Answers.Body: <p>Another benefit of block-level scope is that because variables defined in a block go out of scope at the end of that block, their values are then eligible for garbage collection (assuming no other variables point to them).</p>
  
  <p>Expand your example to use some really big object that takes up a lot of memory:</p>
  
  <pre><code>if (someBooleanCondition){
      ReallyBigObject rbo = getReallyBigObject();
      rbo.doSomeProcessing();
  } else {
      ReallyBigObject rbo = getOtherReallyBigObject();
      rbo.doSomeOtherProcessing();
  }
  
  //rbo is out of scope and can be garbage collected!
  </code></pre>
  
  <p>With the way Java works, the really big object goes out of scope at the end of the <code>if</code> or <code>else</code> block it's declared in. That means it can be garbage collected, and that's awesome because we don't have to worry about it anymore!</p>
  
  <p>It's possible to have function-level scope (like JavaScript), so the students' intuition isn't <strong>wrong</strong>. It's just that Java chose a set of rules to allow garbage collection to happen sooner rather than later.</p>
  
  <p>But as a result, this also means that you can't use a variable outside of its scope.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-07-21T17:18:52.293Z
Answers.LastActivityDate: 2017-07-21T20:17:23.513Z
Answers.LastEditDate: 2017-07-21T20:17:23.513Z
Answers.UserDisplayName: Kevin Workman
Answers.Body: <p>I explain this with a saying:</p>
  
  <blockquote>
    <p>What happens in the block stays in the block</p>
  </blockquote>
  
  <p>Now, this isn't exactly how it works, but the idea is that the students remember that what they create in a block, is accessible only inside the block. </p>
  
  <p>So:</p>
  
  <pre><code>if(shouldGoIntoBlock){
      int x=10;//created in the block
      System.out.println("printed from if body: "+ x);
  else{
      int x=-7;//created in the block
      System.out.println("printed from else body: "+ x);
  }
  System.out.println("printed outside of blocks: "+x);
  </code></pre>
  
  <p>And they see in eclipse that the last print errors in compile time. That shows them that <code>what's created in the block stays in the block</code>. </p>
  
  <p>Next, I show them that there will also be an error if we define <code>x</code> before the <code>if-else</code> block, but the last line won't error. So they see that order of creation matters. Afterwards, I show them that creating <code>x</code> <strong>after</strong> the <code>if-else</code> block and <strong>before</strong> the last print, then there are no errors.</p>
  
  <p>The vast majority of the students understand the idea this way.</p>
  
  <p>They don't learn the stack segment of the computer at that time, but they see that order matters, and that makes learning about the stack easier for them when they learn it later in the year.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-05-31T06:12:58.940Z
Answers.LastActivityDate: 2017-05-31T06:12:58.940Z
Answers.UserDisplayName: ItamarG3
Answers.Body: <p>I teach my students a bit about the stack as part of the very first lesson of the year.  As a result, <em>they are aware coming in</em> that primitive variables are stored next to one another in the stack during runtime, and that the names of the primitive variables that we use in our code are removed by the compiler, and replaced with memory locations.  They are also aware by this time that the stack is populated in the order in which variables are declared.  Thus, I show them the following code, and ask a simple question:</p>
  
  <pre><code>if (someBooleanCondition){
      int v = 18;
      int x = 7;
  } else {
      int x = 0;
      int v = o;
  }
  </code></pre>
  
  <p>If we now wanted to run <code>System.out.println(x)</code>, it is true that <code>x</code> has a <em>value</em>, but what is its <em>location</em>?  If <code>someBooleanCondition</code> is true, <code>x</code> will be at the higher of the two addresses, whereas if it is false, <code>x</code> will be at the lower of the two addresses  How would the compiler know where to direct <code>System.out.println</code> to find <code>x</code>?</p>
  
