#🔒 Python for loop range confusion

118 messages · Page 1 of 1 (latest)

quiet ivy
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Hey guys n-1 concept is use in for loop
right ?
but , for i in range(5,-1,-1): as -1 -1 = -2
not make sense

uncut waspBOT
#

@quiet ivy

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blissful pecan
#

hello

quiet ivy
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hello

blissful pecan
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alright, lets just get it out of the way

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n-1 concept does not mean anything, but let me explain how it works

quiet ivy
blissful pecan
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first lets get some basics down, you know cpp?

quiet ivy
blissful pecan
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you meantioned it in the other channel

quiet ivy
blissful pecan
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i ask because i want to know how much you know about other languages and python

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so i can adjust how i explain it

quiet ivy
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ok

blissful pecan
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so? what do y ou know about programming?

quiet ivy
blissful pecan
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you are not very helpful now

quiet ivy
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i am lazy rn

blissful pecan
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please answer

quiet ivy
blissful pecan
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so you dont know any programming?

quiet ivy
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hmm

blissful pecan
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that is perfectly fine

quiet ivy
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idk definition

blissful pecan
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no worries, ill explain it simple and do it in steps

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do you know what a list is?

quiet ivy
quiet ivy
blissful pecan
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yes, so a list of five elements can look like this

quiet ivy
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list is a collection of elements

blissful pecan
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[0, 1, 2, 3, 4]

quiet ivy
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yes

blissful pecan
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and we can get elements out of a list by indexing it

quiet ivy
#

ik

blissful pecan
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we use square brackets as well for this

quiet ivy
#

ik

blissful pecan
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[0, 1, 2, 3, 4][3] gives us 3

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we can also slice the list into parts

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we can get [0, 1, 2] from the list in this way

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it looks like this

quiet ivy
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yes

blissful pecan
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!e

mylist = [0, 1, 2, 3, 4]
print(mylist[0:3])
uncut waspBOT
#

@blissful pecan :white_check_mark: Your 3.12 eval job has completed with return code 0.

[0, 1, 2]
blissful pecan
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is this part clear?

quiet ivy
quiet ivy
blissful pecan
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the slice has three parts

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start, end, step

quiet ivy
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like for ?

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i know

blissful pecan
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start,. end and step all have default values

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start will by default start at the beginning

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end will by default be the end or last element

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and the step is by default 1

quiet ivy
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ok

blissful pecan
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if we want to get every even element from a list, we can use [0, 5, 2] as our slice

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!e

mylist = [0, 1, 2, 3, 4]
print(mylist[0:5:2])
uncut waspBOT
#

@blissful pecan :white_check_mark: Your 3.12 eval job has completed with return code 0.

[0, 2, 4]
blissful pecan
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since start and end has default values, we can also call it without them

quiet ivy
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ok

blissful pecan
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!e

mylist = [0, 1, 2, 3, 4]
print(mylist[::2])
uncut waspBOT
#

@blissful pecan :white_check_mark: Your 3.12 eval job has completed with return code 0.

[0, 2, 4]
blissful pecan
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with me so far?

quiet ivy
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ok ok

blissful pecan
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alright

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lets look how we can do this in reverse

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lets inspect what negative indexes look like

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mylist[-1] what is this value?

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any idea?

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!e

mylist = [0, 1, 2, 3, 4, 5]
print(mylist[-1])
uncut waspBOT
#

@blissful pecan :white_check_mark: Your 3.12 eval job has completed with return code 0.

5
quiet ivy
blissful pecan
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well, i wanted the value, is it clear why it gives out the value 5 ?

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[0] gives the first index, [-1] gives the last

quiet ivy
blissful pecan
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and [1] gives the second ellement, what about the second to last element?

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what index would that be?

quiet ivy
quiet ivy
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-2

blissful pecan
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correct, -2

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so the list can be indexed forward and backword

quiet ivy
blissful pecan
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that means that you can use the step in a slice to be negative

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!e

mylist = [0, 1, 2, 3, 4, 5]
print(mylist[::-1])
uncut waspBOT
#

@blissful pecan :white_check_mark: Your 3.12 eval job has completed with return code 0.

[5, 4, 3, 2, 1, 0]
quiet ivy
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ok

blissful pecan
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so this new list goes from five to zero

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if we want to write that using range, we have to keep the order in mind

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range(5, -1, -1) in other words

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!e

print(list(range(5, -1, -1)))
uncut waspBOT
#

@blissful pecan :white_check_mark: Your 3.12 eval job has completed with return code 0.

[5, 4, 3, 2, 1, 0]
blissful pecan
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i used list here to see the same result as we have been talking about

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you normally would never do that when your using range

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range just like a slice has start, stop and step

quiet ivy
blissful pecan
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the rules are the same, so if you want to go backwards, you have to start at five, end up negative one with the step of negative one

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and since range is a sequence, you can iterate over it

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!e

my_range = range(10, -1, -1)
for num in my_range:
    print(num)
uncut waspBOT
blissful pecan
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if you want the range to go the other, normal way

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!e

my_range = range(0, 11, 1)
for num in my_range:
    print(num)
uncut waspBOT
blissful pecan
#

though, since start defaults at 0, and step defaults at 1, you can only do range(10)

quiet ivy
quiet ivy
blissful pecan
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did i clear up things now?

quiet ivy
blissful pecan
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excellent

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anything else you wonder about?

quiet ivy
blissful pecan
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good, its great that you revise. once you are done, you can close this channel by typing !close

quiet ivy
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ok

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!close

uncut waspBOT
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