CodeGym/Help with Java Tasks/Please explain why int j doesnt get (j++) at first loop ?...

Level 6

09.08.2022
826views
13comments

Please explain why int j doesnt get (j++) at first loop ?????

Question about the task

Under discussion

Please explain why int j doesnt get (j++) at first loop ????? The program runs correctly i just dont understand why on first loop int j doesnt get j++ When i run debug i see the following steps. Outer loop passes to inner loop and it prints first 8 then goes to line 4 comes back to line 1 ( at this point int i becomes 1, BUT int j DOESNT GET INCREASED BY 1 as i thought it should because of j++) goes to line 3 again (prints 8 in a new row) goes back to line 2 (ONLY NOW int j INCREASES BY 1)

for (int i = 0; i < 10; i++) {
for (int j = 0; j <= i; j++) {
System.out.print('8'); }
System.out.println(); }
        }
    }

Comments (13)

Popular
New
Old

You must be signed in to leave a comment

Behrooz Hussaini

Level 17 , Russia, Russian Federation

4 February 2023, 07:54

I am so frustrated, Not sure if the IDE debugging tool has an issue or I don't understand. The condition is true but it skips the println() and moves to the outer loop. Buy why?

Albert ENG

Level 6

24 August 2022, 10:47

according to this diagram j++ should add increment before it loops again or eixit the loop, sorry guys but non of you explanaition cleared it out for me im still confused why j++ doesnt add 1 after it exits the loop. Anonymous #11045644 says that inner loop with j doesnt run at first repetition. BUT it runs ! i see in debug mode it runs and prints 8, then it goes back to line 2 and here where i expect j++ to increase j and make it j=1 according to this diagram. it would make sense, since j wont be = to i and loop will exit and will go to line 4. BUT loop either way exists and goes to line 4 without j++, thus loop should be considered TRUE and keep looping, but its not. For example in this similar code, my logic works. After first 8 is printed the loop goes back to line 4 and j++ start working, thus j gets increased by 1.

int m = 3;
int n = 4;
      for (int i = 0; i < m; i++) {
          for (int j = 0; j < n; j++) {
              System.out.print('8'); }
          System.out.println();

Guadalupe Gagnon

Level 37 , Tampa, United States

24 August 2022, 13:34

In for loops you have 3 statements: ({1st Statement}; {2nd Statement}; {3rd Statement}) None of these statements are required, but the semi colons in between are required. - The first statement allows you to create variables to be used only inside the loop. You can also change or set outside variables. This statement is only run one time when the loop is formed - the second statement must be a boolean condition. This condition is checked prior to the code in the loop being run. If this check returns false then the loop ends immediately. If there is no boolean check in this then it will default to just 'true' - the third statement allows you to change the value of already declared variables, such as increasing the counter. This statement is run only when the code in the loop is run. If the second statement, the boolean, is false then it will not be increased. So, as I said, none of the statements are required so you can have a valid for loop that looks like this:

for( ; ; ) {
   System.out.println("This will print infinitely");
}

And all of these are valid for loops:

int x = 10;

// fairly standard implementation of a for loop
System.out.println("This prints i 5 times (0 - 4): ");
for(int i = 0; i < x; x--) {
   System.out.println("The value of i is: " + i++);
}
System.out.println();

// this has a boolean check at the end of the loop instead of at the beginning
System.out.println("This sets x back to 1 and then prints it 5 times (1-5)");
for(x= 1;  ;x++) {
   System.out.println("The value of x is: " + x);
   if(x > 4) break;
}

// this will print infinitely as no boolean condition exists to break the loop
// un comment it to see this if you want
/*System.out.println("This will print infinitely:");
for( ; ; x++) {
   System.out.println(x);
}*/

Guadalupe Gagnon

Level 37 , Tampa, United States

24 August 2022, 13:43

I am not sure where your confusion is. The code I shared in my first post breaks down exactly what is going on. The first loop cycle, when the outer loop has i = 0; the inner loop looks like this:

// i = 0
for (int j = 0; j <= 0; j++) {
   System.out.print('8');
}

This first step is that the variable j is created and set to 0. Next the boolean expression is checked, and 0 is less than or equal to 0 so this is true. The code in the block is executed and '8' is printed. next j is increased by one. Next the boolean is checked and 1 is not greater than or equal to 0 so this fails and the inner loop exits. The last line in the outer loop is: System.out.println(); which is executed, creating a new line, and the inner loop block concludes. Next i is increased by one then the outer boolean is checks (1 < 10) which is true and then the outer block starts over. This creates a new 'for' loop for the inner that would look like this:

// i = 1
for (int j = 0; j <= 1; j++) {
   System.out.print('8');
}

So first the variable j is created and set to 0. Next the boolean expression is checked (0 <= 1) and is true so '8' is printed. then j is increased by one. Next the boolean expression is checked (1 <= 1) and is true so '8' is printed again. Then j is increased by one. Next the boolean expression is checked (2 <= 1) and fails and this loop exits. Then the new line is printed and the outer loop cycle ends and i is increased by one. This keeps going on (see my first post) for every loop cycle until the outer loop's boolean (i < 10) fails and the code finally concludes.