  <p>Thus, the scope of variables declared during <code>if</code> statements must be local to that <code>if</code> statement, and cannot be used outside of it.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-05-31T00:15:03.513Z
Answers.LastActivityDate: 2017-05-31T00:15:03.513Z
Answers.UserDisplayName: Ben I.
Answers.Body: <p>I like to use an analogy from <em>Finding Nemo</em>.  I tell them their programming language has the same problem Dory has - short-term memory loss.  When a variable is declared within a block, Java will forget about the variable (just like Dory) after reaching the end of the the block.  So a block declares the "scope" of Dory's short term memory.</p>
  
  <p>I've been concerned that students would think that the variables assigned in a block would lose their value after the block has ended, but I've never run into that problem for some reason.</p>
  
  <p>What's interesting about your example is that it won't even compile without the print statement!  Java doesn't allow you to have a single declaration statement in an if or loop.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-05-31T07:32:18.070Z
Answers.Text: I don't get what you mean in the 2nd paragraph. Do you mean that the variable still exist but have a value of undefined?
Answers.UserDisplayName: ctrl-alt-delor
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-06-01T03:55:45.887Z
Answers.Text: I like it. I don't understand it, since I have no context for Dory (never watched Finding Nemo). That's part of the problem, finding context that matches what the students know.
Answers.UserDisplayName: Gypsy Spellweaver
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-06-01T18:24:03.600Z
Answers.Text: Here's a scene from the movie that you could show for reference:  https://www.youtube.com/watch?v=tESffhWs8l0
Answers.UserDisplayName: Matthew Fahrenbacher
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-05-31T04:18:12.533Z
Answers.LastActivityDate: 2017-07-21T16:41:22.130Z
Answers.LastEditDate: 2017-07-21T16:41:22.130Z
Answers.UserDisplayName: Matthew Fahrenbacher
Answers.Body: <p>You should be able to copy/paste block of codes that does not use other variables. But in your examples what if you want to copy/paste that to a code that already declared x? It would not work</p>
  
  <pre><code>String x = "Ben I.";
  ... a lot of code
  ...
  ...
  if (someBooleanCondition){
      int x = 7;
  } else {
      int x = 0;
  }
  System.out.println(x);
  </code></pre>
  
  <p>In that case you want to print Ben I.
  In block scope very often you need to use temporary variables to help you in some calculations and if you declared them in block then you want them to be cleared from stack when you get out of the block.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-07-21T17:39:26.750Z
Answers.LastActivityDate: 2017-08-04T12:43:27.167Z
Answers.LastEditDate: 2017-08-04T12:43:27.167Z
Answers.UserDisplayName: CodesInTheDark
Answers.Body: <p>Java, like C/C++ has block scope.  When you create a variable in a block, the stack pointer moves to make room for it.  When the block ends, the stack pointer moves back. This makes the variable inaccessible.  It will be overwritten the next time the stack pointer moves over it.</p>
  
  <p>Python and JavaScript have function scope.  Note, however that in ECMA 6, the <code>let</code> construct gives variables block scope.  </p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-06-01T13:40:17.443Z
Answers.LastActivityDate: 2017-06-01T13:40:17.443Z
Answers.UserDisplayName: ncmathsadist
Answers.Body: <p>Something that might be helpful is using computers. Files created on the school computer in the classroom don't exist anywhere else. The file can be copied somewhere else, just like passing it back from a function copies the data, not the variable, to the caller.</p>
  
  <p>To demonstrate inner block access to variables at higher level you can use Google Drive, or something similar. A file on Google Drive can be edited using the web interface, so the local computer has access to the data. Still, the data belongs to Google Drive, so any other computer can also access it and change it. Just like all the loops and blocks in the current subroutine have access to data that belongs to the routine, or anything that is has access to from an even higher level. (The last part might be language dependent, and not apply in the language you utilize, and can be dropped.)</p>
  
  <p>A second option is just to use the directory structure of the computer itself. Creating <code>/home/mine/Documents/computer_class/project_1/notes.txt</code> will show <code>file not found</code> if you try to access <code>/home/mine/Documents/computer_class/notes.txt</code>. [Or for Windows <code>C:\Users\mine\Documents\Computer Class\Project 1\notes.txt</code> can't be edited using <code>C:\Users\mine\Documents\Computer Class\notes.txt</code>.]</p>
  
