Before I get started, if you need some background on what a heap is:

https://en.wikipedia.org/wiki/Heap_(data_structure)

This discussion is about a *minimum heap*.

I recently spent the last week helping one of my paying students to write a minimum-heap data structure in Python3.

Given the following class definition for a Node:

```
class Node:
"""
Class definition shouldn't be modified in anyway
"""
__slots__ = ('_key', '_val')
def __init__(self, key, val):
self._key = key
self._val = val
def __lt__(self, other):
return self._key < other._key or (self._key == other._key and self._val < other._val)
def __gt__(self, other):
return self._key > other._key or (self._key == other._key and self._val > other._val)
def __eq__(self, other):
return self._key == other._key and self._val == other._val
def __str__(self):
return '(k: {0} v: {1})'.format(self._key, self._val)
__repr__ = __str__
@property
def val(self):
"""
:return: underlying value of node
"""
return self._val
```

Be able to define the missing methods on a Minimum-Heap data structure:

```
class Heap:
"""
Class definition is partially completed.
Method signatures and provided methods may not be edited in any way.
"""
__slots__ = ('_size', '_capacity', '_data')
def __init__(self, capacity):
self._size = 0
self._capacity = capacity + 1 # additional element space for push
self._data = [None for _ in range(self._capacity)]
def __str__(self):
return ', '.join(str(el) for el in self._data if el is not None)
__repr__ = __str__
def __len__(self): # allows for use of len(my_heap_object)
return self._size
# DO NOT MODIFY ANYTHING ABOVE THIS LINE
# Start of Student Modifications
def _percolate_up(self):
pass
def _percolate_down(self):
pass
def _min_child(self, i):
pass
def push(self, key, val):
pass
def pop(self):
pass
@property # do not remove
def empty(self):
pass
@property # do not remove
def top(self):
pass
@property # do not remove
def levels(self):
pass
```

Further, there is a function we wish to write outside of the `Heap`

class itself, `def most_common_x(vals, X):`

, which will take a list of values, `vals`

, and then return a `set`

of the `X`

most common values in the list.

```
def most_common_x(vals, X):
pass
```

I'm going to walk through each method, one by one, just as I did with my student.

It is important to test your code as you go along, and to not just rely on automatic testing solutions. Know how to manually test and verify that the output of a method is what you want before assuming that something works.

Let's begin!

### push()

`push()`

depends on `_percolate_up()`

, otherwise we could just write:

```
def push(self, key, val):
self._data[self._size] = Node(key,val)
self._size += 1
```

and be done.

This much would allow us to create a really simple heap:

```
h = Heap(4)
h.push(4, 'a')
h.push(2, 'b')
h.push(3, 'c')
h.push(1, 'd')
```

At this point, we should start looking at `_percolate_up()`

.

### _percolate_up()

`_percolate_up()`

is all about swapping the bottom node with its parent nodes until its parents are no longer greater than it.

```
4
/ \
2 3
/
1
```

Take the heap above. If we were to call `_percolate_up()`

on it, it should result in a heap that looks more like:

```
1
/ \
2 3
/
4
```

- So, we'd need to start at the end of the heap.
- Lets create an index and set it equal to the last valid index in our data list.
- Now, lets loop through our list until we've reached the root node:
- Grab the node for our current index. This is
`child`

. - Calculate the
`parentIndex`

, which is the index`i`

minus`1`

divided by`2`

as a whole`int`

. - Grab the
`parent`

node given its index. - If both
`child`

and`parent`

are not`None`

:- If the
`child`

is less than the`parent`

:- Swap them

- If the
- Update the index to point to
`parent`

- Grab the node for our current index. This is

```
def _percolate_up(self):
i = self._size - 1
while i/2 > 0:
child = self._data[i]
parentIndex = (i-1)//2
parent = self._data[parentIndex]
if child is not None and parent is not None:
if child < parent:
tmp = parent
self._data[parentIndex] = child
self._data[i] = tmp
i = parentIndex
```

### _percolate_down()

Turns out that `push()`

also depends on `pop()`

, which depends on `_percolate_down()`

.

Turns out that `_percolate_down()`

depends on `_min_child()`

!

### _min_child()

This one is just a matter of patiently handling each case.

- If empty
- If the "child" index is out-of-bounds
- If the given index has no Node
- If node has no children
- If node has only left child
- If node has only right child
- If node has both children

This actually depends on `empty()`

but we'll do that in a minute.

```
def _min_child(self, i):
if self.empty:
return -1
if i*2+1 > self._size:
return -1
if self._data[i] is None:
return -1
left = i*2+1
right = i*2+2
left_val = self._data[left]
right_val = self._data[right]
if left_val is None and right_val is None:
return -1
if left_val is not None and right_val is None:
return left
if left_val is None and right_val is not None:
return right
if self._data[left] < self._data[right]:
return left
return right
```

### back to _percolate_down()

So, this time, stepping through branches of the tree in the opposite way to `_percolate_up()`

:

- While we're not at the end of the heap:
- Grab the minimum child
- If valid and the current node is greater than the minimum child:
- Swap them

- Update the index to point to the minimum child's index

```
def _percolate_down(self):
i = 0
while i*2 <= self._size-1:
mc = self._min_child(i)
if mc == -1:
break
if self._data[i] > self._data[mc]:
tmp = self._data[i]
self._data[i] = self._data[mc]
self._data[mc] = tmp
i = mc
```

### empty()

Super easy let's knock it out:

```
def empty(self):
if self._size == 0:
return True
return False
```

### pop()

Now that we've written `_percolate_down()`

and `empty()`

, we can take care of this one.

