Python for Beginners, Part 20: For Loops

Let's talk about 'for' loops and why they are so important! Anything that requires repetition. 'For' loops are your friends! In case you missed it, here are the previous parts of the series: Python for Beginners, Part 1: How to Download and Install Python Python for Beginners, Part 2: Hello World Exercise Python for Beginners, Part 3: The Turtle Module Python for Beginners, Part 4: Interactive Shell vs. Script Python for Beginners, Part 5: Variables Python for Beginners, Part 6: Strings Python for Beginners, Part 7: Fun Fun Functions Python for Beginners, Part 8: Function Arguments Python for Beginners, Part 9: Loops Python for Beginners, Part 10: Primitive Data Python for Beginners, Part 11: String Slicing Python for Beginners, Part 12: List of Things Python for Beginners, Part 13: Append List-Method Python for Beginners, Part 14: E-1 Circle of Squares Python for Beginners, Part 15: Solution Circle of Squares Python for Beginners, Part 16 - Dictionaries Python for Beginners, Part 17 - Boolean Algebra Jiu-Jitsu Python for Beginners, Part 18 - Logical AND, OR, NOT Python for Beginners, Part 19 - Conditionals and Control Flow Transcript 0:00 hey guys what's up in this video I want 0:02 to talk about for loops and how they 0:05 work okay so we're going to get into 0:07 loops you saw that we did a little bit a 0:09 lot earlier in when we were using the 0:13 turtle module so hopefully there are a 0:15 little intuitive to you by now but now 0:18 let's use them for more and guess 0:19 realistic purposes right things to do 0:23 with numbers and manipulation of data so 0:27 not only can you use it for visual 0:29 graphics but you can use it for anything 0:32 now that's what I'm going to teach you 0:33 so if you're making your own game you'll 0:35 be able to use it after learning these 0:37 things or if you were creating your own 0:40 script maybe you're at a job whatever is 0:44 the case you're going to become more 0:46 familiar with loops and it's going to 0:48 help you do a lot of powerful things 0:52 okay 0:52 so let's get started how loops work 0:55 essentially it's a way of doing 0:57 something multiple times whenever you 0:59 think hmmm I need this done a lot of 1:02 times what can I do for example if 1:05 somebody told you print out numbers from 1:08 1 to 2 1:09 you'll be like ok I'll just write print 1:11 1 in print too good but now what if that 1:16 same person told you print numbers from 1:18 1 through 10 you'll say okay I'll just 1:20 write print 1 print to print 3.4 print 5 1:23 all up to print 10 whoo a lot of typing 1:27 but hey it wasn't so bad now let's say I 1:31 tell you to print up to 1000 could you 1:37 type that of course you can would it 1:40 take you maybe 2 hours 3 hours maybe 1:42 you'll copy paste you'll do tricks it'll 1:45 be harder now if I tell you type in a 1:47 million is going to get much much much 1:49 harder okay so for loop helps us 1:51 overcome the burden of doing repetitive 1:54 tasks so let's try something okay let's 1:58 say I want to do something five times 2:00 that's how I would say it okay I want 2:02 you to look at it less from Python 2:04 syntax specific perspective of I want to 2:07 zoom in microscopically and what what is 2:10 I it's hurting my eyes and what is range 2:12 and what does it do and I'm 2:13 getting a headache over here and where 2:15 do I put a colon where I want you to 2:17 read this as do something five times 2:20 okay that's how I want you to read it do 2:24 something five times that's what this is 2:26 saying and here I'm going to say print 2:28 hello so if I told you do do print hello 2:33 five times that's how you would think of 2:35 it you would go okay print hello five 2:38 times you'll say do something five times 2:40 and that something is print hello 2:43 okay I want you to get in an intuitive 2:46 sense of this so then you can reproduce 2:48 these and do them on your own away from 2:51 the screen okay so for example now let's 2:54 run this code and let's see what it does 2:56 and you can see print it out hello 2:58 multiple times now you might be asking 3:00 what is I if you don't already know you 3:03 can call it whatever you want you can 3:05 call it banana the code will still run 3:07 what we need to do is break this one 3:10 break this down one line at a time if we 3:13 need to look at it more introspectively 3:15 and see what's going on so what is range 3:18 of five well let's check it out let's 3:20 see what range of five is so I'm going 3:22 to print this out range of five is 3:25 really a list of numbers from zero 3:27 through four okay so to use loops you 3:31 need to know what a list is since we've 3:33 covered what a list is that's there you 3:36 go so range of five really evaluates to 3:40 this okay when you write the reason why 3:43 you should write lists here is because 3:45 python hides it it secretly writes it 3:47 and you can't tell what it is and 3:50 especially since you're starting out 3:52 this is going to confuse you when it 3:53 says that okay when you get more 3:55 advanced you'll know why it says this 3:57 but for now I'm gonna not hide