  <p>One thing to bear in mind is that not all languages enforce variables to be scoped at that granular of a level, or at all. In addition, how that scope is enforced, or implemented, can also be language dependent. If the language in use has been dissected by the class, then using its internal process can explain <em>why</em> it happens in that case.</p>
  
  <p>Unless the students will <em>always</em> be using that language, even after school, I think it's important to also make them aware that those rules may, or may not, apply, and that when they don't apply <em>side effects</em> can become an issue. In some cases the value isn't <code>undefined</code>, as in non-existent, rather it can be <code>undetermined</code>, as in <em>something</em> is there, but not sure what.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-06-05T02:18:35.137Z
Answers.Text: If we were to use this analogy, how would we deal with variables declared prior to the `if`, which are accessible inside of it, or instance variables?
Answers.UserDisplayName: Ben I.
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-06-05T02:24:18.653Z
Answers.Text: Depends on which version. For Google Drive, the local computer (inside `if` has access to Google Drive (outside `if`) but Google Drive has no access to local computer. For the file system version emulate the `if` block's access with a symlink/shortcut. The file (variable) still belongs to the higher level, but the inner block has been granted access (probably by reference, either to the outer block's scope, or to the data itself).
Answers.UserDisplayName: Gypsy Spellweaver
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-06-05T01:56:28.943Z
Answers.LastActivityDate: 2017-06-05T01:56:28.943Z
Answers.UserDisplayName: Gypsy Spellweaver
Body: <p>One of the problems that my students will stumble upon every year is trying to create or initialize variables within an <code>if</code>/<code>else</code> block.  Why, oh why, can't they perform an operation like this?</p>
  
  <pre><code>if (someBooleanCondition){
      int x = 7;
  } else {
      int x = 0;
  }
  System.out.println(x);
  </code></pre>
  
  <p>After all, they reason that <code>x</code> has been created and initialized <em>no matter what</em>. This year, I discovered a nice way of explaining why this operation does not work. I will be posting my answer below, and I am also curious to see if others have different explanations that they use with their students (or corrections to mine).</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-09-22T08:49:18.497Z
Comments.Text: Scope of variables, and distinction between  declaration and initialization. Anyway students stumble every year, because they are  learning.
Comments.UserDisplayName: Michel Billaud
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2017-05-31T07:40:17.723Z
Comments.Text: [I have not tried this.] When showing how variables work, using the box metaphor, burn the box(and its content) at the end of its block. (you may want to do this out-side.)
Comments.UserDisplayName: ctrl-alt-delor
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2017-05-31T07:42:56.060Z
Comments.Text: Suggested better title “How to illustrate scope problem when creating variables inside `if` statements.” — with current title my first thought was “Yes you can”.
Comments.UserDisplayName: ctrl-alt-delor
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2017-05-31T09:45:49.513Z
Comments.Text: Its certainly a scope problem, and there's no problem with initialising (except for only initialising in one path).
Comments.UserDisplayName: Sean Houlihane
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2017-05-31T10:29:16.787Z
Comments.Text: That's a much better title. Thanks, @richard.
Comments.UserDisplayName: Ben I.
Tags: <java><ap-computer-science-a><scope>
Title: How to illustrate scope problem when creating variables inside `if` statements in Java
ThreadText: # How to illustrate scope problem when creating variables inside `if` statements in Java
  
  (Asked by: username_0 on 2017-05-30)
  
  One of the problems that my students will stumble upon every year is trying to create or initialize variables within an `if`/`else` block. Why, oh why, can't they perform an operation like this?
  
  ```
  if (someBooleanCondition){
      int x = 7;
  } else {
      int x = 0;
  }
  System.out.println(x);
  
  ```
  
  After all, they reason that `x` has been created and initialized *no matter what*. This year, I discovered a nice way of explaining why this operation does not work. I will be posting my answer below, and I am also curious to see if others have different explanations that they use with their students (or corrections to mine).
  
