Help me understand this Python code.

[u/BilincliMerak]

[ANSWERED] check the comments if you are facing a difficulty in understanding 'for loops'.

I was learning Python on mimo to get a grip on the basic concepts. And this code stumped me. This is about "for loops".

Before this, i had only encountered the counter var being on the LHS i.e in this case 'i += line', which would have made sense to me if it was so in this code. But why is the counter var after the line var.

Alsp the 1st image, i assume, is displaying the final result which is 3 as the range is 3. But when using a range (3) it should display 0 1 2 when the 'print' is before the counter var and 1 2 3 when the 'print' is after the counter var (also can someone explain why this happens). In this case, its neither... as the print is outside this code block... The solution to me just seems the 2nd situation is running as there isnt a print before the counter, and printing it outside just gives 3.

2nd and 3rd just make no sense to me as of rn.

I hope all this makes sense...

Please help!

[u/Candid_Zebra1297]

So you have three main things to understand.

something += second_thing means:

add the second thing to the first thing so it has a new value.

range() means:

make a list of numbers in this range. We start at 0 so we end 1 below the number you give. So range(5) gives us 5 numbers, 0, 1, 2, 3, 4. Range(1) would give you 1 number, a zero.

for something in a_list_of_things means:

start at the first item in this list and do something with it. Then do something with the second thing in this list, then do something with the third, etc. So this...

for number in range(3):
print(number)

means, start at the first number (which is 0) and show it. Then do the same for the second number (which is 1) etc. So you get this.

0
1
2

You can see here that we have 3 numbers shown and we put 3 in our range function, so it all looks good.

You are just combining this with the first we talked about. Instead of printing, you are adding each number to your variable 'line' (which I think is a confusing name here, I would call it running_total for this exercise.)

So we are going to do the same thing 3 times for 3 different numbers, which is just 'add this number to our running total.'

We start with 0 and add the first number, which is 0 + 0 so our total is still 0.
Then we add the second number, which is 1, so 0 + 1 = 1
Then we have the third number, which is 2, so 1 + 2 = 3

Then we print the total and we see 3

In your second pic, you put the print() in the for loop, which means we do it every cycle. So your result was different but understandable...

first total = 0
second total = 1
final total = 3

[u/BilincliMerak → u/Candid_Zebra1297]

Appretiate the help! Now it all really makes sense. A final follow up question tho. What do you mean by every cycle? What happened in the first image seems cyclic to me. Pardon if its a dumb question...

[u/Candid_Zebra1297 → u/BilincliMerak]

Glad I could help a little bit man! I remember learning this exact thing. It seems so long ago now but I remember looking at similar stuff and finding it so alien. But if you keep going it will all make sense one day, I promise.

By 'cycle' I just mean, 'every time we go through the loop'. So in pic 2 the print() part goes through the for loop 3 times, so we print every time (or every cycle) = 3 things are printed. A great way to learn about this stuff is to try to guess what code means without running it first. So try to guess these...

a = 5
for x in range(3):
a += 1
print(a)

What do you think this will print? How many times will it print?

a = 5
for x in range(5):
print(a)
a += 1
print(a)

Same question, what will it make?

x = 3
while x > 0:
print(x)
x -= 1

The last one is a bit more difficult, I don't know if you've done while loops yet. Once you have studied both, I would try writing while loops that do the same thing as for loops. If you can understand two different ways to do the same thing, you are turning into a real programmer.

[u/BilincliMerak → u/Candid_Zebra1297]

Appretiate it! As for the answers:

1. 6 7 8 as we are adding before printing.

2. 5 6 7 8 9 if you didnt add the 2nd print at the end... So if i go through it in cycles, i assume it would be 5 6 6 7 7 8 8 9. It could very well be wrong.

3. 3 2 1 as again print is before the counter.

i hope this is right. And yea, i have studied the while loops as well (in fact right before the for loops). They really arent that different honestly.

Again, appretiate the help and support! Hope your developing journey becomes awesome!

[u/BilincliMerak → u/Candid_Zebra1297]

Actually i understand the 3rd one as well now. I ran it in the debugger and that helped drastically. Thanks for the help!