I am learning Python and I am trying out some exercises. I am trying to sum all the odd numbers from 0 to 9 using list comprehension, for and if in one line.
I tried the following code:
for idx in range(10): s = 0 if idx == 0 else s = [(i % 2) for i in range(10)][idx] * range(10)[idx] + s but I get the following error:
SyntaxError: can't assign to conditional expression I don't quite understand it.
Your advice will be appreciated.
very short oneliner:
sum(range(1,10,2)) but the real formula is for any n:
((n+1)//2)**2 With that one, you're able compute the sum for a very big n very quickly.
Back to the point, you cannot accumulate using a list comprehension, or it's very difficult/hacky to do, so sum is required here. So the most logical if requirements are "use a comprehension notation and an if" is:
sum(x for x in range(1,10) if x % 2) Note that there's no need to put an extra [] in that case. It's a generator comprehension (avoids generating an extra list, sum doesn't need to have all the info at once.
This is how you may do it
sum([x for x in range(1,10,2)]) Explaining why your code failed
Here is your code as given in the question
for idx in range(10): s = 0 if idx == 0 else ([(i % 2) for i in range(10)][idx] * range(10)[idx] + s) in the else part you may give the value to be assigned to s ie, s= is not required.You may re-write it as
for idx in range(10): s = 0 if idx == 0 else ([(i % 2) for i in range(10)][idx] * range(10)[idx] + s) The expression syntax for ternary operator in python is as follows
condition_is_true if condition else condition_is_false Eg usage
value1 = 10 value2 = 20 a = 3 b = 4 value = value1 if a > b else value2 print value #20 sum([x for x in range(1,10,2)] is incorrect syntax BTW.You can try this:
sum([x for x in range(10) if x % 2 != 0]) First you create a list of number from 0 to 9 with range, then you create the list (list comprehension) of the odd numbers (using if x % 2). Finally, you sum the contents of the array using sum.
you can one line it this reduce
reduce(lambda x,y: x+y if y%2 != 0 else x ,range(10)) output
25
functools.reduce in python 3.you can do this ..
a = range(0,10) b = sum([x for x in a if x%2 != 0]) print b output :
25 
numbers = range(0, 10) def addOddNumbers(numbers): for num in numbers: if num % 2 == 1: return sum print sum(numbers) if __name__ == '__main__': addOddNumbers(numbers) 
