declare a variable inside a loop

Under discussion

In the below program, the variable "int x" is declared inside the "while" loop. Will the program generate many variables of int x while the loop is executed over and over again? Should I declare the int x outside the loop? like, int x = ""; then in the loop x = console.nextInt(); Scanner console = new Scanner(System.in); int sum = 0; while (console.hasNextInt()) { int x = console.nextInt(); sum = sum + x; } System.out.println(sum); Thank you

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Thomas

Level 41 , Bayreuth, Germany

11 November 2022, 07:28

I modified your example a little bit and ran the disassembler over it v1:

package disassamble;

public class Test {
    static int[] data = {2, 5, 4, 1};

    public static void main(String[] args) {
        int sum = 0;

        for (int i = 0; i < data.length; i++) {
            int x = data[i];
            sum += x;
        }

        System.out.println(sum);;
    }
}

package disassamble;

public class Test {
    static int[] data = {2, 5, 4, 1};

    public static void main(String[] args) {
        int sum = 0;
        int x;

        for (int i = 0; i < data.length; i++) {
            x = data[i];
            sum += x;
        }

        System.out.println(sum);;
    }
}

The resulting assembler code was pretty much the same despite some line numbers. So it seems the compiler is smart enough to optimize that on its own. But to be honest, declaring a variable isn't necessary at all and the compiler doesn't do that. And a link to the JVM instruction set

Thomas

Level 41 , Bayreuth, Germany

11 November 2022, 07:30

Result 1: javap -v Test.class

public static void main(java.lang.String[]);
  descriptor: ([Ljava/lang/String;)V
  flags: (0x0009) ACC_PUBLIC, ACC_STATIC
  Code:
    stack=2, locals=4, args_size=1
       0: iconst_0
       1: istore_1
       2: iconst_0
       3: istore_2
       4: iload_2
       5: getstatic     #7                  // Field data:[I
       8: arraylength
       9: if_icmpge     28
      12: getstatic     #7                  // Field data:[I
      15: iload_2
      16: iaload
      17: istore_3
      18: iload_1
      19: iload_3
      20: iadd
      21: istore_1
      22: iinc          2, 1
      25: goto          4
      28: getstatic     #13                 // Field java/lang/System.out:Ljava/io/PrintStream;
      31: iload_1
      32: invokevirtual #19                 // Method java/io/PrintStream.println:(I)V
      35: return
    LineNumberTable:
      line 7: 0
      line 9: 2
      line 10: 12
      line 11: 18
      line 9: 22
      line 14: 28
      line 15: 35
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
         18       4     3     x   I
          4      24     2     i   I
          0      36     0  args   [Ljava/lang/String;
          2      34     1   sum   I
    StackMapTable: number_of_entries = 2
      frame_type = 253 /* append */
        offset_delta = 4
        locals = [ int, int ]
      frame_type = 250 /* chop */
        offset_delta = 23

Thomas

Level 41 , Bayreuth, Germany

11 November 2022, 07:31

result 2:

public static void main(java.lang.String[]);
  descriptor: ([Ljava/lang/String;)V
  flags: (0x0009) ACC_PUBLIC, ACC_STATIC
  Code:
    stack=2, locals=4, args_size=1
       0: iconst_0
       1: istore_1
       2: iconst_0
       3: istore_3
       4: iload_3
       5: getstatic     #7                  // Field data:[I
       8: arraylength
       9: if_icmpge     28
      12: getstatic     #7                  // Field data:[I
      15: iload_3
      16: iaload
      17: istore_2
      18: iload_1
      19: iload_2
      20: iadd
      21: istore_1
      22: iinc          3, 1
      25: goto          4
      28: getstatic     #13                 // Field java/lang/System.out:Ljava/io/PrintStream;
      31: iload_1
      32: invokevirtual #19                 // Method java/io/PrintStream.println:(I)V
      35: return
    LineNumberTable:
      line 7: 0
      line 10: 2
      line 11: 12
      line 12: 18
      line 10: 22
      line 15: 28
      line 16: 35
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
         18      10     2     x   I
          4      24     3     i   I
          0      36     0  args   [Ljava/lang/String;
          2      34     1   sum   I
    StackMapTable: number_of_entries = 2
      frame_type = 254 /* append */
        offset_delta = 4
        locals = [ int, top, int ]
      frame_type = 249 /* chop */
        offset_delta = 23

Evilpanda H.

Level 10 , Taiwan, Province of China

11 November 2022, 12:13

Thank you Thomas. I think I got it. Java compiler is smarter than C compiler. Thank you.