it unhide 4:00 it and take out the actual form of what 4:03 it looks like and I know that if I call 4:05 list it'll show you like the actual type 4:07 of what what it's doing under the hood 4:09 so essentially whenever you see a range 4:13 of five 4:13 you should think oh that's just this 4:17 it's a list containing five elements 4:19 starting from zero all the way up to 4:21 four okay so now we can try we can start 4:25 to break all of this 4:26 down we can go okay if range of five is 4:30 really this hmm so range of five is 4:32 actually that zero one two three for the 4:36 first time right let me write this code 4:39 here let me let me change this hello and 4:41 let me change it with an eye okay and 4:44 let's let's do that 4:45 you see zero one two three four hmm what 4:48 happened there so let's see the first 4:51 time we ran the code I is zero so it 4:54 prints out zero the second time around 4:55 the code I went and became the second 4:58 element of the list so I became one then 5:00 where I was here it became one it 5:03 printed out the one then I became two it 5:05 put two here printed out two then I 5:07 became three three four and printed out 5:10 four and there you go you see all of 5:12 that on the right hand side zero got 5:14 printed then one got printed and two 5:16 that's why it's all separated by a line 5:18 okay so that's really what that I there 5:22 does could we call this number yes we 5:27 can and that's actually a better 5:28 variable name so my suggestion to you 5:31 especially if you're starting out with 5:32 programming never ever use one letter 5:36 variables never okay so even if you 5:41 think you have you can use an I here or 5:42 an X or Y don't do it take your time and 5:46 pick up pick better variable names so 5:50 I'm going to change this with number 5:51 because number is that variable this 5:55 variable we created on the fly and I'm 5:58 going to say from zero through four now 6:00 let me tell you something about the 6:02 range function in Python the range 6:04 function take start and stop okay but 6:08 it's kind of like this if you have to 6:11 think about it mathematically it 6:12 includes this point which is why I put 6:15 this here and it goes up to but not does 6:18 not include the stop okay so the range 6:21 function takes in two arguments start 6:23 and stop so if I gave it one or let's 6:26 say I gave it zero and let's say I gave 6:28 it ten you get back let's just do two 6:31 because I'm lazy oh you get back 6:34 zero one and two well what if I wanted 6:37 to print up to from 6:39 2:40 right what if I wanted to do that 6:42 well I can certainly write a list from 6:45 20 to 40 right I can go 20 21 22 23 24 6:51 all the way up to 40 but that would take 6:53 a long time let's use range to generate 6:55 that on the fly let's start with 20 and 6:57 go up to 41 but not including 41 it 7:01 excludes the 41 so this actually 7:05 translates to a list starting from 20 7:07 going up to 40 and if we print it out 7:09 you can see goes from 20 all the way up 7:12 to 40 so loops are great now in the next 7:19 video I want to talk about while loops 7:20 before I get to that I want to tell you 7:22 something so up until now we have done a 7:24 lot of Python syntax stuff learn the 7:27 basics of Python and how it works 7:31 programming specific syntax where do you 7:34 put the colon or did he do this now 7:37 after this point especially after the 7:39 while loops we're gonna get into 7:41 exercises and we're gonna kind of drill 7:43 and kill exercises I'm going to make 7:45 this the best Python course there is for 7:49 learning Python hands down what's 7:51 lacking from a lot of the online video 7:53 courses or exercises and I'm going to 7:56 give you guys so many exercises is going 7:58 to hurt your brain but in a good way 8:00 where as it makes you smarter and you 8:02 get better because without doing 8:04 exercises you will not get better you 8:06 can look at me write code all day long 8:08 you're not going to get better because 8:11 of that you need to do it on your own no 8:13 matter how simple it is 8:14 and while code Academy and all those 8:16 things are great the hand holds you 8:18 quite a bit which means that you're not 8:22 really learning a lot of those things 8:25 okay so we're gonna take a break from a 8:28 lot of these tutorials and do exercises 8:31 really get these ideas down okay what do 8:35 I mean by an exercise let's do an 8:36 exercise right here right now okay live 8:39 I'm gonna leave this here for you guys 8:42 because I want to leave this link into 8:45 the youtube description so you can 8:46 actually go look at these notes okay 8:48 so I'm gonna save this and I'm going to 8:50 leave this as notes 8:52 print numbers from 20 through 40 8:58 including 40 ha right or rather I can 9:03 say through to 41 and I can say 9:08 excluding 41 that's probably a better 9:10 way for me to write it because that's 9:12 how Python thinks a bit okay now let's 9:15 do an exercise let's say that we take 9:18 numbers from a list sum them up and 9:20 return them what do I mean if I give you 9:23 a list containing 1 2 3 you should give 