Guadalupe Gagnon

Level 37 , Tampa, United States

24 August 2022, 14:32

I find sometimes visuals help. This is your code with a bunch of output lines to describe what is going on. Run it and walk through the output to see what goes on. Change the 'loops' variable to something smaller (like 2 or 3) if 10 has way too much info:

int loops = 10;
StringBuilder sb = new StringBuilder();
System.out.println("Creating new for loop 'for (int i = 0; i < m; i++)'");
System.out.print("\tChecking if i(0) < " + loops + ": ");
for (int i = 0; i < loops; i++) {
   System.out.println("TRUE");
   System.out.println("\tCreating new for loop 'for (int j = 0; j <= i; j++)'");
   System.out.print("\t\tChecking if j(0) < " + i + ": ");
   for (int j = 0; j <= i; j++){
      System.out.println("TRUE");
      System.out.println("\t\toutputting an '8'");
      sb.append('8');
      System.out.println("\t\tIncreasing j by 1, the value of j is " + (j + 1));
      System.out.print("\t\tChecking if j("+ (j+1) + ") < " + i + ": ");
   }
   System.out.println("FALSE");

   System.out.println("\tOutputting new line");
   sb.append("\n");
   System.out.println();
   System.out.println("\tIncreasing i by 1, the value of i is " + (i + 1));
   System.out.print("\tChecking if i(" + (i + 1) +") < " + loops + ": ");
}
System.out.println("False");
System.out.println("\nResults of the code:");
System.out.println(sb);

Albert ENG

Level 6

24 August 2022, 17:59

Thanks a lot for your effort Guadalupe As you explainer regarding 3rd statement " This statement is run only when the code in the loop is run. If the second statement, the boolean, is false then it will not be increased." As you said it should run ( i mean j++) because the code in the loop is run ( number 8 is printed) But as you see in the screenshots from my debug it doesnt run. j++ doesnt add 1 to j in the first run. Thats what i dont understand, why it doest increase j by 1

in picture 2 it print 8 and goes back to loop (line 8) where i expet j++ to work and increase j by 1. But it jumps out of inner loop and goes to line 10 in picture 3

Guadalupe Gagnon

Level 37 , Tampa, United States

24 August 2022, 18:11

j is increased by one, otherwise the loop does not break. The debugger doesn't show the change until the next line of code. You can see this when i increases to 1. It is not shown when the debugger is on line 1 but when the debugger is inside the loop at line 2. Because the loop ends and j is out of scope and the debugger doesn't show that it has changed value. Debug these two loops:

System.out.println("start"); // put break point here
for(int i = 0; i != 1; i++)
   System.out.println("Doing something");

for(int i = 0; i != 2; i++)
   System.out.println("Doing something");

Stepping through you will see that the first loop never shows 'i' as 1, but it wouldn't break if it wasn't 1 The second loop never shows 'i' as 2, but once again it wouldn't break otherwise. It is the debugger that is confusing you.

Guadalupe Gagnon

Level 37 , Tampa, United States

24 August 2022, 18:27

Here is another example:

int i;
for(System.out.println("Statement1: Creating loop, setting initial value of i to: "+(i = 0)); i < 10; i++, System.out.println("Statement3: now increasing i to: " + i + " -- Statement2: Checking if i is less than 10: " + (i < 10))) {
   System.out.println("code inside loop");
}

Remember my steps for ({statement1}; {statement2}; {statement3}) statement1: this is pre processed before the loop starts statement2: this is run prior to each loop cycle statement3: This is run at the conclusion of every loop cycle The first statement of this code: System.out.println("Creating loop, setting initial value of i to: "+(i = 0)) Will output that the loop is being created and i is being set to 0. This outputs first and only 1 time. The next statement: i < 10 is the boolean expression. I couldn't output the very first check as this statement needs to be a boolean expression and Sys.out.println() is not a boolean. The rest of them are output at the end of statement3. The last expression: i++, System.out.println("Statement3: now increasing i to: " + i + " -- Statement2: Checking if i is less than 10: " + (i < 10)) This first increases i then outputs that it is doing it. It then outputs the result of Statement2 prior to it being executed Run the code and you can follow through the output and see exactly, in order (other than the very first boolean check), how the loop is processed by the compiler.

Albert ENG

Level 6

28 August 2022, 17:36

Guadalupe thanks a lot for your detailed explanation, the effort and patience. You completely cleared it out for me ! Very good teaching skills :)

Anonymous #11045644

Level 9 , Slovenia

13 August 2022, 14:53

Okay so this is how it works: your inner for loop (the one with j) repeats as long as j is smaller or the same value as i. Which means that that in the first iteration of i loop it doesn't run. Because it doesn't run j didn't get declared (it doesn't exist). And even if it did run, once the inner for loop (j) ends variable j would no longer exist. That is because a variable exists as long as the program doesn't exit the block of code where the variable has been declared. In this case i is declared once and once it exists the outer loop (where it has been declared. There no more variable i in the memory. And variable j gets declared 10 times and gets removed from memory 10 times. Note: the chosen variable only gets declared in the first iteration (repetition).

Albert ENG

Level 6

24 August 2022, 11:11

unfortunately i still did not get it, i posted a diagram above in the comment reply which makes me to be confused in this code.

Guadalupe Gagnon

Level 37 , Tampa, United States

9 August 2022, 18:42

Line 2 creates a new loop with j = 0. If you took the outer loop away the code would look like this:

for (int j = 0; j <= 0; j++) {
   System.out.print('8');
}
System.out.println();

for (int j = 0; j <= 1; j++) {
   System.out.print('8');
}
System.out.println();

for (int j = 0; j <= 2; j++) {
   System.out.print('8');
}
System.out.println();

/*...3,4,5,6,7,8...*/

for (int j = 0; j <= 9; j++) {
   System.out.print('8');
}
System.out.println();

Albert ENG

Level 6

24 August 2022, 11:11

unfortunately i still did not get it, i posted a diagram above in the comment reply which makes me to be confused in this code.