FIFO function – manual approach

Question

Python novice here. I am working on an assignment that has me a bit stumped.

The goal is set up a simple FIFO system, without using any imported libraries.

My attempt so far is incorrect and I am looking for some suggestions on how to fix it.

Attempt:

requests = [4, 32, 5, 8, 7, 4, 8] # Will be any random integer inputted by user, provided this list as an example
cache = []

def fifo():
    for page in requests: 
        if page not in cache:
            print(page, "miss")
            cache.append(page) # This isn't right but not sure where to add?
        if page in cache:
            print(page, "hit")
            cache.remove(page) # If a page is already in the cache, it doesn't need to be added again - not sure if this is correct either?
        if len(cache) > 4: # max size of cache = 4
            cache.pop(0) # Removes first indexed item until cache = 4 elements
    print(cache)
    return

# Each page should be printed as a hit/miss if included/not included in the cache
# Ignoring hits/miss being printed, the required output = print(cache) = [32, 5, 7, 8] 
# i.e. the most recent 4 requests, previously requested pages (e.g. 4) shouldn't be included

How should I go about correcting the above? Open to alternative suggestions as I appreciate the above is likely far away from the optimal solution.

Thanks

Asked By: newpythonstudent

||

Source

Answer 1

Given the text in the comments at the bottom of your code, it would appear that this is closer to what is required:

def f(requests, cache):
    for page in requests:
        if page in cache:
            cache.remove(page)
            print(page, "hit")
        else:
            print(page, "miss")
        if len(cache) > 3:
            cache.pop(0)
        cache.append(page)


requests = [4, 32, 5, 8, 7, 4, 8]
cache = []
f(requests, cache)
print(cache)

If would make a bit more sense for the function to operate on individual ‘pages’ though (also showing an approach using a global cache):

cache = []
def f(page):
    global cache
    if page in cache:
        print(page, "hit")
        cache.remove(page)
    else:
        print(page, "miss")
    if len(cache) > 3:
        cache.pop(0)
    cache.append(page)


for page in [4, 32, 5, 8, 7, 4, 8]:
    f(page)


print(cache)

However, note that both these solutions show a different outcome than the question: [5, 7, 4, 8] instead of [32, 5, 7, 8].

The question seems to get the answer wrong itself, unless the actual maximum cache size isn’t 4, but 5. Consider this: how can the cache still contain 4, when it most recently got 32, 5, 8, and 7 and its size would be 4? So, when 4 comes around again, it has no memory of it and should update to [5, 7, 4, 8]. Where are you getting this assignment?

Also note that I wouldn’t want to recommend using a global variable as in the second solution – however, all this seems to be a step up on the way to writing a Cache class of sorts, which would make the cache a variable internal to the class or object, instead of a global. Such a class could look like this:

class Cache:
    def __init__(self, size=4):
        self._size = size
        self._cache = []

    def hit(self, page):
        if (result := (page in self._cache)):
            self._cache.remove(page)
        if self._cache.__len__() == self._size:
            self._cache.pop(0)
        self._cache.append(page)
        return result

    def __str__(self):
        return str(self._cache)


c = Cache(4)
for p in [4, 32, 5, 8, 7, 4, 8]:
    if c.hit(p):
        print('hit', p)
    else:
        print('miss', p)
print(c)

Answered By: Grismar

FIFO function – manual approach

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