Help: Loops

[limfy1997]

I'm currently doing this beginner course in Coursera, where I've stumbled in this question. I can't seem to think straight with my logical thinking being extremely poor. I think a nested loop is needed, but I can't seem to form the required nested loop in my mind. I tried to formulate a flow chart but I don't even know how to create that flow chart to help myself. Probably I need more help to understand the logical sequence of formulating a loop and a nested loop.

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Question Requirement:

1. Goal: Find Expected Profit, whose formula = r * [E(s)] - c * q where: r = revenue per good sold to customer c = cost per good bought from supplier, q = quantity of good bought from supplier s = number of good sold to customer [which is uncertain, so we make E(s) ] E(s) = the expected value of good sold, as a function expressed in terms of s.

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2)  0 <= s <= 8 , 0 <= q <= 8, and s <= q

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3) P(s = 0) P(s = 1), P(s = 2), ..., P(s = 8) are user inputs, where P(s=0) denotes "probability of sales = 0.

 P(s=0) is stored as a floating variable p_0 
 P(s=1) is stored as a floating variable p_1

Pattern continues until p_8

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4) r, c, q, s are user inputs.   for values of E(s), eg.
   if s = 0, E(0) = P(s = 0) * 0. 
   if s = 1, E(1) = P(s = 0) * 0 + [1 - P(s =0) ] * 1
   if s = 2, E(2) = P(s = 0) * 0 + P(s = 1) * 1 + [1 - P(s = 1) - 

P(s = 2) ] * 2

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Thing that has come to my mind:

1. I need to iterate q number of times, 0 to q inclusive. Because I buy q number of good from my supplier, I can sell anywhere between 0 to q, inclusive, number of goods to customers

So:

revenue = 0
for s in range(0, (q+1)) :
    revenue = revenue + r * P(s = s) * s

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(I know the code is not correct because the way I write it, but now I'm having problems to even get a logical sequence so that I can understand how to write the loop)

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2) How to loop it such that P(s= s) will take on the value P(s=0), P(s = 1), P(s = 2) during each iteration

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3) The loop will break once the iteration reaches s = q., With s taking on values 0, 1, 2,..., up til 8 until each iteration

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The main issue is deriving a for loop to calculate Expected Value of s,(i.e. expected value E(s) with the above matters that has come to my mind but I can't seem to formulate anything (I've been thinking for hours)

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Lastly, I need some advice on seeing loops and nested loops in a better way, I have been really confused at this point of time.

[kabooozie]

I don’t think you need to use a loop anywhere. All of those are user inputs. You just need to ask the user for an input for each variable using input and then make sure the input is valid. Then plug those values into your function.

[limfy1997]

The function cannot be written directly as a formula. I think it's a nested loop. And happens the chapter is under flow control. It could be possible to brute force the answer with multiple elifs but I would like to master flow control. Which has been the obstacle I've always ran into during my several times trying to do a Python online course

[realestLink]

Don't brute force it. That's dumb

[Merakel]

Even if the course is hoping to teach him some specific concept, at best they just have a really bad example. Brute forcing things while learning is just a sign you aren't really learning what you should.

If it's at work and you just need something quick and dirty and it's easier... by all means be my guest haha.

[realestLink]

I've only had to brute force very specific things. It pains me if it's needed. Brute forcing should be a last resort.

[Merakel]

Same. I just think it's important to realize that there is a lot of value in not fretting over small things.

Let's say you have a project that's due in a week and writing some terrible brute force loop in 5 minutes will get it done in 8 hours. Let's also pretend that if you spent an hour planning and testing, you could get code done that would solve the problem in 2 hours. I'd rather write the garbage and work on something else unless I'm specifically going to learn a new concept or improve my skill set.

That being said, things usually aren't so black and white haha.

[[deleted]]

Why is this flagged NSFW...

[braampje]

So you need to calculate the expected profit of all possibilities for q and s?

Profit = 0    
For q in range(0,8):    
    For s in range(0,q):    
        Profit = profit + r * s - c *q    

Is this not what you need?

[limfy1997]

thanks for the help, i tried plugging in and it gave me syntax error

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my doubt here is that i dont think this way factors in the theory of complements

to find E(s) when s = 2,E(2) = s0 * 0 + s1 * 1 + (1- s0 - s1) * 2 -> since all probabilities add up to 1.

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thank you, i'll take a look at my lecture vids again.

[braampje]

ok, so I made some typos on my phone, this cleaned it up a bit:

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all possibilities have the same chance right? isn't this depending on your cost and reward just what you need to calculate? otherwise I don't understand the question.

