Why is my do/while loop in c++ not allowing me to input 'amount' more than once?

I am attempting to develop a vending machine in C++ in which the user can insert as many 50 cent, 20 cent, and 10 cent coins as they would like to then move on to the ‘purchase’ of a product.

So far, my primitive code runs smoothly; the issue I am currently facing is that I can only ‘insert’ coins into the vending machine once, even though I think the ‘while’ condition in the ‘do/while’ statement is being executed.

Below you can find my code:

`

#include <iostream>
using namespace std; 

int main() {
    int productid;
    string order, finished;
    int amount;
        cout << "Welcome to the coffee machine! We have a variety of beverages we can offer. \n Here is our selection: \n 1) Coffee- 50 cents \n 2) Tea- 60 cents \n 3) Cappuccino- 80 cents\n";
        cout << "Please select your drink of choice by entering its ID: (1, 2, or 3)";
        cin >> productid;
        if (productid != 1 &&  productid != 2 && productid != 3){
            cout << "That is an invalid entry; please try again. \n";
            cin >> productid;
        }
        cout << "Please insert your coins. \n This vending machine only accepts 50 cent coins, 20 cent coins, and 10 cent coins. \n ";
        cout << "When finished, please input '0'. ";
        cin >> amount;
        if (amount != 50 && amount != 20 && amount != 10 && amount != 0){
            cout << "That is an invalid coin; please insert coins again.\n";
            cin >> amount;
        }
        do {
            amount += amount;
        }
        while (amount != 0);

        return 0;
}

`

I was expecting to be able to insert coins until I input ‘0’, but the code terminal says ‘Process finished with exit code 0’ after I insert coins once, thereby not allowing me to continue to insert coins.

If anyone has any suggestions as to how I could fix this so that the user can continuously insert coins until they input ‘0’ I would greatly appreciate it.

Please feel free to leave any suggestions as to how to proceed as well.

Thank you

EDIT: This problem has been solved thanks to the insight of everyone that commented and offered solutions. Thank you for being a welcoming community and being patient with very beginner programmers like me. I don’t feel as intimiated to ask questions anymore.

>Solution :

You need to put the do { ... } while(...); around the entire block you’d like to repeat. Also, you need a separate variable for the sum.

    int amount, sum = 0;

    // ...

         cout << "Please insert your coins. \n This vending machine only accepts 50 cent coins, 20 cent coins, and 10 cent coins. \n ";
        cout << "When finished, please input '0'. ";
        do {
            cin >> amount;
            while (amount != 50 && amount != 20 && amount != 10 && amount != 0){
                cout << "That is an invalid coin; please insert coins again.\n";
                cin >> amount;
            }
            sum += amount;
        }
        while (amount != 0);

I’ve also changed an if to a while in your code for the case when the user makes multiple mistakes.

To solve these cases yourself, it’s recommended that you either use a debugger and step through your code; or add some logging into the code an check what’s going on (e.g., what the loop repeats).