service?" << endl;
cout << "As of now,I am only able to EXIT from this program, or calculate a simple road trip" << endl;
cout << "Which would you like to do, exit, or calculate?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Calculate"||"calculate"||"Road trip"||"road trip"){
    dataInput();
}

if(answer=="Exit"||"exit"||"Escape"||"escape"||"Quit"||"quit"){
    return 0;
}
}

void Answers::dataInput(){
cout << "How many miles to the gallon does your car get?: ";
cin >> mpg; cout << "\n";

cout << "What are the average gas prices along your trip?: ";
cin >> gasPrices; cout << "\n";

cout << "How far away in miles is your destination?: ";
cin >> distance; cout << "\n" << endl;

doubleChecking();
}

void Answers::doubleChecking(){
cout << "Alright, so if I'm not mistaken:" << endl;
cout << "Your car gets " << mpg << " miles per gallon" << endl;
cout << "The average price of gas along the trip is $" << gasPrices << endl;
cout << "And your destination is approximately " << distance << " miles away\n" << endl;

cout << "Is this information correct?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Yes"||"yes"||"Y"||"y"){
    calculations();
}

if(answer=="No"||"no"||"N"||"n"){
    incDataInput();
}
}

void Answers::incDataInput(){
cout << "Oh? Well that wont be a problem" << endl;
cout << "So what information was entered incorrectly?" << endl;
cout << "Was it MPG, gas prices, or distance?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="MPG"||"mpg"||"Miles per gallon"||"miles per gallon"){
    incDataMpg();
}

if(answer=="Gas"||"gas"||"Gas prices"||"gas prices"){
    incDataGasPrices();
}

if(answer=="Distance"||"distance"){
    incDataDistance();
}
}

void Answers::incDataMpg(){
cout << "So tell me again, how many miles to the gallon does your car get?: ";
cin >> mpg; cout << "\n";

cout << "Your car gets " << mpg << " miles per gallon, is that correct?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Yes"||"yes"||"Y"||"y"){
    doubleChecking();
}

if(answer=="No"||"no"||"N"||"n"){
    incDataMpg();
}
}

void Answers::incDataGasPrices(){
cout << "So tell me again, what's the average price of gas along your road trip?: ";
cin >> gasPrices; cout << "\n";

cout << "The average price of gas along the trip is $" << gasPrices << ", is that correct?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Yes"||"yes"||"Y"||"y"){
    doubleChecking();
}

if(answer=="No"||"no"||"N"||"n"){
    incDataGasPrices();
}
}

void Answers::incDataDistance(){
cout << "So tell me again, approximately how many miles away is your destination?: ";
cin >> distance; cout << "\n";

cout << "Your destination is approximately " << distance << " miles away, is that correct?: ";
cin >> answer; cout << "\n" << endl;

if(answer=="Yes"||"yes"||"Y"||"y"){
    doubleChecking();
}

if(answer=="No"||"no"||"N"||"n"){
    incDataDistance();
}
}

Answers::calculations(){
int extraMoney = distance*.05;
int total = (distance/mpg)*gasPrices;
cout << "So according to my calculations, in order for your car to travel the " << distance << " miles needed" << endl;
cout << "It'd be smart to have at least $" << total << "for your overall road trip, but if I were you, I'd consider" << endl;
cout << "making it $" << total+extraMoney << "Leaving you " << extraMoney << " extra dollars to spend on food an necessities for the trip" << endl;
cout << "I think it should cover " << distance << " miles"; cout << "\n" << endl;

goodbye();
}

Answers::goodbye(){
cout << "Well, hasn't this been fun? We should do it again sometime. I hope I was able to help you out" << endl;
cout << "That being said, would you like me exit this program?: ";
cin >> answer;

if("Yes"||"yes"||"Y"||"y"){
    return 0;
}

if("No"||"no"||"N"||"n"){
    finalGoodbye();
}
}

Answers::finalGoodbye(){
cout << "Listen, it's nice that you wanna keep me around and all, but literally all I can do is calculate road trips" << endl;
cout << "... and quit out of myself" << endl;

cout << "That being said, would you like me exit this program?: ";
cin >> answer;

if("Yes"||"yes"||"Y"||"y"){
    return 0;
}

if("No"||"no"||"N"||"n"){
    finalGoodbye();
}
}

#包括“Answers.h”
#包括
使用名称空间std；
答案：：答案（）{
}
答案：：简介（）{
不能必须这样
如果（回答==“每加仑汽油”|回答==“每加仑汽油”|回答==“每加仑英里数”|回答==“每加仑英里数”）

对于所有其他的ifs也是如此。否则，您将检查指向“mpg”的指针和所有其他字符串指针是否不等于0。因此，该条件的计算结果始终为true。
answer==“Yes”| |“Yes”|“Y”|“Y”
不会像您认为的那样。基本上，它将始终返回true，因为在“or”中您刚刚输入的
const char*
值总是被认为是真的

为了使此逻辑正常工作，您必须将其替换为
answer==“Yes”| | answer==“Yes”| answer==“Y”| | answer==“Y”
。更好的是，
toupper（answer.c|str（）[0]）=='Y'
。这同样适用于其他字符串比较
if
语句。
类似于
something==somethingelse | | | somethingthird | | |..
的表达式不会执行您认为它会执行的操作，因为您的比较在编译器看来是这样的

