C++ : The if statement not functioning correctly and proceeds to Invalid output

Question

刚开始学习 C++ 并尝试制作一个自动订购系统，但在继续使用它之前尝试运行时我遇到了这个问题:

#include <iostream>
using namespace std;

char acoustic, fender, hartwood, electric, gibson, ibanez, drums, pearl, 
roland, piano, casio;
char rolandpi, equip, string, headphone, amp, mixer, micro, tuner, pick, 
music, Yes, No;

int pay1 = 0, pay2 = 0;

int main()
{
    cout << "Welcome to the Music Shop" << endl <<endl;

    cout << "a. Acoustic Guitar" << endl;
    cout << "a.1 Fender Acoustic Guitar   - P6,900.00" << endl;
    cout << "a.2 Hartwood Acoustic Guitar - P6,300.00" << endl <<endl;

    cout << "b. Electric Guitar" << endl;
    cout << "b.1 Gibson Electric Guitar   -P8,500.00" << endl;
    cout << "b.2 Ibanez Electric Guitar   -P25,000.00" << endl <<endl;

    cout << "c. Drums" << endl;
    cout << "c.1 Pearl Drum Kits          -P27,000.00" << endl;
    cout << "c.2 Roland Electronic Drums  -P24,000.00" << endl <<endl;

    cout << "d. Piano" << endl;
    cout << "d.1 Casio Digital Piano      -P19,000.00" << endl;
    cout << "d.2 Roland Piano             -P120,000.00" << endl <<endl;

    cout << "e. Music Equipments" << endl;
    cout << "e.1 Guitar String            -P113.00" << endl;
    cout << "e.2 Headphones               -P1,600.00" << endl;
    cout << "e.3 Amplifier                -P2,800.00" << endl;
    cout << "e.4 Digital Mixer            -P4,750.00" << endl;
    cout << "e.4 Vocal Microphone         -P860.00" << endl;
    cout << "e.5 Guitar Tuner             -P537.00" << endl;
    cout << "e.6 Guitar Pick              -P360.00" << endl <<endl;

    cout << "Choose the music instrument or equipment you want to buy: ";
    cin >> music;

    switch (music)
    {
        case 'a':

        cout<< "Acoustic Guitar" << endl;
        cout<< "1. Fender Acoustic Guitar   - P6,900.00" << endl;
        cout<< "2. Hartwood Acoustic Guitar - P6,300.00" << endl <<endl;
        cout<<"Choose from the available Acoustic Guitars:";
        cin>>acoustic;

        if (acoustic == 1){
            cout<<"Enter your payment:";
            cin>>pay1;
            if (pay1=6900){
                cout<<"You have succesfully purchased Fender Acoustic Guitar"<<endl;
            }
                else if (pay1>6900){
                    pay1 -= 6900;
                    cout<<"Your change is:"<<pay1<<endl;
                }
                    else (pay1<6900);
                    {
                        cout<<"You do not have enough money"<<endl;
                    }
        }
        else if (acoustic == 2){
            cout<<"Enter your payment:";
            cin>>pay1;
            if (pay1=6300){
                cout<<"You have succesfully purchased Fender Acoustic Guitar"<<endl;
            }
                else if (pay1>6300){
                    pay1 -= 6300;
                    cout<<"Your change is:"<<pay1<<endl;
                }
                    else (pay1<6300);
                    {
                        cout<<"You do not have enough money"<<endl;
                    }
        }
        else {
            cout<<"Invalid"<<endl;
        }
    }
}

我已经检查了我的 if else 语句，但找不到问题所在。

从“从可用的原声吉他中选择”输入 1 或 2 后，它一直继续“无效”。

Answer 1

你读了一个char这里:

cout << "Choose from the available Acoustic Guitars:";
cin >> acoustic;

但随后您将其与 int 进行比较一行之后:

if (acoustic == 1) {

相反，这应该是:

if (acoustic == '1') {

还有这个:

if (pay1 = 6900) {

应该是:

if (pay1 == 6900) {

否则，无论输入什么，您都会得到不正确的输出，因为 pay1 = 6900套pay1至 6900并返回 6900 , 隐式转换为 true .

还有这一行:

else (pay1 < 6900);

需要改成

else if (pay1 < 6900)

因为否则(pay1 < 6900)不是if的条件声明，而是 (pay1 < 6900); else 中发生了什么case(只计算一个 bool 值并丢弃它)，导致以下 "You do not have enough money"要始终打印的消息。