30天挑戰目標是搭建一個簡單的command line系統，那要有個首頁讓user可以做選擇對不對？
今天要用這個來聊聊如何做一個簡單的介面，包含基本指令順便練習一下把function分出來。

物件導向就是分類大法:

• 函數function（裝功能的分類箱）
• 結構struct（裝variable的分類箱）

今天會用到function裝我們的menu。

（我是使用mac系統，所以有些指令可能與windows會有所不同）
（這篇沒有使用namespace std喔）

第一步：目錄Menu

有四種功能，看帳戶餘額、提款、存款、轉帳。

注意：這個menu是一個function唷！因為沒有retun value所以是void

當然也可以直接寫進int main裡，只是單純練習一下如何分類，關於function是什麼請搭直通車Day07函數&Day08函數參數

void menu(){
    std::cout << "******MENU******" << std::endl;
    std::cout << "1.Check deposit" << std::endl;
    std::cout << "2.Deposit" << std::endl;
    std::cout << "3.Withdraw" << std::endl;
    std::cout << "4.Transfer" << std::endl;
    std::cout << "5.Exit" << std::endl;
    std::cout << "6.Help" << std::endl;
    std::cout << "****************" << std::endl;
}

接下來在選項的方面，可以有幾種選擇，用之前講過的if...else，或是用switch。
switch把所有可能的情況都列出來，然後符合情況以後要記得加break，說明不用再繼續跑第二個switch了，
不然會繼續跑下去喔。
Switch用法如下:

switch (option) {    //option是我們之前的variable int option, 所以是數字
        case 1:   //所以case 1，就等於我們打 1 的時候會執行的情況
            std::cout <<"Balance: $"<< balance<< std::endl;
            break;  //停止switch不再往case 2跑
        case 2:      //case 2，就等於我們打 2 的時候會執行的情況
            std::cout << "Deposit: " << std::endl;
            double deposit;
            std::cin >> deposit;
            balance = balance + deposit;
            break;
        case 3:
            std::cout <<"Withdraw: $" << std::endl;
            double withdraw;
            std::cin >> withdraw;
            if(withdraw < balance)
                balance = balance - withdraw;
            else
                std::cout << "NOT enought money" << std::endl;
            break;
        case 4:{
            std::string transfto_name;
            double amount;
            std::cout <<"Transfer your money to whom?\n";
            std::cin >> transfto_name;
            std::cout <<"How much do you want to transfer? \n";
            std::cin >> amount;
            std::cout << "You transfered " << amount <<" to " << transfto_name<< std::endl;
            if(amount < balance)
                balance = balance - amount;
            else std::cout <<"Not enought money in you account to tranfer. \n";
            break;}
//        case 5:
//            std::cout <<"Exit now \n";
            break;
        case 6:
            std::cout << "Need help? Don't worry we're here 24/7!"<< std::endl;
            std::cout << "If you have trouble with our ATM service, please don't hesitate to contact us. Call:2003-449-229"<< std::endl;

很好很好，最後要做個exit功能，防止這個程式無限制循環跑下去對不對？
如果這樣直接放進main，也可以跑，但打完指令後會用無限重複回到menu頁面，如下所示：

    menu(); //顯示menu
    std::cout << "Type the option to proceed" << std::endl; 
    std::cin >> option;

為了要讓程式知道何時要結束顯示上面的東西，我們需要用到do....while，最終程式如下：

#include <iostream>
#include <unistd.h>
#include <cstdlib>
#include <curses.h>
#include <stdlib.h>

void menu(){
    std::cout << "******MENU******" << std::endl;
    std::cout << "1.Check deposit" << std::endl;
    std::cout << "2.Deposit" << std::endl;
    std::cout << "3.Withdraw" << std::endl;
    std::cout << "4.Transfer" << std::endl;
    std::cout << "5.Exit" << std::endl;
    std::cout << "6.Help" << std::endl;
    std::cout << "****************" << std::endl;

}


int main() {
    int option;
    double balance = 500;
    
    do {
    menu();
    std::cout << "Type the option to proceed" << std::endl;
    std::cin >> option;
    switch (option) {
        case 1:
            std::cout <<"Balance: $"<< balance<< std::endl;
            break;
        case 2:
            std::cout << "Deposit: " << std::endl;
            double deposit;
            std::cin >> deposit;
            balance = balance + deposit;
            break;
        case 3:
            std::cout <<"Withdraw: $" << std::endl;
            double withdraw;
            std::cin >> withdraw;
            if(withdraw < balance)
                balance = balance - withdraw;
            else
                std::cout << "NOT enought money" << std::endl;
            break;
        case 4:{
            std::string transfto_name;
            double amount;
            std::cout <<"Transfer your money to whom?\n";
            std::cin >> transfto_name;
            std::cout <<"How much do you want to transfer? \n";
            std::cin >> amount;
            std::cout << "You transfered " << amount <<" to " << transfto_name<< std::endl;
            if(amount < balance)
                balance = balance - amount;
            else std::cout <<"Not enought money in you account to tranfer. \n";
            break;}
//        case 5:
            std::cout << "Need help? Don't worry we're here 24/7!"<< std::endl;
            std::cout << "If you have trouble with our ATM service, please don't hesitate to contact us. Call:2003-449-229"<< std::endl;
            break;
        case 6:   
//            std::cout <<"Exit now \n";

    }
    }while (option != 6);   //當輸入的選項不是6的時候 重複跑上面的menu()、輸入選項。
                          //當輸入6(exit）時，停止上面的menu，main裡沒有東西可以跑成程式就結束了
    
    
}

do{menu(),回到選單}...while{輸入的數字option不是6}...

當輸入的選項不是6的時候 重複跑上面的menu()、輸入選項。
當輸入6(exit）時，停止上面的menu，main裡沒有東西可以跑成程式就結束了

Reference: CodeBeuaty(youtube), w3school, geeksforgeeks