9:26 me back 6 because 1 plus 2 plus 3 is 6 9:29 so if I gave you this list you should 9:34 give me back a 6 but now let's also 9:37 start adding together everything we've 9:40 learned so far 9:41 let's create a function okay that takes 9:44 in something and then does it so we're 9:46 going to build up to that 9:47 ok so first let's just write something 9:50 using a for loop that sums up all the 9:52 numbers in the list and then returns 9:54 them back to us okay or prints them out 9:56 back to us so let's do this let's say 9:59 count is equal to 0 so we're going to 10:02 use some variable to keep track of every 10:06 number so essentially then we can say 10:08 that if when we go through the loop we 10:11 can say okay I want you to add 1 to 10:15 count then when I want to go through the 10:17 loop again I wanna I want to say add 2 10:20 to count then when I go through the loop 10:21 again I want to say add 3 to count okay 10:25 and so in the start you'll have a count 10:28 being 1 then you'll have it being 1 plus 10:30 2 so 3 and then you'll have it being 2 10:33 plus 3 so 6 so you should get you know 10:36 arm back 6 so let's try it so I will say 10:41 for number in range let's say range what 10:45 will I say to range to create a list 10:47 from 0 to 3 or 1 to 3 basically I would 10:51 say 1 2 4 ok this creates a list of 1 2 10:57 & 3 10:57 ok 10:58 so I'm going to say range one through 11:01 four okay now let's do how can we 11:07 increment count okay I know that number 11:11 is going to be one the first time then 11:12 two then three that's not a problem so 11:14 number is good what we're getting with 11:16 number but how can I increment it to 11:18 count every single time here's what I 11:20 will do also count is equal to count 11:22 plus number okay what does this mean 11:26 here's what this means this says I want 11:31 the new count to be so the new count to 11:34 be here new count is equal to old count 11:38 plus number okay but so this is saying 11:43 my new variable count is going to be 11:46 what my old variable count was plus 11:50 number okay so the first time we go 11:52 through the loop the number is going to 11:53 be one old count is going to be zero 11:56 okay and so it's going to say now count 12:00 is one okay then when we go through the 12:03 loop the second time number is going to 12:05 be two oh by the way this count is going 12:07 to turn to one right because we 12:09 redefined what count is then what's 12:13 going to happen is that a number is 12:16 going to be two so essentially you're 12:18 going to get back half plus number which 12:21 is going to evaluate to count plus two 12:23 and number is two and count was one so 12:27 this is going to evaluate to three store 12:29 count as three then you're going to get 12:31 back count plus number here again right 12:33 and then you're going to get so the 12:36 number is now this time going to be 12:38 three and so you get back count which is 12:42 three plus three this evaluates to a six 12:45 and our loop ends okay so let's just 12:48 undo this damage that we have done and 12:52 let's check it out let's print count so 12:56 we run through this entire loop right 12:59 and once we're done running through this 13:00 entire loop we print out count it should 13:04 be 13:05 in our in this case six so let's check 13:08 it okay and you can see that count does 13:12 indeed give us back a six well that's 13:15 great that we wrote a four loop that 13:17 runs for the numbers one two and three 13:19 what if we want to write this for loop 13:22 and we want it to run right we want it 13:26 to run and sum up lists for any list 13:29 given to it any size list it could be a 13:31 size one list size 30 list size a 13:35 bajillion lists whatever okay so let's 13:38 see how we do it okay because practice 13:41 is key right how do you go from 13:43 conscious competence to in conscious 13:46 competence 13:47 that's how Jordan Belfort says it right 13:50 the guy in walls wolf of Wall Street the 13:53 movie oh-hoo-hoo wolf of Wall Street was 13:55 bait the person who wolf of Wall Street 13:57 was based on he defines two concepts 14:00 conscious competence and unconscious 14:02 competence what is conscious competence 14:04 that's you thinking about something and 14:08 then being good at it what's in 14:09 conscious competence you get so damn 14:11 good at what you do that you don't even 14:14 have to think about it and you do it 14:15 okay so how do we go from conscious 14:18 competence you're looking at your notes 14:19 he's really thinking about it and how 14:21 does a function work to like you just 14:24 writing code is practice okay there's no 14:27 there's there's no substitution for that 14:30 the only thing that you can do is hard 14:33 work okay there's no getting around it 14:35 there's no you can't watch somebody 14:37 else's videos or my videos and just sit 14:40 there and try to synthesize information 14:41 be like why am I not getting it you 14:43 can't just take notes all day from your 14:45 professor you have to try to do this 14:48 stuff on your own end of rant let's 14:52 continue so let's write this function 