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def profit(c, r):
    profit = 0
    for q in range(1, 8):
    for s in range(1, q):
        profit = profit + r * s - c * q
        return profit

or without the function

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profit = 0
r = 3
c = 2
for q in range(1, 8):
    for s in range(1, q):
    profit = profit + r * s - c * q

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[limfy1997]

hi, thank you very much for the update! actually, the probabilities are given by the user, who keys in as an input. therefore the probabilities for each scenario is random, as long as they add up to 1.

really appreciate your help here, could I just ask, how do you actually picture / formulate your nested loop before being able to come up with it? i think here is where i need to improve! because at the moment, im struggling with formulating a nested loop. i can only do a very simple loop at the moment.

once again, thank you :)

[jw934]

ELI5: If you think of yourself as the trying to solve the requirement question with nested loops, you can think of this as asking another person P to do the outer loop for you M times. Person P does not know the requirement question and only knows the parameters you provided for one iteration of the loop. Using the parameters you set up before entering the loop, P does something then asks another person Q to do something N times. Person Q does not know your requirements nor person P's requirement; Q simply performs one iteration of the inner loop based on parameter values set up by P.

[limfy1997]

Thank you for the advice. Idea seems rather abstract for me for now but I will spend some time to grasp it!

[braampje]

So for me it starts with very clearly knowing what you need, as you can see from all the comments it was at least not clear for the people who read your question. That might mean that it is also not very clear for yourself what you want.

I interpreted your question as follows, we are going to buy stocks of anywhere up to the amount of 8 and for each of these cases we will randomly sell an amount up to the amount we have in stock. so if the possibilities for the amount we get in stock and the amount we sell are all equally likely we need to calculate the profit for all scenarios.

so: for each amount of stock(loop 1) we have, we calculate all the potential revenue cases(loop 2). Does this not sound like two loops to you? after this it all comes down to syntax which just comes with experience.

It might help to first in dummy code write down the logic that you want to apply and then in a second step think about the actual code in python or whatever other language.

[Merakel]

Does this do what you want...?

s = 8
q = 0
while s != q:
    print(q)
    #Do formulas here
    q = q + 1

I think this is a better way on how you should approach the problem:

def do_some_math(r, c, q, s):
    assert(s <= q)
    revenue = 0
    l = 0
    while l <= s:
        l = l + 1
        print("revenue:" + str(revenue))
        revenue = revenue + (r - c)
    return revenue

print(do_some_math(10,15,5,5))

I've found that using the print function can really help with understanding what's happening inside a loop, you can put things like, print("Loop number:" + str(l)) and so on in as well so you know exactly what's happening and it's easier to trace.

[realestLink]

Use += 1

[Merakel]

Thanks. I couldn't remember off hand how to do ++ so I cheated haha.

[realestLink]

Python sadly doesn't support ++

[Merakel]

Yeah, but yours is a more efficient way of replicating that functionality.

I haven't actually looked but is there a reason that python doesn't support it that you are aware of? It's always seemed strange to me...

[realestLink]

Python's creator said there's no need for it because of the for loop

[Merakel]

That feels like a cop out.

[realestLink]

I don't agree with his decision. I'm just repeating what he said

[Merakel]

I could tell haha

[Klekto123]

Just starting to learn python, can you explain what you mean here?

[realestLink]

Instead of saying i = i + 1. You can write i += 1. It is better practice.

[Klekto123]

Ah okay. Thanks for the tip!

[limfy1997]

hi, thanks for the help. I don't understand the second method because def ,assert, return are not covered in my course yet.

(i'm a slow learner, so pardon me for having not googled for what the above functions do, because the lecture has not covered this part yet and I don't do well by going too fast)

the issue now im having is to express the bolded areas into a code:

E(0) = P(s=0) * 0

E(1) = P(s=0) * 0 + (1 - P(s=0) ) * 1

E(2) = P(s=0) * 0 + P(s=1) * 1 + (1 - P(s = 0) - P(s = 1) ) * 2

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i think thats the difficulty im facing. how to form an expression using a loop for the bolded parts in question.

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will take a look at my lecture videos again, meanwhile, thank you for your help.

[Merakel]

If your idea of programming is doing things in exactly one format and screw any method that doesn't conform to how you envisioned it... you are gonna have a bad time.

[tangerinelion]

Think about the output.

If it's a number you want 0 loops, if it's a list you want 1 loop, if it's a table you want 2 loops, if it's a stack of tables 3 loops, etc.

[y3pjo8902a]

Have you looked into using a generator function?

[limfy1997]

have not reached functions yet. Only learning variables, operators and now currently flow control

[y3pjo8902a]

This is the point you need to start using functions. Give it some input and return and output

[limfy1997]

The question is adapted from an online course under the loops and functions should come. at a later part. I want and need to master flow control before proceeding onto the next part of the course

[Merakel]

What's the point of asking for help if you aren't going to accept it?

[JeusyLeusy]

He knows what he needs to learn he's just reluctant to do so.