=== James is now known as Guest29919 === SergioMeneses_ is now known as SergioMeneses === Mrokii_ is now known as Mrokii === ZachK_ is now known as zkriesse === Eric is now known as Guest36399 === zumbi_ is now known as zumbi === Mohan_chml is now known as mohi1 === kdrx-[-_-]\ is now known as kuadrosx === ChanServ changed the topic of #ubuntu-classroom to: Welcome to the Ubuntu Classroom - https://wiki.ubuntu.com/Classroom || Support in #ubuntu || Upcoming Schedule: http://is.gd/8rtIi || Questions in #ubuntu-classroom-chat || Event: Beginners Team Dev Academy - Current Session: Introduction to Python: Part 3 - Instructors: pedro3005 [18:01] Why, hello everyone [18:01] er.. is anybody out there? [18:01] *tumbleweed rolls by* [18:03] Alright, let's go on [18:03] * pedro3005 checks logs [18:03] Well, last time we were talking about lists [18:03] a quick refresher to you guys [18:04] numbers = [1, 2, 3, 4, 5] [18:04] so we have various methods within lists to help us deal with them, as was explained [18:04] http://docs.python.org/tutorial/datastructures.html#more-on-lists you can see them here if you forgot [18:05] (i know you did, don't lie) [18:05] Well, anyway, today I would like to talk a bit more on list slicing [18:05] We can get a specific element or number of elements from lists using that [18:05] take the numbers list for example [18:05] numbers[0] is 1 [18:06] we always start counting at 0 [18:06] numbers[1] is 2 and so forth [18:06] we can use the colon to indicate more than 1 element [18:06] say I want everything from the beginning to the 3rd element? [18:07] numbers[0:3] will yield that [18:07] a list with [1, 2, 3] [18:08] I can use -1 to go back [18:08] numbers[-1] is the first element of the list counting from right to left [18:09] and I can also define the step with an extra colon [18:09] for instance [18:09] numbers[0:5:2] returns [1, 3, 5] [18:09] because it grabs every other number from position 0 to element 5 [18:10] All of this list slicing also works with strings [18:10] >>> "abcd"[0] [18:10] 'a' [18:11] Mohan_chml asked: Is there any difference in using for loop and slicing OR it is just code efficiency? [18:11] it makes code smaller and neater [18:11] and easier to understand too [18:12] For dealing with strings, we have this method called split() [18:12] it splits a string (returning a list of strings) by a certain character [18:12] for instance, space [18:13] "a b c d".split(" ") will return ['a', 'b', 'c', 'd'] [18:13] split() is the same as split(" ") [18:13] you can also split by comma, for instance [18:13] "a,b,c,d".split(",") will do the same [18:14] the inverse of the split function is join [18:15] Otto56 asked: Does split accept multiple seperator characters or does it then split on teh sequence / sub-string? [18:15] No, your example would split by ":;" [18:15] eg split(";:") [18:15] err, I mean ";:" [18:16] You can use regular expressions for what you want [18:16] (but I don't know regexp :P) [18:16] or you can call subsequent split methods [18:16] wait.. no you can't [18:16] never mind that [18:16] As I was saying, the join function [18:17] you call it with a string and a list and it will join each element of this list with a string between them [18:18] let me exemplify [18:18] ", ".join(['a', 'b', 'c', 'd']) [18:19] that's pretty much it about join [18:19] any questions? [18:20] there are a whole bunch of string methods, and it's not my goal to go over each of them [18:20] many are self-explanatory [18:20] you have for instance str.startswith() and str.endswith() [18:20] check the python docs for all [18:20] http://docs.python.org/library/stdtypes.html#string-methods [18:20] I would like to introduce a new concept here [18:21] before we go on to more data structures, like dicts etc. it's important to learn functions [18:22] All this time, we've been using various functions [18:22] raw_input(), split(), they're all functions [18:22] and we call functions