(answer=="Calculate")||"calculate"||"Road trip"||"road trip"

换句话说，它将
answer
与字符串
“Calculate”
进行比较，并将逻辑or运算符的结果与文字字符串常量一起使用，文字字符串常量将是非空指针，因此始终为true，这意味着比较将始终为true

相反，你需要这样做

answer=="Calculate" || answer=="calculate" || answer=="Road trip" || answer=="road trip"

if语句的反应就像第一个if语句中列出了一个函数[sic；例如，这个函数：

]无论我输入什么都是正确的。我确信编码是正确的，但我觉得我可能遗漏了什么

那么是哪一个呢？代码正确与否

事实上，代码与您的意图不符。您似乎想要：

if(answer=="Calculate"||answer=="calculate"||answer=="Road trip"||answer=="road trip"){

您最初的问题是
answer==“Calculate”
是否为真（可能是真的，也可能不是真的），或者
“Calculate”
是否为真（始终是真的），或者两个其他条件也是真的。
使代码编译。
首先，虽然代码编译的G+编译器没有选项，但它没有用VisualC++来编译，当使用选项<代码> -F NO-MS-ExtExt/Cuth>禁用微软代码中使用的语言扩展：
时，它不使用G++编译。
[C:\my\forums\so\169]
> doskey /macros
g++=g++ -std=c++14 -pedantic-errors -Wall -Wextra -fexec-charset=cp1252 $*

[C:\my\forums\so\169]
> g++ main.cpp Answers.cpp -fno-ms-extensions
Answers.cpp:12:16: error: ISO C++ forbids declaration of 'intro' with no type [-fpermissive]
 Answers::intro() {
                ^
Answers.cpp:137:23: error: ISO C++ forbids declaration of 'calculations' with no type [-fpermissive]
 Answers::calculations() {
                       ^
Answers.cpp: In member function 'int Answers::calculations()':
Answers.cpp:147:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^
Answers.cpp: At global scope:
Answers.cpp:149:18: error: ISO C++ forbids declaration of 'goodbye' with no type [-fpermissive]
 Answers::goodbye() {
                  ^
Answers.cpp:163:23: error: ISO C++ forbids declaration of 'finalGoodbye' with no type [-fpermissive]
 Answers::finalGoodbye() {
                       ^
Answers.cpp: In member function 'int Answers::intro()':
Answers.cpp:26:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

[C:\my\forums\so\169]
> _
…始终执行
dataInput（）
，而不考虑
answer
的值

是，C++操作符被设计成支持这样的表达式：
if( x1 != x2 || y1 != y2 )  // The points are different

if( contains( answer, Strings{ "Calculate", "calculate", "Road trip", "road trip" } ) )
{
    // Whatever.
}

if( answer=="Calculate" ||
    answer == "calculate" ||
    answer == "Road trip" ||
    answer == "road trip"
    )
{
    // Whatever.
}

< > >在<<强>布尔运算符< /St> > < <代码> > <代码>之前，对关系运算符如<代码>！>代码>＝代码>和代码>进行评估。即关系运算符具有更高的<强>优先< /强>。您可以在所有地方找到包括C++运算符的优先表，包括
。
与Python不同，运算符链没有特殊的规则，因此
answer
检查代码被解释为

if( ((((answer=="Calculate") || "calculate") || "Road trip") || "road trip") ) {
    dataInput();
}

这是因为在任何可能的上下文中，一个简单的字符串文字衰减为指向第一个
char
的指针，而在这里，这是可能的，因为一个指针进一步衰减（隐式转换）为布尔
false
和
true

空指针产生
false
，任何其他指针产生
true

由于文本中的指针不是空指针，因此上述内容相当于

if( ((((answer=="Calculate") || true) || true) || true) ) {
    dataInput();
}

现在您可以看到，这是如何完全独立于
answer
的值的

如何表达“s是这些字符串中的一个吗…？”。
意识到这是为了检查给定字符串是否在给定字符串集中，可以编写一个名为

包含的函数，并像这样使用它：

if( x1 != x2 || y1 != y2 )  // The points are different

if( contains( answer, Strings{ "Calculate", "calculate", "Road trip", "road trip" } ) )
{
    // Whatever.
}

if( answer=="Calculate" ||
    answer == "calculate" ||
    answer == "Road trip" ||
    answer == "road trip"
    )
{
    // Whatever.
}

定义这样一个函数很简单：

#include <string>
#include <unordered_set>
using namespace std;

using Strings = unordered_set<string>;

auto contains( string const& item, Strings const& set )
    -> bool
{ return (set.count( item ) > 0); }

---

但这很尴尬，冗长，而且不可伸缩。

你必须写：如果（答案==“计算”| |答案==“计算”| |答案==“公路旅行”）…）否则会有一个比较（答案==“计算”）还有很多const char*值永远不会计算为false…谢谢你的反馈，我很感激！我要哭了…我不敢相信这么简单，非常感谢你，你是个救命恩人。非常感谢你的反馈，你让它更容易理解！感谢你的反馈！感谢你的反馈！我感谢你的反馈！我感谢inpuT