python - strange result when removing item from a list -

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• remove items list while iterating 18 answers

i've got piece of code :

numbers = range(1,50)  in numbers :     if < 20 :          print "do something"          numbers.remove(i)  print numbers 

but result i`m getting :

[2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49]

of course i'm expecting numbers below 20 not appear in results, i'm assuming i`m doing wrong remove

you're modifying list while iterate on it. means first time through loop, i == 1, 1 removed list. for loop goes second item in list, not 2, 3! that's removed list, , for loop goes on third item in list, 5. , on. perhaps it's easier visualize so, ^ pointing value of i:

[1, 2, 3, 4, 5, 6...]  ^ 

that's state of list initially; 1 removed , loop goes second item in list:

[2, 3, 4, 5, 6...]     ^ [2, 4, 5, 6...]        ^ 

and on.

there's no way alter list's length while iterating on it. best can this:

numbers = [n n in numbers if n >= 20] 

or this, in-place alteration (the thing in parens generator expression, implicitly converted tuple before slice-assignment):

numbers[:] = (n in in numbers if n >= 20) 

if want perform operation on n before removing it, 1 trick try this:

for i, n in enumerate(numbers):     if n < 20 :         print "do something"          numbers[i] = none numbers = [n n in numbers if n not none] 

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