这是我第一次使用Stackoverflow。 我正在制作一个程序来找出一辆汽车的MPG。我想知道如何才能使cin语句只接受正整数？而且，如果输入的输入无效，可以重置吗？我不确定这是否合理。我没必要上课。我只是好奇如何做到这一点。这是代码。
#include <iostream>
using namespace std;
int main()
{
double tank, miles, mpg;
cout << "Hello. This is a program that calculates the MPG ( Miles Per Gallon) for your\n" ;
cout << "vehicle\n" << endl;
cout << "Please enter how many gallons your vehicle can hold\n" << endl;
cin >> tank;
cout << endl;
cout << "Please enter how many miles that have been driven on a full tank\n" <<endl;
cin >> miles;
cout << endl;
mpg = (miles)/(tank);
cout << "Your vehicle recieves " << mpg << " miles per gallon\n" << endl;
system ("pause");
return 0;
}
答案 0 :(得分：4)
iostreams不是构建复杂UI的工具包。除非你想编写自己的相当复杂的流来包装通常的流，否则你无法获得它（a）只接受正整数或（b）礼貌地与输入其他东西的用户互动。
你应该只是阅读cin中的行，并在你看到你得到的内容后打印你自己的错误提示等。
答案 1 :(得分：1)
cout << "Hello. This is a program that calculates the MPG ( Miles Per Gallon) for your\n" ;
cout << "vehicle\n" << endl;
do
{
cout << "Please enter how many gallons your vehicle can hold\n" << endl;
cin >> tank;
cout << endl;
} while (tank <= 0 && ((int)tank != tank));
do
{
cout << "Please enter how many miles that have been driven on a full tank\n" <<endl;
cin >> miles;
cout << endl;
} while (miles <= 0 && ((int)miles != miles));
如果在运行语句后执行此操作，如果答案为0或更低或不是整数，它将重新运行它们。如果您使用变量ints而不是double，那么您可以删除while语句中的“＆amp;＆amp;（（int）miles == miles）”部分。
答案 2 :(得分：0)
但是，在命令行环境中有几种标准方法可以做到这一点。
您可以将cin语句捕获到一个循环中，该循环在输入有效输入之前不会释放。这是验证CLI输入的“标准”方式，而不仅仅是签名号码。
do
{
cout << "\nPlease enter...";
cin >> tank;
}
while (tank < 0)
while语句中的条件是验证数据的位置。您还可以创建一个if语句来解释输入无效的原因。
另一种方法是简单地强制值为正，只需转到tank = fabs(tank);，它取tank变量的绝对值（即正数）。
答案 3 :(得分：0)
So this is my code for an infinite loop
1: So main will call the "Get_number()" function
2: Get number will accept an int from the user
3(A): If int is greater than 0, go into loop
3(B): Else, display to user "Invalid Input" and then call the function
"Get_number()" again creating an infinite loop until the user
enters a value greater than 0
#include <iostream> // Access the input output stream library
#include <fstream> // Access to the fstream library (used to read and write to files)
#include <chrono> // Needed to access "std::chrono_literals"
#include <thread> // Needed to access "namespace std::this_thread"
using std::fstream; // this will allow us to use the fstream (we'll be able to read and write to files)
using std::ios; // needed for iostream (used to be able to tell fstream to read and/or write to a file and that it's reading/writing a binary file)
using std::cout; // need this statment to access cout (to display info to user)
using std::cin; // need this statment to access cin (to gather info from user)
using std::endl; // need this statment to access endl (will end the line)
using namespace std::this_thread; // This will allow me to use "Sleep_For" or "Sleep_Until"
using namespace std::chrono_literals; // This will allow the use of measurements of time such as ns, us, s, h, etc.
//Prototypes***************************************************************************************************
void shellSort(int read[], int readLength); //Making Prototype (Declaring our function) so that compiler knows not to worry about it
void Get_number();
void Write_to_file(int user_input_of_how_many_random_numbers_to_generate); //Making Prototype (Declaring our function) so that compiler knows not to worry about it
void Read_from_file(int user_input_of_how_many_random_numbers_to_generate);//Making Prototype (Declaring our function) so that compiler knows not to worry about it
//*************************************************************************************************************
void main()
{
Get_number();
system("pause>>void"); // will let the console pause untill user presses any button to continue
}
/**************************************************************************************************************
* Purpose: This function will gather a positive integer from the user and use it to generate that many
* random numbers!
*
* Precondition: None
*
*
* Postcondition:
* Would've gathered the number of random numbers the user wanted to generate and then gone into the
* Write_to_file and Read_from_file function
*
**************************************************************************************************************/
void Get_number()
{
int user_input_of_how_many_random_numbers_to_generate = 0; //make variable that will accept the int value the user wants to generate random numbers
cout << "Please Enter A Number Greater Than Zero:" << endl; // displays to user to enter a number greater than zero
cin >> user_input_of_how_many_random_numbers_to_generate; // will accept the value the user inputted and place it in the "user_input_of_how_many_random_numbers_to_generate" variable
system("cls"); // Will clear the screen
if (user_input_of_how_many_random_numbers_to_generate > 0) // if user input is greater than zero, enter this
{
Write_to_file(user_input_of_how_many_random_numbers_to_generate); // will bring up the "Write_to_file" function
Read_from_file(user_input_of_how_many_random_numbers_to_generate); // will bring up the "Read_from_file" function
}
else // else enter this
{
cout << "invalid input!" << endl; // display to user "invalid input"
sleep_for(2s); // system will pause for 2 seconds allowing the user to read the message of "invalid input"
system("cls"); // console will be cleared
Get_number(); // Get_number function will be entered creating an infinate loop untill the user's input is valid!
}
}
答案 4 :(得分：-3)
而不是
cin >> miles;
尝试
while ( (cin >> miles) < 0 )
cout << "Please enter how many gallons your vehicle can hold\n" << endl;
这将重复这个问题，直到输入为正。你也可以为其他问题做到这一点。
请注意，输入流不适用于输入过滤。你必须为此提供自己的逻辑。