sometimes with parameters, like split(" ") [18:22] that's calling split with " " as argument [18:23] We can define our own functions [18:23] This is useful for a procedure that you're doing various times in your code [18:23] to avoid writing the same thing over and over, you define a function [18:23] for instance, let's define a function that prints a help message [18:24] def print_help(): [18:24] print "You're on your own" [18:24] now that that's done, when you call print_help() it'll display that message [18:25] you can add more print commands to it [18:25] We also learned about boolean expressions, and we can make functions that answer boolean questions [18:26] for instance, let's create a function that checks if a number is negative [18:26] def is_negative(number): [18:26] if number < 0: [18:26] return True [18:26] return False [18:27] let's analyze this [18:27] the function is_negative accepts a parameter, called number [18:27] if this number is smaller than 0, it returns True [18:28] the thing is, when a function returns a value, it immediately stops executing and goes back to the main code [18:28] so it wouldn't hit return False [18:28] but if the if block was not run, that is, number > 0, it would just read return False [18:28] ;) [18:28] you could otherwise rewrite that function as [18:28] def is_negative(number): [18:28] if number < 0: [18:28] return True [18:28] else: [18:28] return False [18:28] but it's one line too big :) [18:29] we can write functions that accept more than one parameters [18:29] for instance, let's make a function that multiplies two values [18:30] def mul(x, y): [18:30] return x * y [18:30] but what if I wanted a function to double a number? [18:30] I could try writing it as this (it is _WRONG_) [18:30] def double(x): [18:31] x = 2 * x [18:31] why is this wrong? [18:32] because the variable we're dealing with inside the function is a copy of the variable you called the function with [18:35] if we only alter the copy, it won't alter the original variable [18:35] Otto56 asked: Is it possible to pass values in by reference? [18:35] like in C? no [18:35] we can make global variables [18:35] can I brb for some minutes? family calls [18:36] sorry everyone [18:44] back [18:44] For this problem, we have two solutions [18:45] if we're dealing with a specific variable, we can make it global [18:45] a = 2 [18:45] def double_a(): [18:45] global a [18:45] a = a * 2 [18:46] Or, we can return the new value and set the old variable equal to that [18:46] def double(x): [18:46] return 2 * x [18:46] a = 4 [18:46] a = double(a) [18:46] that would yield 8 [18:47] a function can return and accept any object [18:47] and everything is an object [18:48] you can for instance define a function that completes a sentence with the period, for instance [18:49] err, too many for instance's [18:49] but let's show that [18:49] def complete_sentence(sentence): [18:49] if sentence.endswith("."): [18:49] return sentence [18:50] return sentence + "." [18:50] you could modify that function to check that the first letter is a capital letter etc [18:50] (left as an exercise for the reader :P) [18:51] a function can even accept another function as argument (do I hear some functional programming?) [18:52] for instance, imagine we have functions that return some numeric value. e.x. minus_one(x), minus_two(x) ad infinitum. (what they do is fairly obvious, and so is the implementation) [18:53] we want to make a generic function which calls it a number is negative and record the number of steps (for some freakish reason. my examples suck. sorry) [18:53] give me a minute to write that === harrisonk_away is now known as harrisonk [18:55] >>> def call(f, x): [18:55] ... if x < 0: [18:55] ... return 0 [18:55] ... steps = 0 [18:55] ... while x: [18:55] ... x = f(x) [18:55] ... steps += 1 [18:55] ... return steps [18:55] ... [18:55] let's analyze that step by step [18:55] (day by day...) [18:55] sorry, let's