计算字符输入的次数-Do-While循环Java

Question

I'm a bit confused as to why I need the do-while loop in this piece. What I am trying to do is count the number of times a user inputs a character. The count should stop when the user types a ".". When I try to run the program without the do-while, the count of x is multiplied by 3. I've read online that the console input is line buffered. The way I understand it, is a an entire line is created by one character and as such there are actually more characters? Is my understanding correct? And if possible, could I have an explanation of the do-while loop?

public class readCharLengths {
public static void main(String args[]) 
    throws java.io.IOException {
        char typed, ignore;
        int x=0;  
        for ( ; ; ) {      //infinite loop to keep reading in values

            typed = (char) System.in.read();      //get typed character

            do {                                    //confused here
            ignore = (char) System.in.read();       //confused here
            } while (ignore != '\n');               //confused here

                    if (typed =='.') break;       //break loop when . is typed
            x++;                                  //increment x

        }

     System.out.println("The number of characters is: " + x);   //print x

}
}

Answer 1

It depends on the circumstances around how a user is giving input into the console. If they insert one character at a time, each on a separate line. Or if they input a string of characters, basically like a sentence, and then you count the number of characters until a period is reached.

If it is one character per line, I would suggest:

    int x = 0;
    Scanner in = new Scanner(System.in);
    while (true) {
        String input = in.nextChar();
        if (input.equals('.') {
            break;
        } else {
            x++;
        }
    }

If it is a string of characters, I would suggest:

    int x = 0;
    Scanner in = new Scanner(System.in);

    String input = in.nextLine();

    for (int i=0; i<input.length(); i++) {
        if (input.charAt(i).equals('.') {
            break;
        } else {
            x++;
        }
    }

Answer 2

Well, there are a couple of things about this.

First, this might not do what you want because the do/while loop will consume every character after the first one without incrementing the counter until it hits the LF. So, your count will always be 1 with the do/while loop in place assuming you type at least one character and then hit Enter. However, if your intention is that the user is allowed to enter only one character at a time (followed by Enter), then see the second item.

Second, console input defaults to buffered until you press Enter. So, if you take the do/while loop out, enter a single character and press Enter, you'll actually get three characters on Windows--the character you typed, CR and LF. Assuming you're testing with single characters, that would explain the multiplication by 3.

Answer 3

To answer what a do...while loop is, I'm going to go into some very basic programming concepts. I don't mean to be condescending if I come across that way: I just don't know your skill level with programming, so I'm assuming you are very new to it.

First, let me explain the concept of a loop condition. A loop condition is code that, when it is evaluated while your program runs, will either come out to true or false . So, in the case of a regular while loop, the format is:

while (loop condition) {
    run some code, maybe multiple times
}

When your code reaches the while line of code it evaluates the condition and decides whether to run the code inside the loop's braces.

Let's say your condition is "x must be less than 5". You might have a while loop like this:

int x = 0;
while(x < 5) {
   System.out.println("A");
   x++;
}

This code will print A on 5 lines because x = 0 when it reaches the while statement the first time, and since 0 < 5 evaluates to true , the loop's code is going to execute. When the end of the loop is reached, it jumps up to the loop condition and evaluates it again. This time, x = 1 because we added 1 to x. Since 1 < 5 , the loop happens again. This happens a total of 5 times and then when it evaluates the loop condition, you have it try 5 < 5 , which is false . It skips the loop code and continues the program at this point.

A do...while loop is based on the same idea. Instead of checking the loop condition at the start of the loop, it is checked at the end. So...

int x = 0;
do {
  System.out.println("A");
} while(x < 0);

This code will output A once. Even though the loop condition is not met (since x starts at 0), when you use a do...while, the loop code is always executed at least once. At the end of the loop, the condition is checked and since 0 < 0 is false , the loop does not repeat.

---

So, on to how System.in.read() works. You have it right that it buffers the input until a linebreak is reached. But do you actually know what that means?

Here's how it works. You start typing stuff into the console. Say you type aabbcc and then press enter. Your program waits when it calls System.in.read() until user input is sent back to the program. It does not continue running when the first a is typed though! Your program only gets to continue running once you press enter (that is, a linebreak is sent).

The idea of buffering input is that all the letters you typed are stored and available once your program starts running again. So when your program runs after you have entered aabbcc , all of those letters are available (and also the linebreak for the enter key you pressed).

With your code, that means that the first time you call System.in.read() , you get back the character a , but there is still abbcc and newline waiting. It enters the loop and reads each character. So it reads a , then b , then b , then c , then c , then newline. At this point the loop condition is false and the loop ends.