Difficulties with a++, ++a

Question

I start programming about one month ago, and i have some difficulties with the operators ++a, a++ in java. Can someone explain me line by line this program?

import java.util.Scanner; 
public class Number { 
 public static void main(String[] args) { 
 Scanner keyboard = new Scanner(System.in); 
 int number = keyboard.nextInt(); 
 int division1 = (number++) % 10; 
 number = number / 10; 
 System.out.println(number % 10+division1); 
 } 
} 

Answer 1

int division1 = (number++) % 10;

Is equivalent to :

int division1 = number % 10; 
number += 1;

While

int division1 = (++number) % 10;

Would be equivalent to :

number += 1;
int division1 = number % 10; 

---

Basically, your code is equivalent to :

int number = new java.util.Scanner(System.in).nextInt(); 
System.out.println( ((number + 1) / 10) % 10 + number % 10 ); 

Answer 2

++a is a pre-incrementation. Which means that a is incremented before returning the value of a .

a++ is a post-incrementation. Which means that a is incremented after returning the value of a .

In other words, a++ gives the current value of a and then increment it. While ++a directly increment a . If a=42 then System.out.println(a++) gives 42 while System.out.println(++a) gives 43 and in both cases, a=43 now.

OP also asked for a line by line explanation of that code:

import java.util.Scanner;

public class Number { 
 public static void main(String[] args) { 
   Scanner keyboard = new Scanner(System.in); 
   int number = keyboard.nextInt(); 
   int division1 = (number++) % 10; 
   number = number / 10; 
   System.out.println(number % 10+division1); 
 } 
}

I guess, only the code inside the main function need some explanations :

   // Create a Scanner object that read from the standard input.
   Scanner keyboard = new Scanner(System.in);

   // Read an integer.
   int number = keyboard.nextInt(); 

   // put (number % 10) into variable division1 and then increment `number`.
   int division1 = (number++) % 10;

   // Divide number by 10.
   number = number / 10; 

   // Print that expression :
   System.out.println(number % 10+division1);

The line int division1 = (number++) % 10; might not be very clear. It would be simpler to read like that:

int division1 = number % 10;
number += 1;

Now, the explanation of what the function does:

If number = 142 , we put 2 into variable division1 , then number is incremented and divided by 10. So number gets the value 14 ((142+1) / 10). And now we print number % 10 + division1 which is 4 + 2 = 6.

Here some examples of results (I've compiled the code myself):

3 => 3
9 => 10
10 => 1
248 => 12

Answer 3

Literally line-by-line explanation:

import java.util.Scanner; // Import Scanner
public class Number {     // Create Java class called Number
    public static void main(String[] args) {  // Define the main function
        Scanner keyboard = new Scanner(System.in); // Initialize the scanner
        int number = keyboard.nextInt();   // Read an integer from standard input
        int division1 = (number++) % 10;   // Take the unit digit of number, then increase number by 1
        number = number / 10; // Divide number by 10
        // Take the tens digit of the original number (or tens digit + 1, if the unit was 9), sum with the unit digit previously stored in division1
        System.out.println(number % 10+division1);
        // The result would sum the last two digit of number if it doesn't end with 9, or sum the last two digit +1 if the unit was 9.
    } 
}

Some cases:

f(1) = 1
f(124) = 2+4 = 6
f(39) = 4+9 = 13
f(5248) = 4+8 = 12

Answer 4

++a is a pre-incrementation. Which means that a is incremented before the expression is evaluated.

a++ is a post-incrementation. Which means that a is incremented after the expression is evaluated.

One thing where beginners might be confused is what is this 'expression' after which the increment takes place It is actually the ; When you use a++, it is after the ";" when the increment actually takes place and when you use ++a; it is just as you inserted a statement before this one saying a=a+1 and then your expression comes.

Answer 5

I think, the interesting part for you are the following lines:

int number = keyboard.nextInt(); 

Here, you get an input from the user. Say, he enters 12

int division1 = (number++) % 10; 

division1 will become 2, because 12 mod 10 equals 2. After this, number will be incremented by 1, so it is now 13

Let's now assume there would be a ++number instead of number++ . In this case, we would increment number before the modulo operation. So, first, number becomes 13, then we assign 3 to division1 .

And that's the basic thing about a++ and ++a - the time, when the value is incremented. a++ means, all operations use a as it is and as a last step, a is incremented. ++a works in the other order.