  **Tags:** java, ap-computer-science-a, scope
  
  > **Comments:**
  > **username_2**: [I have not tried this.] When showing how variables work, using the box metaphor, burn the box(and its content) at the end of its block. (you may want to do this out-side.)  
  > **username_2**: Suggested better title “How to illustrate scope problem when creating variables inside `if` statements.” — with current title my first thought was “Yes you can”.  
  > **username_3**: Its certainly a scope problem, and there's no problem with initialising (except for only initialising in one path).  
  > **username_0**: That's a much better title. Thanks, @richard.  
  > **username_1**: Scope of variables, and distinction between  declaration and initialization. Anyway students stumble every year, because they are  learning.  
  
  ---
  
  ## 7 answers
  
  ### Answer by username_6 on 2017-05-31. Score: 8
  
  I like to use an analogy from *Finding Nemo*. I tell them their programming language has the same problem Dory has - short-term memory loss. When a variable is declared within a block, Java will forget about the variable (just like Dory) after reaching the end of the the block. So a block declares the "scope" of Dory's short term memory.
  
  I've been concerned that students would think that the variables assigned in a block would lose their value after the block has ended, but I've never run into that problem for some reason.
  
  What's interesting about your example is that it won't even compile without the print statement! Java doesn't allow you to have a single declaration statement in an if or loop.
  
  > **Comments:**
  > **username_2**: I don't get what you mean in the 2nd paragraph. Do you mean that the variable still exist but have a value of undefined?  
  > **username_7**: I like it. I don't understand it, since I have no context for Dory (never watched Finding Nemo). That's part of the problem, finding context that matches what the students know.  
  > **username_6**: Here's a scene from the movie that you could show for reference:  https://www.youtube.com/watch?v=tESffhWs8l0  
  
  ---
  
  ### Answer by username_5 on 2017-05-31. Score: 6
  
  I explain this with a saying:
  
  > What happens in the block stays in the block
  
  Now, this isn't exactly how it works, but the idea is that the students remember that what they create in a block, is accessible only inside the block.
  
  So:
  
  ```
  if(shouldGoIntoBlock){
      int x=10;//created in the block
      System.out.println("printed from if body: "+ x);
  else{
      int x=-7;//created in the block
      System.out.println("printed from else body: "+ x);
  }
  System.out.println("printed outside of blocks: "+x);
  
  ```
  
  And they see in eclipse that the last print errors in compile time. That shows them that `what's created in the block stays in the block`.
  
  Next, I show them that there will also be an error if we define `x` before the `if-else` block, but the last line won't error. So they see that order of creation matters. Afterwards, I show them that creating `x` **after** the `if-else` block and **before** the last print, then there are no errors.
  
  The vast majority of the students understand the idea this way.
  
  They don't learn the stack segment of the computer at that time, but they see that order matters, and that makes learning about the stack easier for them when they learn it later in the year.
  
  
  ---
  
  ### Answer by username_0 on 2017-05-31. Score: 5
  
  I teach my students a bit about the stack as part of the very first lesson of the year. As a result, *they are aware coming in* that primitive variables are stored next to one another in the stack during runtime, and that the names of the primitive variables that we use in our code are removed by the compiler, and replaced with memory locations. They are also aware by this time that the stack is populated in the order in which variables are declared. Thus, I show them the following code, and ask a simple question:
  
  ```
  if (someBooleanCondition){
      int v = 18;
      int x = 7;
  } else {
      int x = 0;
      int v = o;
  }
  
  ```
  
  If we now wanted to run `System.out.println(x)`, it is true that `x` has a *value*, but what is its *location*? If `someBooleanCondition` is true, `x` will be at the higher of the two addresses, whereas if it is false, `x` will be at the lower of the two addresses How would the compiler know where to direct `System.out.println` to find `x`?
  
  Thus, the scope of variables declared during `if` statements must be local to that `if` statement, and cannot be used outside of it.
  