- If the heap is empty, return
`None`

- Our return-value is just the head node, or the front of our data list
- Swap the front node with the end of our heap / the last value / node
- Decrement size
- Clear the last element (effectively we swap the smallest and biggest elements, nullify the last node, then...)
- Percolate the head node down to its new position

```
def pop(self):
if self.empty:
return None
rv = self._data[0].val
self._data[0] = self._data[self._size-1]
self._size -= 1
self._data[self._size] = None
self._percolate_down()
return rv
```

### return to push()

With both `pop()`

and `_percolate_up()`

taken care of, it is time we took care of `push()`

:

- Create a new
`Node`

in the last position in our heap - Increment heap-size
- Percolate the new node up into its appropriate position
- If we've reached capacity, pop the smallest node out

```
def push(self, key, val):
self._data[self._size] = Node(key,val)
self._size += 1
self._percolate_up()
if self._size == self._capacity:
self.pop()
```

### top()

Also super-easy. Just grab the top node so long as heap is not empty.

```
def top(self):
if self.empty is False:
return self._data[0].val
return None
```

### levels()

This one is a bit longer.

Levels is to return a list-of-lists, with each heap-level's nodes in each list (so, one list per level).

Edge-case 1: Empty heap. Return an empty list.

- Start at the beginning of the heap
- Create a new list for this level
- If it is the first list we are adding to:
- Add the root node to the levelOne list
- Add the levelOne list to our return list

- Else:
- Calculate the indices of the
`startIndex`

and`endIndex`

of this level in our heap's data list - Add each node to our levelList
- If the levelList is not empty, add it to our return list

- Calculate the indices of the

```
def levels(self):
ourList = []
if self.empty:
return ourList
prevStartIndex = 0
while prevStartIndex < len(self._data):
thisLevel = []
if len(ourList) == 0:
thisLevel.append(self._data[0])
prevStartIndex = 0
ourList.append(thisLevel)
else:
startIndex = (prevStartIndex * 2) + 1
endIndex = startIndex * 2
j = startIndex
while j < len(self._data) and j <= endIndex:
node = self._data[j]
if node is not None:
thisLevel.append(self._data[j])
j += 1
prevStartIndex = startIndex
if len(thisLevel) != 0:
ourList.append(thisLevel)
return ourList
```

Lastly, the function outside of the `Heap`

class:

### most_common_x(vals, X)

- Create an empty dictionary
- For each element in vals:
- Count it in the dictionary by incrementing its value for an element key

- Create a new Heap
- Given the dictionary keys:
- Grab the value off the dictionary for each key, this is the number of times that element appeared in the list
`vals`

- Push it onto the heap

- Grab the value off the dictionary for each key, this is the number of times that element appeared in the list
- Pop off enough nodes such that remaining Nodes are X in count
- Add remaining nodes to a return
`set`

Code with some comments:

```
def most_common_x(vals, X):
# step 1 - build the dictionary
d = {}
for element in vals:
# check to see if there is a count for our given element
# if not, return 0 for default
count = d.get(element, 0)
# increment count
count += 1
# re-assign new count back to dictionary
d[element] = count
# step 2 - build the heap from the dictionary
d_keys = d.keys()
heapSize = len(d_keys)
heap = Heap(heapSize)
for key in d_keys:
count = d[key]
heap.push(count, key)
# step 3 - grab the leaf nodes and add to our return set
returnSet = set()
while len(heap) > X:
heap.pop()
while not heap.empty:
returnSet.add(heap.pop())
return returnSet
```

This is, admittedly, a quick write-up, but I can always flesh it out further if you need clarification, just ask! :D

# If you need a Computer Science tutor, code reviewer, or just someone to pair program with, hit me up

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## Discussion

You might want to look at my code. It is in C#, but I have a generic heap, and all you need to make a heap is a compare function. You don't need two values when you add to a heap. You can have multiple heaps of the same type but they can use different priority algorithms based on the delegate you pass in on construction.

github.com/ZacharyPatten/Towel

This was a student of mine's homework project. His professor's specifications required the Heap to store Node objects with key-values on them. I understand that you can do this many different ways, but thank you for the clarification. I like the idea of initializing with a different delegate to indicate algorithm changes :)

If you have to store keys and values, all you do is (in C# using my example) this:

Heap<(KeyType, ValueType)>

So there should only ever be one generic type, even if you need to store multiple values, you just pass in a type with multiple values.

Just wanted to clarify that. Let me know if that doesn't make sense and I will gladly elaborate.

But yeah. Hope the student had fun figuring out the algorithms of a heap. :)

Yeah, your template looks a lot like the templating stuff I'd use in C++ and Java (I don't know C# but I understand it is similar enough).

We both had fun working through the methods :D It is definitely good practice.