c++ - C++中的输入限制和错误检查

Question

我的以下C++程序有问题。它根据用户输入的信息，使用Widmark公式计算并打印出血液中的酒精浓度。存在一些输入错误检查，例如，在询问他们喝了哪种饮料之后是否输入了随机字符，或者在询问重量之后是否输入了大于400的任何数字。然后出现一条错误消息(这是故意的)，并要求用户再次输入。

但是，它们是输入错误检查的问题，例如，如果在询问性别后输入x> 1个字符，则错误消息将出现x次(应该只出现一次)。如果要求输入重量后输入了任何非整数字符，则错误消息将无限次出现，从而导致无休止的循环(输入非浮点值后，公升的公升与醉酒一样)。

有什么方法可以限制输入，以便错误消息在任何无效输入之后仅在之后出现一次？

#include<iostream> // for std::cin and std::cout
#include<iomanip> // for std::fixed and std::setprecision

float BAC (char gender, int weight, char drink, float litres) // function to calculate blood alcohol concentration
{
 // local variables
 float bac, rate, vol; // blood alcohol concentration, widmark rate, volume percentage

 // determine widmark rate
 if (gender == 'm' || gender == 'M') // not case-sensitive
 {
 rate = 0.68;
 } 
 if (gender == 'f' || gender == 'F')
 {
 rate = 0.55;
 }
 if (gender == 'c' || gender == 'C')
 {
 rate = 0.75;
 }

 // determine volume percentage
 if (drink == 'b' || drink == 'B') 
 {
 vol = 5;
 } 
 if (drink == 'w' || drink == 'W')
 {
 vol = 12;
 }
 if (drink == 's' || drink == 'S')
 {
 vol = 38;
 }

 // calculating blood alcohol concentration
 bac = ((litres * 1000) * (vol / 100) * 0.8) / (weight * rate);
 // ((litres * 1000) * (vol / 100) * 0.8) --> formula for alcohol mass
 // (litres * 1000) --> conversion to millilitres

 return bac;
}

int main () // main function for user input/output
{
 // variables to be entered
 char gender, drink;
 int weight;
 float litres;

 // getting user input
 std::cout << "Are you male (m), female (f), or a small child (c)?" << std::endl;
 do
 {
 std::cin >> gender;
 if (gender != 'm' && gender != 'M' && gender != 'f' && gender != 'F' && gender != 'c' && gender != 'C') // check input
 {
 std::cout << "\nInvalid input: Please enter 'm', 'f' or 'c' (capital letters are allowed)." << std::endl;
 }
 }
 while (gender != 'm' && gender != 'M' && gender != 'f' && gender != 'F' && gender != 'c' && gender != 'C');
 // repeat as long as valid input is entered

 std::cout << "\nWhat is your weight (in kg)?" << std::endl;
 do
 {
 std::cin >> weight;
 if (weight < 10 || weight > 400)
 {
 std::cout << "\nInvalid input: Please enter a number bigger than or equal to 10 and smaller than or equal to 400." << std::endl;
 }
 }
 while (weight < 10 || weight > 400);

 std::cout << "\nWhich beverage did you drink? beer: (b), wine: (w), schnapps: (s)" << std::endl;
 do
 {
 std::cin >> drink;
 if (drink != 'b' && drink != 'B' && drink != 'w' && drink != 'W' && drink != 's' && drink != 'S')
 {
 std::cout << "\nInvalid input: Please enter 'b', 'w' or 's' (capital letters are allowed)." << std::endl;
 }
 }
 while (drink != 'b' && drink != 'B' && drink != 'w' && drink != 'W' && drink != 's' && drink != 'S');

 std::cout << "\nHow many litres did you drink?" << std::endl;
 do
 {
 std::cin >> litres;
 if (litres < 0 || litres > 10)
 {
 std::cout << "\nInvalid input: Please enter a number bigger than or equal to 0 and smaller than or equal to 10." << std::endl;
 }
 }
 while (litres < 0 || litres > 10);

 // print the result
 std::cout << std::endl << std::endl << "Your blood alcohol concentration is: "
 << std::fixed << std::setprecision(2) // show only 2 decimal places after the dot
 << BAC (gender, weight, drink, litres) // function call
 << " thousandths." << std::endl;

 return 0;
}

Answer 1

我确定您已经意识到，您的问题是读取每个输入后std:cin流中的“剩余字符”。现在，尽管您实际上无法“刷新”一个input stream，但是可以在每次输入之前使用如下代码清除所有“等待中”的输入字符:

std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
std::cin >> weight;

有关此的更多讨论，请参见此处: How do I flush the cin buffer?

您可能需要考虑的另一点是，当任何输入操作失败时会发生什么。您应该使用“无效”值初始化变量，以确保用户实际提供了良好的数据:

char gender = '?', drink = '?';
int weight = -1.0;
float litres = -1.0;