4.7 6 powers of two python

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0 points

about 7 years

my_list = [0,1,2,3,6,5,7]

for i in range(0,len(my_list),2**i): print my_list[i]

this code gives an error

Answer 55e1c6b5d3292fba3e0000c2

The code is fine apart from the for loop line. Read that line again. for i in range(0, len(my_list), 2**i) means to take the next value from the range and then assign it to i. So first, Python has to generate the range. This means that i can only be assigned a value when the range has been generated. This is how things will go in your case:

1. Python sees the for loop
2. Python tries to generate a range (it doesn’t know what i is yet)
3. It notices that you have passed i to the range() method, it doesn’t know what i is so it generates a NameError (which basically means that it doesn’t know what i is a name of)

This is how they should have gone:

1. Python sees the for loop
2. It tries to generate a range. Generation is successful.
3. It assigns the first value of the range to i

As you can see, in your case, i has not been created yet and you are already trying to pass it in as an argument to the range() function. If you tell me what you are trying to do then I could fix your code.

points

about 7 years

Answer 55e28f289113cb1cd400054d

points

about 7 years

Answer 55e3b13b937676a569000576

Try the following:

import math l = list(range(0,100)) for i in range(0, int(math.log(len(l),2))+1): print l[2**i - 1]

Note that the last line is indented but does not show in the preview.

points

about 7 years

best way with powers of 2 is using bit-shifting on 1, which is way faster than exponentiation in that case.

That said, I wouldn't recommend a while loop but rather a for loop, or even better: generate your list of values using a list comprehension (which avoids all the variables and undesired side effects, infinite loops because of while, etc...) and one-liner:

print([1<

result:

[1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536]