关于字符输入的问题

Question

This is what I'm trying to do...

char input[4];
cin >> input;
cout << "Input[0]: " << input[0] << "Input[1]: " << input[1] << "Input[2]: " << input[2] << "Input[3] " << input[3]<< "Input[4] " << input[4] <<endl;

However, when I enter "PF" I get an output of this: Input[0]:P Input[1]: Input[2]:(A weird looking square with two zeros on top and a zero and 4 on the bottom) Input[3] Input[4]

Why do I get that weird character instead of F?

Answer 1

cin >> separates inputs by a space, hence when you enter P<space>F , only the P is accepted into input , and F is queued for the next cin >> .

Thus after that cin >> input line, your input will look like

   input[0] = 'P';
   input[1] = '\0';
// input[2] = garbage;
// input[3] = garbage;
// input[4] = buffer-overflow;

Perhaps you want to use cin.getline :

cin.getline(input, 4);
cout << ...;

(Or better, use std::string which has flexible string length.)

Answer 2

Extracting from cin will break on a space, so it's only reading the P . If you want to read a line from the user you should look at getline()

std::string input;
std::getline(std::cin, input);

Answer 3

What you are doing is actually incredibly dangerous. Your input variable is actually degrading into an object of type char* , so it is actually attempting to read a string into your input variable; however, since it doesn't know how many characters your input array can hold, it can possibly overrun input . Therefore, the code as you have it is actually a buffer overflow vulnerability . Instead, you should use:

for (int i = 0; i < 4; i++ ){
   std::cin >> input[i];
}

As to why you get that weird character... the stream extraction operator >> reads words that are separated by spaces, so "PF" actually only reads "P". So, your input variable gets filled with "P" (which is NUL-terminated, so you actually have 'P' then '\\0'). The remaining elements are uninitialized and so have whatever garbage that happens to be there. Also, I should add that if you did want to read string, then std::getline is a very good way to read std::string objects from the input as in:

std::string result;
std::getline(std::cin,result);