CODEWITHSUNDEEP
c++inputcin
tetektoza
tetektoza

Reputation: 300

What exactly empty input means for cin.get()?

I think it's a simple question, but I don't understand the concept in this sample of code, mainly in the while loop:

#include <iostream>

const int ArSize = 10;
void strcount(const char * str);

int main(){
    using namespace std;
    char input[ArSize];
    char next;


    cout << "Enter text:\n";
    cin.get(input, ArSize);
    while(cin){

        cin.get(next);
        while(next != '\n')
            cin.get(next)
        
        strcount(input);
        cout << "Enter next line, empty line ends the program:\n";
        cin.get(input, ArSize);
    }

    cout << "The end\n";
    return 0;
}

...

What I understand is that the while loop continues until cin returns false. It filters out the remaining input that's left in the buffer (because it wasn't the size of ArSize or under, or it was - then it will just filter out the newline character) until it meets the newline character. Then it counts string's characters (not important in this question), and then, let's say someone just presses enter. cin.get() discards newline character in input. So if someone for example enters an empty line of text in the terminal, it reads it as 'failed' input and cin returns false? Because if someone proceeds to the new line, just by pressing enter, it just leaves the newline character in the buffer, and cin.get() can't get it so it returns false. Or am I wrong?

In short - What exactly happens if you just press enter? cin.get() can't get the input because there's only newline in buffer and it counts it as failed input, so it returns false?

Upvotes: 2

Views: 874

Answers (2)

Bemwa Malak
Bemwa Malak

Reputation: 1339

As you can see here from the CPP reference https://en.cppreference.com/w/cpp/io/basic_istream/get

cin. get() is used to read the next character from the keyboard buffer and it returns that character in case it was available to be read and returns EOF otherwise and sets failbit and eofbit (which makes the expression in the if statement evaluates to false).

now let's see the code in action line by line:

while(cin){

this evaluates to true as long as the failbit flag in the cin object is set to goodbit showing no error. (https://en.cppreference.com/w/cpp/io/ios_base/iostate)

 cin.get(next);
     while(next != '\n')
          cin.get(next)

the first line reads the next character from the keyboard buffer and stores it in the next variable and the while loop checks for the newline character which is equivalent to pressing Enter if it is not the next character in the buffer then continue reading and storing in next until it meets a newline character then it exits the loop returning to the outer while loop.

strcount(input);
    cout << "Enter next line, empty line ends the program:\n";
    cin.get(input, ArSize);

then strcount function as I assume is used to count the characters entered by the user in the input array by this line of code before the while loop.

cin.get(input, ArSize);

and then at the last line inside of the while loop, the program reads another input by the user.

Please Note:

the use of these three lines here is to make sure that each line is read at every single loop with no characters read in the second input before the newline character appears even if the number of characters is bigger than the ArSize variable. when that happens the first line before the while loop will read the number of ArSize from the buffer and if there are remaining characters other than the newline it will be read by the three lines until a newline appears so that the next get function will start looking for characters in the buffer after the previous newline.

cin.get(next);
    while(next != '\n')
        cin.get(next)

if there is anything unclear please let me know.

Upvotes: 2

Alan Birtles
Alan Birtles

Reputation: 36389

If cin.get(input, ArSize); reads no characters (i.e. the first character it encounters is a newline) it calls setstate(failbit) putting the stream into a failed state and therefore while(cin) becomes false, ending the loop.

Upvotes: 4

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