On 18/01/2015 13:41, Asokan Pichai wrote:
On Sun, Jan 18, 2015 at 6:50 PM, Sydney Shall <s.sh...@virginmedia.com <mailto:s.sh...@virginmedia.com>> wrote: I am a beginner and I have a question of syntax. Welcome! I am just learning to use list comprehension, which oc course, I find very helpful indeed. However, I am stuck with a specific problem of how to incorporate an else in a list comp-rehension. I cannot do it. The following snippet of code does what I need. But, perhaps I am solving this math error problem incorrectly. The problem is I am occasionally getting exactly zeros when I need to obtain the logarithm of the number. for i in range(len(cap)): Using a for loop like this is generally a poor idea. for aCap in cap: is the more pythonic way. And also will help you translate to a list comprehension easily if cap[i] == 0.0: tmp = math.log(1.0) lncap.append(tmp) Possibly lncap.append(math.log(1.0)) or even lncap.append(0) is simpler else: lncap.append(math.log(cap[i])) Having said that above your present code, let me try to explain the thought process behind constructing the list comprehension for your purpose. It is possible that what you need is not an else in the list comprehension lncap = [ f(aCap) for aCap in cap] will be the starting point; where f() will need to provide either math.log(aCap) or 0 as we saw earlier. You can actually write an auxiliary function f that does just that; like say def f(p): if p == 0.0: return 0.0 return math.log(p) Or write the list comprehension as: lncap = [ (math.log(aCap) if aCap > 0 else 0.0) for aCap in cap] Hope this helps
Thanks. perfect. Thanks especialy for the explanation. -- Sydney _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