  
  ---
  
  ### Answer by username_8 on 2017-07-21. Score: 3
  
  You should be able to copy/paste block of codes that does not use other variables. But in your examples what if you want to copy/paste that to a code that already declared x? It would not work
  
  ```
  String x = "username_0.";
  ... a lot of code
  ...
  ...
  if (someBooleanCondition){
      int x = 7;
  } else {
      int x = 0;
  }
  System.out.println(x);
  
  ```
  
  In that case you want to print username_0.
  In block scope very often you need to use temporary variables to help you in some calculations and if you declared them in block then you want them to be cleared from stack when you get out of the block.
  
  
  ---
  
  ### Answer by username_9 on 2017-06-01. Score: 3
  
  Java, like C/C++ has block scope. When you create a variable in a block, the stack pointer moves to make room for it. When the block ends, the stack pointer moves back. This makes the variable inaccessible. It will be overwritten the next time the stack pointer moves over it.
  
  Python and JavaScript have function scope. Note, however that in ECMA 6, the `let` construct gives variables block scope.
  
  
  ---
  
  ### Answer by username_7 on 2017-06-05. Score: 2
  
  Something that might be helpful is using computers. Files created on the school computer in the classroom don't exist anywhere else. The file can be copied somewhere else, just like passing it back from a function copies the data, not the variable, to the caller.
  
  To demonstrate inner block access to variables at higher level you can use Google Drive, or something similar. A file on Google Drive can be edited using the web interface, so the local computer has access to the data. Still, the data belongs to Google Drive, so any other computer can also access it and change it. Just like all the loops and blocks in the current subroutine have access to data that belongs to the routine, or anything that is has access to from an even higher level. (The last part might be language dependent, and not apply in the language you utilize, and can be dropped.)
  
  A second option is just to use the directory structure of the computer itself. Creating `/home/mine/Documents/computer_class/project_1/notes.txt` will show `file not found` if you try to access `/home/mine/Documents/computer_class/notes.txt`. [Or for Windows `C:\Users\mine\Documents\Computer Class\Project 1\notes.txt` can't be edited using `C:\Users\mine\Documents\Computer Class\notes.txt`.]
  
  One thing to bear in mind is that not all languages enforce variables to be scoped at that granular of a level, or at all. In addition, how that scope is enforced, or implemented, can also be language dependent. If the language in use has been dissected by the class, then using its internal process can explain *why* it happens in that case.
  
  Unless the students will *always* be using that language, even after school, I think it's important to also make them aware that those rules may, or may not, apply, and that when they don't apply *side effects* can become an issue. In some cases the value isn't `undefined`, as in non-existent, rather it can be `undetermined`, as in *something* is there, but not sure what.
  
  > **Comments:**
  > **username_0**: If we were to use this analogy, how would we deal with variables declared prior to the `if`, which are accessible inside of it, or instance variables?  
  > **username_7**: Depends on which version. For Google Drive, the local computer (inside `if` has access to Google Drive (outside `if`) but Google Drive has no access to local computer. For the file system version emulate the `if` block's access with a symlink/shortcut. The file (variable) still belongs to the higher level, but the inner block has been granted access (probably by reference, either to the outer block's scope, or to the data itself).  
  
  ---
  
  ### Answer by username_4 on 2017-07-21. Score: 1
  
  Another benefit of block-level scope is that because variables defined in a block go out of scope at the end of that block, their values are then eligible for garbage collection (assuming no other variables point to them).
  
  Expand your example to use some really big object that takes up a lot of memory:
  
  ```
  if (someBooleanCondition){
      ReallyBigObject rbo = getReallyBigObject();
      rbo.doSomeProcessing();
  } else {
      ReallyBigObject rbo = getOtherReallyBigObject();
      rbo.doSomeOtherProcessing();
  }
  
  //rbo is out of scope and can be garbage collected!
  
  ```
  
  With the way Java works, the really big object goes out of scope at the end of the `if` or `else` block it's declared in. That means it can be garbage collected, and that's awesome because we don't have to worry about it anymore!
  
  It's possible to have function-level scope (like JavaScript), so the students' intuition isn't **wrong**. It's just that Java chose a set of rules to allow garbage collection to happen sooner rather than later.
  
  But as a result, this also means that you can't use a variable outside of its scope.
  
  
  ---
  
  