Question

Has anyone tried the first list comprehension optional exercise? I'm getting a syntax error and I'm not sure why. My code is:
def number_list(random_list):
num = [x if isinstance(x, int) in random_list]
return num

print number_list([2, 4.01, 6, 9.01, "hello", "why"])

Answer 1

num = [x if isinstance(x, int) in random_list]

this line is no good, you need to rephrase it, probably turning it into a few lines

Answer 2

I get that that is the line that is incorrect, but if we are supposed to use a list comprehension to create the list, then it can't be a few lines, can it?

Answer 3

could you post a link to the exercise?

Answer 4

It's the option exercise 2 the first one in this pdf for the course: http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-189-a-gentle-introduction-to-programming-using-python-january-iap-2011/assignments/MIT6_189IAP11_hw2.pdf

Answer 5

how about [x for x in ln list if x isinstance of(...] - now i'll go read the problem....

Answer 6

it seems to work:
r = ['a', 'w', [1,2,3], 5,'why me?',8]
b = [x for x in r if x isinstance(x,int)]
[5, 8]

Answer 7

ooops [x for x in r if isinstance(x,int)] ...threw an extra x in....

Answer 8

The syntax for list comprehension is
[x for x in SOMETHING if EXPRESSION]
where "SOMETHING" is a list and "EXPRESSION" has a boolean value
for example: [x for x in [1,2,3] if x > 2]

while your list comprehension begins "x if" instead of "x for".

Answer 9

a = ['op', 1.7, 101, 3, 'ate', 10, 35.6]
b = [i for i in a if isinstance(i, int)]
print b

b = [(x,y)for y in range(11) for x in range(-5, 6) if x**2 + 1 == y]
print b

b = [(x,y)for y in range(11)for x in range(-5, 6) if x**2 + y**2 == 25]
print b

Answer 10

I think it is better to say the syntax is
[ EXPRESSION for x in LIST if BOOLEAN EXPRESSION]
>>> a = [1,2,3]
>>> lc = [2*x for x in a]
>>> print lc
[2, 4, 6]

Answer 11

Thanks all for your help. :)