MRAB <pyt...@mrabarnett.plus.com> writes:
> On 02/09/2011 01:35, Bart Kastermans wrote:
>> graph = [[a,b] for a in data for b in data if d(a,b) ==1 and a< b]
>> graph2 = []
>> for i in range (0, len(data)):
>> for j in range(0,len(data)):
>> if d(data[i],data[j]) == 1 and i< j:
>> graph2.append ([i,j])
>
> Are they actually equivalent? Does graph == graph2?
>
> The first version (list comprehension) creates a list of pairs of
> values:
>
> [a, b]
>
> whereas the second version (for loops) creates a list of pairs of
> indexes:
>
> [i, j]
>
> The second version has subscripting ("data[i]" and "data[j]"), which
> will slow it down.
You are absolutely right. I had changed the code from the
equivalent:
graph2 = []
for i in range (0, len(data)):
for j in range(0,len(data)):
if d(data[i],data[j]) == 1 and i < j:
graph2.append ([data[i],data[j]])
But then also tried the equivalent
for a in data:
for b in data:
if d(a,b) == 1 and a < b:
graph2.append([a,b])
Which does away with the indexing, and is just about exactly as
fast as the list comprehension.
That'll teach me; I almost didn't ask the question thinking it might
be silly. And it was, but I thought it for the wrong reason. I tell my
students there are no stupid questions, I should listen to myself
more when I do. Thanks!
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