Re: [Tutor] selecting data from a list
- Original Message - From: Peter Otten __pete...@web.de To: tutor@python.org Cc: Sent: Sunday, January 18, 2015 4:38 AM Subject: Re: [Tutor] selecting data from a list Colin Ross wrote: Hi all, I am attempting to isolate a certain subset of data from the list a and then turn it into any array. To do so, I have written the following code: import numpy as np a = [0,1,2,3,4,5,6,7,8,9,10] b = [10,20,30,40,50,60,70,80,90,100,110] for a in range(len(a)): if a 5: print a a_1 = np.array(a) print a_1 The output is as follows: 6 7 8 9 10 10 As you can see, when I attempt to turn the list of numbers 6 through 10 into an array I am left with it only printing out 10... The desired result is: [6,7,8,9,10} Any guidance would be greatly appreciated. I have a hunch that you may be looking for slicing: a = [0,1,2,3,4,5,6,7,8,9,10] b = [10,20,30,40,50,60,70,80,90,100,110] a[6:] [6, 7, 8, 9, 10] b[3:] [40, 50, 60, 70, 80, 90, 100, 110] If I'm right you should really work through a tutorial. If Peter is right about his hunch, then ignore the following. If not, then you could use a Boolean array to do the selection: import numpy as np arr = np.arange(11) arr array([ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) arr 5 array([False, False, False, False, False, False, True, True, True, True, True], dtype=bool) arr[arr 5] array([ 6, 7, 8, 9, 10]) Regards, Albert-Jan ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] selecting data from a list
On 18/01/2015 00:49, Colin Ross wrote: Hi all, I am attempting to isolate a certain subset of data from the list a and then turn it into any array. To do so, I have written the following code: import numpy as np a = [0,1,2,3,4,5,6,7,8,9,10] b = [10,20,30,40,50,60,70,80,90,100,110] for a in range(len(a)): As others have already offered the usual sound advice I'll just say that using the above type of construct is usually wrong in Python. A useful tip is to make your container names plural, then process the singular name so lets have:- cars = ['Ford, 'Vauxhall', 'Land Rover', 'Jaguar'] for car in cars: doSomething(car) -- My fellow Pythonistas, ask not what our language can do for you, ask what you can do for our language. Mark Lawrence ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
[Tutor] selecting data from a list
Hi all, I am attempting to isolate a certain subset of data from the list a and then turn it into any array. To do so, I have written the following code: import numpy as np a = [0,1,2,3,4,5,6,7,8,9,10] b = [10,20,30,40,50,60,70,80,90,100,110] for a in range(len(a)): if a 5: print a a_1 = np.array(a) print a_1 The output is as follows: 6 7 8 9 10 10 As you can see, when I attempt to turn the list of numbers 6 through 10 into an array I am left with it only printing out 10... The desired result is: [6,7,8,9,10} Any guidance would be greatly appreciated. Thank you. Colin ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] selecting data from a list
On 18/01/15 00:49, Colin Ross wrote: a = [0,1,2,3,4,5,6,7,8,9,10] b = [10,20,30,40,50,60,70,80,90,100,110] for a in range(len(a)): if a 5: print a You have named your iteration cvariable the same as the list you are iterating over. Don't ever do this! In effect you have made your list invisible. Python just sees: for a in range(11): and a becomes each integer in turn. At the end of the loop a is the number 10. a_1 = np.array(a) print a_1 So now you try to create an array using just the number 10. The desired result is: [6,7,8,9,10} You could just rename the iteration variable: for x in range(len(a)): if x 5: print x You would be better using a list comprehension: a_1 = [x for x in range(len(a)) if x 5] print a_1 or to create the array directly: a_1 = array(x for x in range(len(a)) if x 5) should work. BTW I assume you will eventually want the contents of the original list rather than the indexes? If so it woyuld look like: a_1 = array(a[x] for x in range(len(a)) if a[x] 5) or, working on the list directly, and more generally: a_1 = array(item for item in a if test(item) ) where test() is any filter function you care you write. HTH -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.amazon.com/author/alan_gauld Follow my photo-blog on Flickr at: http://www.flickr.com/photos/alangauldphotos ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] selecting data from a list
Colin Ross wrote: Hi all, I am attempting to isolate a certain subset of data from the list a and then turn it into any array. To do so, I have written the following code: import numpy as np a = [0,1,2,3,4,5,6,7,8,9,10] b = [10,20,30,40,50,60,70,80,90,100,110] for a in range(len(a)): if a 5: print a a_1 = np.array(a) print a_1 The output is as follows: 6 7 8 9 10 10 As you can see, when I attempt to turn the list of numbers 6 through 10 into an array I am left with it only printing out 10... The desired result is: [6,7,8,9,10} Any guidance would be greatly appreciated. I have a hunch that you may be looking for slicing: a = [0,1,2,3,4,5,6,7,8,9,10] b = [10,20,30,40,50,60,70,80,90,100,110] a[6:] [6, 7, 8, 9, 10] b[3:] [40, 50, 60, 70, 80, 90, 100, 110] If I'm right you should really work through a tutorial. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] selecting data from a list
Hi Colin, and welcome. My responses are interleaved with your comments below. On Sat, Jan 17, 2015 at 08:49:30PM -0400, Colin Ross wrote: Hi all, I am attempting to isolate a certain subset of data from the list a and then turn it into any array. To do so, I have written the following code: import numpy as np a = [0,1,2,3,4,5,6,7,8,9,10] b = [10,20,30,40,50,60,70,80,90,100,110] Your code below doesn't use either the list a or b. You create these lists, then (almost) immediately throw away a and don't use b at all. This makes is hard to tell precisely what you are attempting to do, since your description of what you want to do and what your code actually does are so very different, I'm having to guess what I imagine you probably want. for a in range(len(a)): if a 5: print a That's a syntax error. Indentation is significant when programming in Python, so be careful to not lose it. That should be: for a in range(len(a)): if a 5: print a except it shouldn't because that is useless. All you are doing is printing out the matching numbers one at a time, then forgetting all about them. In this case, we can write out what the Python interpreter will do, step by step: * get the length of list a (in this case, 11) * generate a new list range(11) -- [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] * re-assign the name a to the first item of this new list, 0 * which then allows the original list to be deleted and memory reclaimed * check whether a is larger than 5 * since it isn't, continue with a = 1, a = 2, a = 3, a = 4, a = 5 * at last we get to a = 6 * which is larger than 5, so print 6 * continue with a = 7 (which is printed), a = 8, etc. * finally the loop ends * which leaves us with list b untouched and never used * and a is set to 10 I'm going to guess what you intend instead: * starting with list a = [0, 1, 2, 3, ... 9, 10] * check each value to see if it is larger than 5 * if so, you want to REMEMBER THAT VALUE for later * and collect all those values. Here is the long way of doing that: a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] collector = [] # will hold the values we collect for later for value in a: if value 5: collector.append(value) # remember it for later print Found, value print collector # optional: convert from a Python list to a numpy array import numpy as np a_1 = np.array(collector) print a_1 Note that I make sure to avoid using the same name for the list a and the individual items inside a. Here's a shorter way to do the same, using a list comprehension: a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] collector = [value for value in a if value 5] a_1 = np.array(collector) print a_1 And here's an even shorter way: a_1 = np.array(range(6, 11)) print a_1 Can you work out what each of those three things are doing? Feel free to ask for help. -- Steven ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
