I don't understand `cin` works in c++

Question

I'm new to C++, and I want learn how the following code generates output?

int main(){
    int a;
    char b;
    cin>>a; 
    cin>>b;
    cout<<a<<b;
}

I input 123 for a and , for b. But how come the line cout<<a<<b give output of 123,?

Looks correct to me. What output do you think you should get? — john, Sep 01 '18 at 06:58
After your input a is 123 and b is a comma. Then you output the same. I really don't understand why you think it should be any different. What did you expect to happen? — john, Sep 01 '18 at 07:08
@john, why does `char b[] = {}`; instead of `char b;` affects `cout< — iGian, Sep 01 '18 at 11:56
@iGian Your version has undefined behaviour because you are reading input into an empty array. — john, Sep 02 '18 at 07:56
@john, thanks. Still stuck. Actually, if I use `int a;` and `char b[] = "b";` then input just one char to `cin >> b;`, the `cout<>b`, the `cout<>a;` is not. I'd expect `cout< — iGian, Sep 02 '18 at 09:12
@iGian You might be right about a backwards override but the only C++ answer is that it's undefined behaviour. Incidentally you need one more character in the array than characters input because `cin>>b` adds a terminating nul character to the array as well as the input characters. — john, Sep 02 '18 at 09:18

score 1 · Answer 1 · answered Sep 01 '18 at 08:56

In cin>>a>>b when you enter 123, a is an integer, c++ starts to find an integer. It detects 123 but , isn't an integer so cin fails to detect this. When a cin fails it goes to the next command and the next command is to read a character b and it reads ,. I hope this was helpful!

score -1 · Answer 2 · answered Sep 01 '18 at 07:33

-1

This happening because you have two different data types declared as "a" is type "int" while "b" is type "char" and moreover C++ takes '123' as single int-input. Hope this helps. Quick Fix: Avoid Char Input after int as when you press return (enter-key) char is inputted to it's ascii-value. Code Snippet:

int a;
char b;
cin>>a;
cout<<a<<endl;
cin>>b;
cout<<b<<endl;

answered Sep 01 '18 at 07:33

Ali Asadullah Shah

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`cin>>b` skips whitespace so b will be the next non-whitespace character after 123. It won't be the ASCII value of the enter key. – john Sep 01 '18 at 08:05
"op" stands for what?? – Ali Asadullah Shah Sep 01 '18 at 08:06
@AliAsadullah Original Poster – john Sep 01 '18 at 08:07
You are right @Ali Asadullah, It should print 123 then char as the input sequence is little confusing here. – Nasrullah Shah Sep 02 '18 at 08:55

I don't understand `cin` works in c++

2 Answers2