Modifying / extracting multiple list items simultaneously using variable from range

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Modifying / extracting multiple list items simultaneously using variable from range - ehammarlund - Dec-06-2017

I am a beginner(**) trying to figure out how to put multiple items in/out of lists using variables. To demonstrate, I'm trying to make a prime sieve. [u]My comments/questions bolded below.

highnum=1000000 #the number you’re checking to, here it's a million, could be anything
erik=list(range(1,highnum) #create a list to sieve it
def sieve(x):
	erik[(2x)::x]=False #

I CAN’T FIGURE OUT HOW TO ASSIGN MULTIPLE LIST VALUES AT THE SAME TIME. CAN IT BE DONE? It doesn’t seem to want to accept a variable in the list definition so perhaps not…?

I ALSO TRIED:

erik.insert[(2x)::x]=False

THAT DID NOT WORK EITHER. SIGH.

IF I CAN GET IT TO WORK, THEN IN THEORY I CAN DO THIS:

for y in range (1,((highnum/2)+1)):
	sieve(y)
sieveoutput=(Erik[] where erik!= False)

#WILL THIS WORK/WHY WON’T THIS WORK? HELP!

OR DO I NEED TO DO IT LIKE THIS?

For z in range(1,highnum):
	iferik(z) != False:
		sieveoutput.append erik[z]

Sorry about the very low-level questions;I literally started to learn Python three days ago. Thanks in advance for any pointers.

RE: Modifying / extracting multiple list items simultaneously using variable from range - hshivaraj - Dec-06-2017

def sieve(x):
    erik[(2x)::x]=False

Im not sure what your trying to do here. In fact what your trying to do there is syntactically wrong!

Quote:I CAN’T FIGURE OUT HOW TO ASSIGN MULTIPLE LIST VALUES AT THE SAME TIME. CAN IT BE DONE? It doesn’t seem to want to accept a variable in the list definition so perhaps not…?

Did you mean something like this?

>>> a = list(range(100))
>>> 
>>> a = [False] * 100
>>> 
>>> a
[False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False]

RE: Modifying / extracting multiple list items simultaneously using variable from range - ehammarlund - Dec-06-2017

Thanks! I am trying to begin at index value= 2x (whatever x is as per the range loop) and replace the number in the list with "False" throughout the whole list, every X steps.

As an example for x=3, the list
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20
would become(new changes underlined)
1,2,3,4,5,False,7,8,False,10,11,False,13,14,False,16,17,False,19,20

and if I then ran that same function for x=5 it would become (new changes underlined):
1,2,3,4,5,False,7,8,False,False,11,False,13,14,False,16,17,False,19,False

(yes, I know that my actual code includes x=2 and x=4, but I left them out because they make it hard to illustrate what I'm doing.)
Eventually the list would only contain "False" and primes.

I can then either output a list of primes if I want them or can leave the list as-is for a very quick way to check the primacy of a number.
If I succeed, I'll choose which output to use later on. I don't know yet whether there's a material speed difference in
a) testing "is list[index]==False"
versus
b) testing "is [item] contained in list"

RE: Modifying / extracting multiple list items simultaneously using variable from range - nilamo - Dec-06-2017

Quote: As an example for x=3, the list
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20
would become(new changes underlined)
1,2,3,4,5,False,7,8,False,10,11,False,13,14,False,16,17,False,19,20

You can assign to a list slice, but only using another list of the same length of the slice you're replacing. So for this, it'd look something like:

>>> items = list(range(21))
>>> items
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
>>> replace = 3
>>> items[::replace]
[0, 3, 6, 9, 12, 15, 18]
>>> slice = items[::replace]
>>> items[::replace] = [False for _ in range(len(slice))]
>>> items
[False, 1, 2, False, 4, 5, False, 7, 8, False, 10, 11, False, 13, 14, False, 16, 17, False, 19, 20]

RE: Modifying / extracting multiple list items simultaneously using variable from range - ehammarlund - Dec-06-2017

aaaaaah. That explains so much--thanks a ton!!