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Oct 23, 2021
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Tree traversal in smalltalk #453

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Tree traversal in smalltalk
Neverik Oct 5, 2018
0970e19
Fixed indent
Neverik Oct 5, 2018
69d5118
Kinda some changes
Neverik Oct 5, 2018
c0a07b2
Shortening text
Neverik Oct 5, 2018
3395e0a
Shortening code
Neverik Oct 5, 2018
8e3300b
typo fix
Neverik Oct 5, 2018
d56619e
Merge branch 'master' into tree_traversal_smalltalk
Neverik Oct 7, 2018
f051340
Merge branch 'master' into tree_traversal_smalltalk
Neverik Jan 3, 2019
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101 changes: 101 additions & 0 deletions contents/tree_traversal/code/smalltalk/tree_traversal.st
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Original file line number Diff line number Diff line change
@@ -0,0 +1,101 @@
Object subclass: #Node
instanceVariableNames: 'children data'
classVariableNames: ''
package: ''

Node>>children
"Children getter."
^ children

Node>>children: newChildren
"Children setter."
children := newChildren.

Node>>data
"Data getter"
^ data

Node>>data: newData
"Data setter"
data := newData.

Node>>dfsRecursive
"Recursive depth first search."
data isNil ifFalse: [
Transcript show: data.
Transcript cr.
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You do this a lot:

Transcript show: something.
Transcript cr.

That can, and should, be simplified to this:

Transcript show: something; cr.

I'm not gonna leave a comment everywhere you do it, because that'd be a lot.

].
children collect: [ :each | each dfsRecursive].
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What exactly is an each, and why does children contain those, instead of... children? (I'm trying to tell you to rename each to child, because the parameter isn't representing an "each", it's representing one of the several children)


Node>>dfsRecursivePostorder
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Why not take a visitor and return an array instead of just printing all the elements?

Node>>dfsPostorder: ary into: visitor
  children inject: ary into: [ :child | child dfsPostorder: visitor ]
  data isNil ifFalse: [ ary add: visitor value: data. ]

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I did this taking reference from the Python example.

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Probably printing will be more clear (I guess).

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@Neverik Fair enough! This isn't a necessary change, just an idea.

"Recursive depth first search (post-order)."
children collect: [ :each | each dfsRecursivePostorder ].
data isNil ifTrue: [ ^ self ].
Transcript show: data.
Transcript cr.

Node>>dfsInOrderBinaryTree
"Recursive depth first search on a binary tree in order."
children size = 2 ifTrue: [
(children at: 1) dfsInOrderBinaryTree.
Transcript show: data.
Transcript cr.
(children at: 2) dfsInOrderBinaryTree.
^self.
].
children size = 1 ifTrue: [
(children at: 1) dfsInOrderBinaryTree.
Transcript show: data.
Transcript cr.
^self.
].
children size = 0 ifTrue: [
Transcript show: data.
Transcript cr.
^self.
].
Transcript show: 'This is not a binary tree!'.
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This block of ifs can be simplified a bit:

children size > 2 ifTrue: [
  Transcript show: 'This is not a binary tree!'; cr.
  ^self.
]
children size = 2 ifTrue: [
  (children at: 1) dfsInOrderBinaryTree.
]
Transcript show: data; cr.
children size >= 1 ifTrue: [
  (children at: 0) dfsInOrderBinaryTree.
]
^self.

Transcript cr.
children length

Node>>dfsStack
"Depth-first search with a stack."
| stack top |
stack := Stack new.
stack push: self.
[stack size > 0] whileTrue: [
top := stack pop.
Transcript show: top data.
Transcript cr.
top children reverseDo: [ :child |
stack push: child
].
].

Node>>bfs
"A breadth-first tree search using queues."
| queue current |
queue := LinkedList with: self.
[ queue size > 0 ] whileTrue: [
current := queue first.
queue removeFirst.
Transcript show: current.
Transcript cr.
current children collect: [ :child |
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Messy indenting here, too; again, please switch to all spaces.

queue addLast: child
].
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I think that period is unnecessary.

]

| test |
test := Node new: 1 children: { Node new: 2.
Node new: 3 children: { Node new: 4.
Node new: 5. } }.
test dfsRecursive.
Transcript cr.
test dfsRecursivePostorder.
Transcript cr.
test dfsInOrderBinaryTree.
Transcript cr.
test dfsStack.
Transcript cr.
test bfs.
14 changes: 14 additions & 0 deletions contents/tree_traversal/tree_traversal.md
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Expand Up @@ -32,6 +32,8 @@ This has not been implemented in your chosen language, so here is the Julia code
[import:3-27, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:1-5, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="st" %}
[import:1-20, lang:"st"](code/smalltalk/tree_traversal.st)
{% endmethod %}

Because of this, the most straightforward way to traverse the tree might be recursive. This naturally leads us to the Depth-First Search (DFS) method:
Expand Down Expand Up @@ -66,6 +68,8 @@ Because of this, the most straightforward way to traverse the tree might be recu
[import:31-35, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:7-10, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="st" %}
[import:22-28, lang:"st"](code/smalltalk/tree_traversal.st)
{% endmethod %}

At least to me, this makes a lot of sense. We fight recursion with recursion! First, we first output the node we are on and then we call `DFS_recursive(...)` on each of its children nodes. This method of tree traversal does what its name implies: it goes to the depths of the tree first before going through the rest of the branches. In this case, the ordering looks like:
Expand Down Expand Up @@ -108,6 +112,8 @@ Now, in this case the first element searched through is still the root of the tr
[import:37-41, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:12-15, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="st" %}
[import:30-35, lang:"st"](code/smalltalk/tree_traversal.st)
{% endmethod %}

<p>
Expand Down Expand Up @@ -145,6 +151,8 @@ In this case, the first node visited is at the bottom of the tree and moves up t
[import:43-62, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:17-31, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="st" %}
[import:37-59, lang:"st"](code/smalltalk/tree_traversal.st)
{% endmethod %}

<p>
Expand Down Expand Up @@ -192,6 +200,8 @@ In code, it looks like this:
[import:64-73, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:33-41, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="st" %}
[import:61-73, lang:"st"](code/smalltalk/tree_traversal.st)
{% endmethod %}

All this said, there are a few details about DFS that might not be idea, depending on the situation. For example, if we use DFS on an incredibly long tree, we will spend a lot of time going further and further down a single branch without searching the rest of the data structure. In addition, it is not the natural way humans would order a tree if asked to number all the nodes from top to bottom. I would argue a more natural traversal order would look something like this:
Expand Down Expand Up @@ -231,6 +241,8 @@ And this is exactly what Breadth-First Search (BFS) does! On top of that, it can
[import:65-74, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import:43-51, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="st" %}
[import:75-87, lang:"st"](code/smalltalk/tree_traversal.st)
{% endmethod %}

## Example Code
Expand Down Expand Up @@ -272,6 +284,8 @@ The code snippets were taken from this [Scratch project](https://scratch.mit.edu
[import, lang:"php"](code/php/tree_traversal.php)
{% sample lang="crystal" %}
[import, lang:"crystal"](code/crystal/tree-traversal.cr)
{% sample lang="st" %}
[import, lang:"st"](code/smalltalk/tree_traversal.st)
{% endmethod %}


Expand Down