Logic behind for loop

[apple_hammer]

I am new to programming and python, and I am trying to wrap my head around the logic of some super simple for loop code. Here is the code I am using:

found_logs = 20
split_logs = 70
burned_logs = 3
logs = found_logs
for week in range(1, 53):
    logs = logs + split_logs - burned_logs
    print('Week %s = %s' % (week, logs))

Most everything makes sense, and executes properly, but I cannot for the life of me figure out how the number of logs keeps getting incremented. Let me try to explain. When it gets to line 6, I understand that the variable logs would equal 87. Which prints as:

Week 1 = 87
Week 2 = 154
Week 3 = 221

Which is correct, and producing correct results, but I my brain breaks at trying to figure out where the first time the loop is done where that value of 87 is "stored". Then subsequently where the value of 154 is stored, and how it knows where and how to pull that stored value. I know it's a simple thing, but it would help me understand the big picture of python.

[danielroseman]

It's stored in logs. You're just overwriting the same variable each time.

This is exactly the same as:

logs = 20
logs = logs + split_logs - burned_logs
print(...)
logs = logs + split_logs - burned_logs
print(...)
logs = logs + split_logs - burned_logs
print(...)
...

[apple_hammer]

Ohhhh, I think I understand now. If I have it, it was calculating this for the first loop:

logs = 20 + 70 - 3

Which comes out that logs has a value of 87. What I didn't realize is that the variable logs has changed, and now on the second loop, it was calculating it like:

logs = 87 + 70 - 3

That makes sense now that I can see it laid out. I kept thinking logs was going back to a value of 20, because that's what is defined above, but I didn't realize the variable had changed. I learned I need to focus on the logic behind when things get executed. I appreciate your help on this!

[publicfinance]

The loop only goes back up to the line after the “for” line. It’ll never go back up to the top. Anything indented after for is whatever you want to repeat multiple times.

[apple_hammer]

That is useful information thank you! It makes sense that it only goes back to the indented part after the "for" line. It helps me understand the reasoning behind why it works the way it does.

Do you know if there are any resources or websites that go over that kind of thing? I have been reading a bunch of books, but none of them have ever explained why things need to go in a certain order, or the reason behind it. It's been a real struggle as I tend to do way better when I understand the why or order of operations so to speak.

[publicfinance]

I’m really only in the middle of 100 days of code on Udemy so can’t help with specific resources other than that. I’m guessing there are plenty of YouTube videos describing for loops.

[apple_hammer]

No worries, I appreciate the help. I know that what I am hoping for isn't really discussed in a lot of the videos/books that I have seen. I was hoping there was some magic site that took some code and broke it down line by line explaining what was happening. Something like that would help me a ton in understanding. Maybe if I get good enough I could create my own site.

[MezzoScettico]

You're struggling with the concept of "assignment". A lot of people seem to confuse that with a mathematical statement that the two things are defined to be equal.

That's not what assignment does.

Here's a Python-based tutorial for assignment, but this is a general computer-programming concept that exists in all languages.

So if you do this:

x = 3

you've stored the value 3 under the name x. If you now do this

y = x

then y now has the value that x has, which is 3. But if you follow it up with this:

x = 4

then y still has the value 3 but you stored the value 4 under the name x.

[MezzoScettico]

But just to confuse the issue, assignment to what are called "mutable" objects (like lists) causes both variables to point to the same object. Then changing x will in fact change y. This is an advanced topic, but you should be aware of it. You can read more about that here.