[Tutor] Doubt in list comprehension

[Mats Wichmann]

On 02/23/2018 06:40 AM, Peter Otten wrote:
> vinod bhaskaran wrote:
> 
>> Hi All,
>>
>> I am a beginner programmer and i wrote a small program (as per a
>> assignment i saw) as below:
>>
>> newlist = []
>> for a in range(2,5):
>>   for b in range (0,3):
>>     newlist.append([a])
>>     a = a + 1
>> print(newlist)
>>
>> it gives the expected output as below:
>> [[2], [3], [4], [3], [4], [5], [4], [5], [6]]
>>
>> but when i try list comprehension i am not able to get it correct....can
>> someone please suggest where the (a=a+1) should be placed in a list
>> comprehension
> 
> You canot sneak a statement like
> 
>>     a = a + 1

I would go further: you should not be sneaking it into your original
code either - it's not a great idea to modify the iteration variable
while inside the loop. It works this time because you're iterating over
a list that has been made for you by range:

>>> print(range(2,5))
[2, 3, 4]

so second time through 'a' gets the second value in the list and it
doesn't break things that you changed 'a' while it was in use, but it's
a bad habit to get into - if you use the same concept in a while loop,
say, you will get unpleasant surprises.

So to further Peter's suggestion - try to figure out how to stop doing
that in your inner loop, and it will be much more clear what to do in
the comprehension form.