Having Trouble Understanding List Comprehensions in Python 2.x

ahzthecat · 12-20-2012, 01:14 AM

Can someone please help me to understand list comprehensions in Python?

I'm working my way through the MIT Gentle Introduction to Python and I'm stuck on the list comprehensions section.

The first exercise I get.

Code:

# **********  Exercise 2.10 **********
oneten=[1,2,3,4,5,6,7,8,9,10]

cubes=[x**3 for x in oneten]

# Test Cases
##### YOUR CODE HERE #####

print cubes

but the next is to use list comprehension to generate all possible results of two coin tosses (hh, ht, th, tt)

So far I have this, which generates only "hh , tt" and a headache...

Code:

flips=["h", "t"]

allresults=[x+x for x in flips ] 

print allresults

Do I use a loop inside a list comprehension statement, or am I completely on the wrong track?

millgates · 12-20-2012, 02:36 AM

Without ever seeing Python in my life, I would try

Code:

allresults=[x+y for x in flips for y in flips ]

ahzthecat · 12-20-2012, 07:04 PM

Thanks millgates!

I read some more and arrived at this solution

Code:

coina=["h", "t"]
coinb=["h","t"]

allresults2=[x+y for x in coina for y in coinb]

then I went back and tried your suggestion, and it worked a treat. Silly me for not trying that first.
Good thing is, I feel like I understand comprehension syntax a little better now. Cheers!