continue [18:55] Well, if x is already smaller than 0 (negative), it took 0 steps, right? [18:55] so we return that [18:56] we continue the function (if it's positive) by initializing the number of steps at 0 [18:56] while x, that is, while x > 0 [18:56] hmmm [18:56] maybe I should change that to while x >= 0 [18:56] yes [18:57] >>> def call(f, x): [18:57] ... if x < 0: [18:57] ... return 0 [18:57] ... steps = 0 [18:57] ... while x >= 0: [18:57] ... x = f(x) [18:57] ... steps += 1 [18:57] ... return steps [18:57] ... [18:58] that is correct now [18:58] Same thing [18:58] while x is bigger or equal to 0, that is, non-negative [18:58] we run these steps [18:58] first, we set x to be f(x), that is, we call the function upon x [18:58] we raise the step count by 1 and continue iterating [18:58] when it is done, we return steps [18:59] so, consider we had the function minus_one() and the value 4 [18:59] call(minus_one, 4) [18:59] returns 5, as expected [18:59] we type minus_one like that, without parenthesis () [18:59] because we're not calling the function, we're passing it as argument [19:00] python has the built-in function map() but we can rewrite it for fun [19:00] one min [19:03] >>> def new_map(f, values): [19:03] ... final = [] [19:03] ... for x in values: [19:03] ... final.append(f(x)) [19:03] ... return final [19:03] ... [19:04] so, what does this do? [19:04] it accepts a list and calls the function f on each element of the list, returning a new list with the returned values [19:04] >>> new_map(minus_one, [1, 2, 3]) [19:04] [0, 1, 2] [19:04] (again, python already has the function map()) [19:05] Otto56 asked: What is the python terminology for this? I understand it as a function pointer or delegate [19:05] hm.. I'm not aware of a specific terminology [19:05] it's a function [19:05] it's not a pointer to a function, it IS a function [19:05] forget pointers :P [19:06] (how I hate them) [19:06] anyway [19:06] We could write the map() function more easily using list comprehension, but I didn't go over that [19:07] any questions? does everyone understand the functions? === harrisonk is now known as harrsonk_away [19:17] So let's go back to lists with list comprehension [19:17] this is useful, I promise [19:18] list comprehension is a method of constructing a new list by some iteration [19:18] it's useful for instance to unpack strings [19:19] let's say I want to get a string "abcd" and turn it into a list with each character, i.e. ['a', 'b', 'c', 'd'] [19:19] newlist = [x for x in "abcd"] [19:20] but I can do things with this first x [19:20] let's say I want to ensure that all characters are lower-case [19:20] we have the function lower() [19:20] newlist = [x.lower() for x in "AbcD"] [19:21] or, imagine this [19:21] we have a list of lists [19:21] [[1, 2], [3, 4], [5, 6]] [19:22] we want a simple list with all these values multiplied, as in, 1 * 2, 3 * 4, 5 * 6 [19:23] we can do that with: [19:23] newlist = [x * y for x, y in oldlist] [19:23] (given oldlist = [[1, 2], [3, 4], [5, 6]]) [19:24] we're unpacking each element of the list (another list) [19:24] let me exemplify a simpler case [19:24] imagine we have a = [1, 2] [19:24] we can do [19:24] x, y = a [19:24] we are unpacking a [19:24] x = 1, y = 2 [19:25] so, as I mentioned, we can rewrite the map function using list comprehensions [19:25] def new_map(f, x): [19:26] return [f(x) for val in x] [19:26] errrrr [19:26] def new_map(f, values): [19:26] return [f(x) for x in values] [19:27] we can use this to improvise multiple-argument maps [19:27] for instance, the built-in function max() returns the max value of the list it is called with [19:28] hmm, no , forget that [19:28] hehe [19:28] someone give me a function with two arguments :P [19:29] ohh [19:29] we have the cmp() function. it accepts two integers [19:29] it returns negative if x < y, zero if x == y, and positive if x > y (given cmp(x, y)) [19:29] we can map that [19:30] a = [[1, 2], [3, 3], [4, 3]] [19:30] vals = [cmp(x, y) for x, y in a] [19:30] >>> vals [19:30] [-1, 0, 1] [19:31] questions? [19:32] we can add boolean expressions to list comprehensions [19:32] say I want all positive values from a list [19:32] a = [1, 2, -5, 0, 4] [19:33] newlist = [x for x in a if x > 0] [19:34] we can use all sorts of expressions there [19:34] for instance, we can call int() but only if it's a digit [19:35] >>> a = ['a', "2", "c", "4"] [19:35] >>> [int(x) for x in a if x.isdigit()] [19:35] [2, 4] [19:35] we can use this to strip out of a string all upper-case characters for instance [19:36] message = "HELLO there MY friend" [19:36] >>> [x for x in message.split() if not x.isupper()] [19:36] ['there', 'friend'] [19:37] this one is a bit more complex so I'll go over it [19:37] I wanted to separate message by each word it had, so I called message.split(). if you recall, that defaults to split(" "), that is, grab by each space [19:37] so we're effectively getting each word [19:38] if not x.isupper() would be equal to saying if x.isupper() == False [19:38] but shorter and in my opinion clearer [19:39] because it's closer to natural language [19:39] Questions? [19:42] Alright [19:42] Now, as I promised last class, I will solve a couple problems [19:42] first I must introduce a simple built-in function, range() [19:42] it does arithmetic progressions [19:43] >>> range(10) [19:43] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [19:43] range(10) means that stop = 10. it stops at 10 without including 10. And it defaults to starting at 0, but you can specify [19:43] >>> range(1, 10) [19:43] [1, 2, 3, 4, 5, 6, 7, 8, 9] [19:43] now we started at 1 [19:44] we can also specify the step [19:44] >>> range(1, 10, 2) [19:44] [1, 3, 5, 7, 9] [19:44] picking every other number [19:44] the step may be negative to count descending [19:44] >>> range(10, 0, -1) [19:44] [10, 9, 8, 7, 6, 5, 4, 3, 2, 1] [19:45] What is this useful for, you ask [19:45] well, many things [19:45] let's take for instance this problem [19:45] Add all the natural numbers below one thousand that are multiples of 3 or 5. [19:46] well, we'll be working with the range of 1 - 1000 (we can skip 0, since it's not a multiple) [19:46] let's ask ourselves, how can we translate "multiple of 3 or 5" to python? [19:47] well, if x is a multiple of 3, when we divide x by 3 we get 0 rest [19:47] right? [19:47] so we use the % operator [19:47] meaning that if x is a multiple of 3, x % 3 == 0 [19:47] now it's easy! [19:47] first, we get a variable to store the sum [19:47] value = 0 [19:48] for num in range(1, 1000): [19:48] if num % 3 == 0 or num % 5 == 0: [19:48] value += num [19:49] after the loop is done, value will hold the sum of all the natural numbers below one thousand that are multiples of 3 or 5 [19:50] Questions? [19:58] The old classic FizzBuzz (let's see if I have enough time) [19:59] we must loop from 1 to 100, print Fizz for multiples of 3, Buzz for multiples of 5, and FizzBuzz if both [19:59] for num in range(1, 101): # gotta account for 100 [20:00] if num % 3 == 0: [20:00] err, wait [20:00] answer = "" === ChanServ changed the topic of #ubuntu-classroom to: Welcome to the Ubuntu Classroom - https://wiki.ubuntu.com/Classroom || Support in #ubuntu || Upcoming Schedule: http://is.gd/8rtIi || Questions in #ubuntu-classroom-chat || [20:00] lol damn [20:01] ok [20:01] FizzBuzz is left as homework! [20:01] :P [20:01] I already gave a hint [20:01] ;) === pedro_ is now known as pedro3005 [20:41] For those interested, i started a home page for pedro3005's Intro to Python class: https://wiki.ubuntu.com/BeginnersTeam/FocusGroups/Development/Academy/IntroToPython [22:02] pedro3005: question: in the above class you referred several times to f(), and you passed it in as an argument as: somefunc(f,x), but it was never defined. is that a function defined by python or one that you would define (but you just didn't in