当循环再次运行时，为什么此输出没有“请重试”？

Question

Hello I am trying to learn c++ and I wanted to give a little practice with a program. However I'm having trouble using cout within the loop.

This is the loop I'm trying to output text from. When the user enters a number that isn't valid it is supposed to say "Sorry try again!"

while (datecheck)
{
    bool check(false);
    if (check)
        std::cout<<"Sorry try again!"<<std::endl;
    std::cin>>c;
    if (c >= 1)
    {
        if (b == 2 && c <= 28)
            datecheck = false;
        if (b == 2 && a % 4 == 0 && c <= 29) 
            datecheck = false;
        if (b == 4 || b == 6 || b == 9 || b == 11 && c <= 30) 
            datecheck = false;
        if (c <= 31) 
            datecheck = false;
    }
    check = true;
}

When it outputs and I purposely keep myself in the loop it doesn't output anything

Year: -20
-20
-20

Answer 1

You declare a fresh new variable check at every iteration. And you initialize that variable to false every time. So move that declaration before the while loop.

Change this:

while (datecheck)
{
    bool check(false);
    ...
    check = true;
}

to this:

bool check(false);
while (datecheck)
{
     ...
    check = true;
}

Answer 2

The problem is with declaration of bool check(false); . This keeps on re-assigning value to false at beginning of each iteration.

A simple fix could be to get-rid of use of check variable and use only datecheck .

bool datecheck(true);
while (true)
{
    std::cin>>c;
    if (c >= 1)
    {
        if (b == 2 && c <= 28)
            datecheck = false;
        if (b == 2 && a % 4 == 0 && c <= 29) 
            datecheck = false;
        if (b == 4 || b == 6 || b == 9 || b == 11 && c <= 30) 
            datecheck = false;
        if (c <= 31) 
            datecheck = false;
    }
    if (datecheck)
    {
        std::cout<<"Sorry try again!"<<std::endl;
    }
    else
    {
        break;
    }
}