如何排除無效輸入，然后在 if else 語句中重新輸入

Question

我是編碼新手，需要幫助

if(x == "D" || x == "d"){
        cout << "How many 5c coins? ==> ";
        cin >> j;
        five += j;
        cout << "How many 10c coins? ==> ";
        cin >> a;
        ten += a;
        cout << "How many 20c coins? ==> ";
        cin >> q;
        twenty += q;
        cout << "How many 50c coins? ==> ";
        cin >> k;
        fifty += k;

我希望用戶輸入一個整數，所以我聲明了 int = j, a, q, k 如何做到這一點，例如，如果用戶輸入字母 P，它將出現錯誤，用戶可以重新輸入

Answer 1

使用循環允許用戶在用戶輸入無效輸入的情況下重新輸入輸入。

你可以做：

bool validinput = true;

if(x == "D" || x == "d"){
        do{
                validinput = true;
                cout << "How many 5c coins? ==> ";
                cin >> j;
                if (cin.fail()) { 
                        cout << "Please enter a valid value.." << endl;
                        cin.clear(); //reset state bits
                        cin.ignore(numeric_limits<streamsize>::max(), '\n'); //discard all characters from the input buffer, or until a newline is read.
                        validinput = false;
                }
        }while(validinput == false);

//repeat same to validate other numeric inputs

Answer 2

您將需要一個循環：

do {
  int num;
  if(cin >> num)
  { /* input is good */ 
    ...
    break;
  }
  else
  { /* error */ }
} while(true);