Simple calculations

exercise No. 2

Working with variables

Q:

This exercise is about two variables a and b representing values being subject to change. Embed the following snippet in a class and execute the beast: 

void main() {

  int a = 4,
      b = 7;

  IO.println("a=" + a);
  IO.println("b=" + b);

  int sum = a + b;
  IO.println("Sum: " + sum);
}

  1. Modify these values and code more sophisticated expressions.

  2. Complete the following still empty println(...) statement to create the expected output:

    Source code Expected output 
    int a = 4,
    b = 7;

IO.println(...); // TODO
    		 
    4+7=11

    Your code is expected to produce correct output irrespective of the two variables a and b's values:

    Source code Expected output 
    int a = -4,  // Changing just values
    b = 100;

IO.println(...); // Remains unchanged
    		 
    -4+100=96

    Tip

    Java™ does not only provide an arithmetic + operator: You have already seen the + operator connecting two strings. Java™ extends this concept of string concatenation to non-string values as well.

A:

A naive solution reads:

int a = -4,
    b = 100;

IO.println(a + "+" + b + "=" + a + b);

This unfortunately does not work. The above expression is being evaluated from left to right thus being fully equivalent to: 

IO.println(((((a + "+")  + b) +  "=") + a)  +  b);
//              ↘ ↙    ↙       ↙     ↙     ↙ 
//             "-4+" ↙       ↙     ↙     ↙ 
//              ↘ ↙       ↙     ↙     ↙  
//             "-4+100" ↙     ↙     ↙      
//                 ↘ ↙      ↙     ↙  
//               "-4+100="↙     ↙ 
//                    ↘↙      ↙  
//               "-4+100=-4"↙ 
//                      ↘↙ 
//                 "-4+100=-4100"

As we shall see in the section called “Operators and expressions” each of these constituents evaluates to a string thus producing: 

-4+100=-4100

Adding a pair of braces grouping (a+b) forces an arithmetic “+” operation:

Source code Output 
int a = -4,
    b = 100;

IO.println(a + "+" + b + "=" + (a + b));
 
-4+100=96

This is equivalent to:

IO.println((((a + "+") +  b) + "=") + (a + b));
//             ↘  ↙     ↙     ↙          ↘↙ 
//            "-4+"   ↙     ↙            96    
//                 ↘↙     ↙            ↙  
//             "-4+100" ↙           ↙  
//                   ↘↙          ↙  
//                "-4+100="   ↙   
//                        ↘↙  
//                   "-4+100=96"

Reverting to the original variable values 4 and 7 still yields a correct output:

Source code Output 
int a = 4,
    b = 7;

IO.println(a + "+" + b+ "=" + (a + b));
 
4+7=11
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