Re: [Tutor] List comprehension possible with condition statements?
"Steven D'Aprano" wrote List comps can include *any* comparison: [x+1 for x in data if (3*x+2)**2 > 100*x or x < -5] Sure, but the wording suggested (maybe wrongly) that the OP was a real beginner and so the concept of an expression was likely to be foreign. Sticking with equalty or inequality seemed likely to be the most familiar test to him/her. Trying to explain the differnce between is and == seemed like it would be a diversion from the primary objective, namely how to filter a list comp. However I guess this discussion has covered that topic too now, so once again we have managed to kill several sparrows with one round of buckshot... :-) -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension possible with condition statements?
On Thu, 4 Mar 2010 05:18:40 am Alan Gauld wrote: > "Steven D'Aprano" wrote > > > Comparisons with None almost always should be one of: > > > > item is None > > item is not None > > Yes, but the reason I changed it (I originally had "is not") is that > != is a more general test for illustrating the use of 'if' within a > LC which seemed to be the real issue within the question. List comps can include *any* comparison: [x+1 for x in data if (3*x+2)**2 > 100*x or x < -5] -- Steven D'Aprano ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension possible with condition statements?
"Steven D'Aprano" wrote Comparisons with None almost always should be one of: item is None item is not None Yes, but the reason I changed it (I originally had "is not") is that != is a more general test for illustrating the use of 'if' within a LC which seemed to be the real issue within the question. -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension possible with condition statements?
On Wed, 3 Mar 2010 07:46:39 pm Alan Gauld wrote: > mylist = [irtem for item in aList where item != None] Comparisons with None almost always should be one of: item is None item is not None The reason is that "item is None" is ONLY ever true if the item actually is the singleton object None (accept no substitutes!). On the other hand, "item == None" might be true for some customer items. So if you actually *do* want to accept substitutes, you can use ==, but that would be an unusual thing to do, and worthy of a comment explaining that you did mean == and it isn't a mistake. Likewise for "item != None" versus "item is not None". -- Steven D'Aprano ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension possible with condition statements?
Dave Angel wrote: Jojo Mwebaze wrote: Hi There, i would like to implement the following in lists assuming x = 3 y = 4 z = None i want to create a dynamic list such that mylist = [ x , y, z ] , if z in not None if z is None then mylist = [x,y] Anyhelp! cheers Jojo Are there any constraints on x and y ? If you want to throw out all None values, then it's a ready problem. You try it, and if it doesn't quite work, post the code. We'll try to help. But if only the third value is special, then there's little point in making a comprehension of one value. Just conditionally append the z value to the list containing x and y. DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor Hello, I wrote a solution which isn't correct. >>> [i for i in mylist if i] [3, 4] The if condition should test like the other examples given to the list. >>> [i for i in mylist if i != None] [3, 4] 'if i' would also leave out the integer 0. T ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension possible with condition statements?
Thanks to everyone, nice ideas! cheers On Wed, Mar 3, 2010 at 10:02 AM, Christian Witts wrote: > Jojo Mwebaze wrote: > >> Hi There, >> >> i would like to implement the following in lists >> >> assuming >> >> x = 3 >> y = 4 >> z = None >> >> i want to create a dynamic list such that >> >> mylist = [ x , y, z ] , if z in not None >> >> if z is None then >> >> mylist = [x,y] >> >> Anyhelp! >> >> cheers >> >> Jojo >> >> >> >> >> >> >> >> >> ___ >> Tutor maillist - Tutor@python.org >> To unsubscribe or change subscription options: >> http://mail.python.org/mailman/listinfo/tutor >> >> > > | for , > > So something like `x**2 for x in [1, 2, 3, 4, 5, None, 9] if x != None` > would iterate over your input set pumping the current item into the variable > x, it will check "if x != None" and if that condition evaluates true it will > perform the function you set out to perform. > > The predicate section acts as a filter to your data set ensuring the > variable you are working with meets certain conditions. If you wanted to > for eg. still accept the None but perform a different function on it Python > does allow it like `x**2 if x else 'Not a number' for x in [1, 2, 3, 4, 5, > None, 9]`. > > Hope that helps. > > -- > Kind Regards, > Christian Witts > > > ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension possible with condition statements?
Jojo Mwebaze wrote: Hi There, i would like to implement the following in lists assuming x = 3 y = 4 z = None i want to create a dynamic list such that mylist = [ x , y, z ] , if z in not None if z is None then mylist = [x,y] Anyhelp! cheers Jojo Are there any constraints on x and y ? If you want to throw out all None values, then it's a ready problem. You try it, and if it doesn't quite work, post the code. We'll try to help. But if only the third value is special, then there's little point in making a comprehension of one value. Just conditionally append the z value to the list containing x and y. DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension possible with condition statements?
Jojo Mwebaze wrote: Hi There, i would like to implement the following in lists assuming x = 3 y = 4 z = None i want to create a dynamic list such that mylist = [ x , y, z ] , if z in not None if z is None then mylist = [x,y] Anyhelp! cheers Jojo ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor | for , So something like `x**2 for x in [1, 2, 3, 4, 5, None, 9] if x != None` would iterate over your input set pumping the current item into the variable x, it will check "if x != None" and if that condition evaluates true it will perform the function you set out to perform. The predicate section acts as a filter to your data set ensuring the variable you are working with meets certain conditions. If you wanted to for eg. still accept the None but perform a different function on it Python does allow it like `x**2 if x else 'Not a number' for x in [1, 2, 3, 4, 5, None, 9]`. Hope that helps. -- Kind Regards, Christian Witts ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension possible with condition statements?
Jojo Mwebaze wrote: Hi There, i would like to implement the following in lists assuming x = 3 y = 4 z = None i want to create a dynamic list such that mylist = [ x , y, z ] , if z in not None if z is None then mylist = [x,y] Anyhelp! cheers Jojo ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor Yes a list comprehension can have if statements. You can get the values of x and y pretty simply: >>> [i for i in mylist if i] [3, 4] T ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension possible with condition statements?
"Jojo Mwebaze" wrote i would like to implement the following in lists assuming x = 3 y = 4 z = None i want to create a dynamic list such that mylist = [ x , y, z ] , if z in not None if z is None then mylist = [x,y] Assuming you actually mean that you don;t want to include any item that is None you can use an if clause at the end of the comprehension: mylist = [irtem for item in aList where item != None] and aList is any list, which could be [x,y,z] in your example. HTH, -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