these examples)? [22:02] nUboon2Age, double(x): return x + x [22:02] is x defined anywhere else in the code? [22:04] sorry i'm still not understanding pedro3005 [22:05] nUboon2Age, [22:05] >>> def double(x): [22:05] ... return x + x [22:05] ... [22:05] >>> double(2) [22:05] 4 [22:06] x is not defined elsewhere, just in that function [22:06] the variable x exists only in the double function, when it is called. x is a copy of whatever value was passed to it [22:06] okay [22:06] makes sense so far [22:06] similarly, we can do this with functions [22:07] for instance, a function that calls f on x (for example purposes) [22:08] >>> def call(f, x): [22:08] ... f(x) [22:08] ... [22:08] >>> def p(x): [22:08] ... print x [22:08] ... [22:08] >>> call(p, "Hello world!") [22:08] Hello world! [22:09] oh, okay, i think i'm starting to get it. [22:10] nUboon2Age, just start to think of things as objects [22:10] a function is also an object [22:10] so it can be passed around [22:11] i'll go back and look at the prior examples to see if i understand them now. [22:11] nUboon2Age, are you good with math? [22:12] pedro3005: not great, just okay [22:12] i think this is the line i was confused by: x = f(x) [22:13] i guess the f(x) was not explicitly defined in the example [22:14] no, by that line we're calling the function that was passed to us [22:14] we don't know what the function does [22:14] we just expect it to return something [22:14] and we set x to be that [22:14] think of it like this [22:14] on that example, we're dealing with numeric functions [22:15] so they're supposed to take a number like 4 and return something like 3 [22:15] so imagine we have x = 4 [22:15] we want to call the function which does whatever and then set x to be that new value [22:15] x = f(x) [22:15] repeat this process until we get a negative value [22:16] okay i got that. [22:17] another thing that i was confused by was "return [f(x) for val in x]" -- the 'for val in x' part isn't clear to me pedro3005 [22:18] nUboon2Age, it was a mistake by me [22:18] oh, what was it supposed to say? [22:18] return [f(x) for x in values] [22:20] nUboon2Age, it kind of makes sense if you read it out loud. [f of x for (each) x in values] [22:20] okay, could you break that down for me please pedro3005? [22:20] so what would an example be pedro3005? [22:21] grab each x inside values and add to the new list we're creating the return value of f(x) [22:21] hm, for instance [22:21] [x for x in [1, 2, 3]] [22:21] that is simply [1, 2, 3] [22:21] for each x in the old list, it puts x in the new list [22:21] but imagine this [22:21] [str(x) for x in [1, 2, 3]] [22:22] that is ['1', '2', '3'] [22:22] for x in the old list, it puts string of x in the new list [22:22] for each* [22:22] similarly, we can do [22:22] [x for x in [-1, 0, 1] if x > 0] [22:22] does x mean each element in the array? [22:23] for each x in the old list, put x in the new list if x is bigger than 0 [22:23] nUboon2Age, yes, we're iterating through it [22:24] and x could be any label, not just x? [22:25] [goomba for goomba in [-1,0,1]if goomba > 0] -- pedro3005? [22:25] nUboon2Age, absolutely [22:25] i'm closer but not quite there pedro3005 [22:26] nUboon2Age, do you understand for loops? [22:26] yes [22:26] the thing that is confusing me i think is [22:26] the first x [22:26] >>> a = [1, 2, 3, 4, 5] [22:26] >>> new_a = [] [22:26] >>> for number in a: [22:26] ... new_a.append(number) [22:26] ... [22:26] >>> new_a [22:27] [1, 2, 3, 4, 5] [22:27] that is the same thing as saying new_a = [x for x in a] [22:27] so i understand ... for x in[blah], but why the first x? [22:28] nUboon2Age, well, again, read it out loud. for x in [blah]. imagine you're the interpreter. Okay, we've got that; for x in [blah]. but what do I do with it? [22:28] oh, i'm starting to get it [22:28] x for x in [blah] -- okay, just return x. [f(x) for x in [blah]] -- okay, call f() with x and return that [22:31]