14:53 that can take in any size list and sum 14:57 it up okay so I'm going to say define 15:00 function okay so let's just write a 15:02 little note at the top so so we know 15:04 what we're doing right a function that 15:09 sums sums all elements of a list and 15:13 returns them okay that's what this is 15:16 doing that's what returns them let's 15:20 just put a little thing here like this 15:24 all right so let's write this function 15:27 I'm going to say define that's the first 15:29 step to defining a function and I'm 15:31 going to say a sum list okay so the 15:33 function is called sum list it takes in 15:35 one argument let's make that argument be 15:38 whatever my list okay that's you can 15:42 call it whatever you want you can call 15:43 ABC X Z but remember my rule try not to 15:47 think the name things as one bit of one 15:50 letter variable try to make it more 15:51 descriptive so I'm assuming I'm going to 15:54 get a my list or a list as input so I'm 15:57 going to call it my list so I'm going to 16:00 hit enter now what's the next part okay 16:03 hmm 16:04 let's think about it so the next part I 16:06 want to put here is I want to have a 16:11 counter variable right that's what we 16:13 had when we were writing this for loop 16:15 and I want to say for number in well 16:20 what's the list we're working with here 16:22 the list we were working with is one 16:24 through four right kind of like this 16:26 list so imagine if we had that list as 16:29 my list well we know it's called my list 16:31 whatever the list we get is called my 16:34 list and so just so you guys know what 16:38 we are kind of doing is eventually what 16:40 we want to be able to do is it's a sum 16:43 list and pass it to list one two three 16:46 and this should give us back a six okay 16:50 so we want to say assert that sum list 16:54 is this is equal to six 16:59 okay so this is a nice little test for 17:01 you guys which asserts things for 17:02 example if I write an assertion that's 17:04 false it's going to give me back false 17:06 so if I said five is equal to six my 17:09 code is going to yell at me hold on 17:11 I'm obviously having an error here 17:13 because I'm writing funky business but 17:15 you but you can say yes you can see I 17:17 get an assertion error it goes hey five 17:19 is not equal to six but if I go six is 17:22 equal to 17:22 six you can see that assertion gives me 17:25 a nice little three here right which is 17:28 which is or sorry does this give me 17:32 three there no I'm sorry yeah what so 17:35 assertion basically just doesn't give 17:37 you an error okay 17:38 when assertion works it just doesn't 17:40 give you an error okay so certian 17:44 doesn't give me back anything it just 17:46 gives me back no error when it works and 17:48 it gives me a big red angry 17:50 scary-looking error when it when it's 17:53 wrong so I want to have this assertion 17:56 here and why this assertion will be 17:58 helpful to me is it's going to test my 18:00 code okay 18:02 let's have another assertion and let's 18:07 say for and this should give me back 10 18:10 so I'm also teaching you guys how to 18:12 test your code so once I write this 18:14 function you should be able to call the 18:16 function by saying some list passing it 18:19 any list in this case this list 1 2 3 18:23 and it should give you back a 6 and it 18:25 should give you back a 10 when you run 18:26 this code if it doesn't give you an 18:28 error that means it's passed all of 18:30 these test cases if it fails it means it 18:32 failed one of these test cases and your 18:35 code is wrong all right let's keep going 18:37 so I want to say for a number in my list 18:40 right that's the list I get so remember 18:44 this list is going to be if we pass this 18:47 test case right if we give it this test 18:49 case and list is going to be 1 2 3 18:50 really right so let's just say my list 18:54 and what I want to do is what do I want 18:57 to do now I want to go count is equal to 18:59 count plus 1 ok so even though we're 19:02 doing it this exercise right now I'm 19:03 gonna have you guys do it again in the 19:05 exercises portion just so you can do it 19:07 from scratch for now just watch along 19:09 and get the concepts and how it works 19:11 ok so I increment count by what should 19:15 increment count by I don't want to 19:17 increment it by 1 that would be wrong I 19:19 want to increment it by the actual 19:24 number right so I want to say number 19:26 here oh I'm sorry I wrote this wrong I'm 19:28 silly I meant to 19:30 I meant incrementing by number up there 19:32 hopefully that didn't throw you off too 19:34 much okay 19:35 so but for you guys when you guys look 19:38 at the notes it's going to be fixed okay 19:40 I change it to number sorry about that 19:42 all right 19:43 so whatever count was plus the number 19:47 from list okay great and now at the end 19:52 we have to say return count okay just 19:57 format it a little bit nicer let's 19:59 remove some of these extra spaces and 20:02 let's save it again 20:04 and now I'm going to run it and if I 20:06 don't get back an assertion error which 20:08 I didn't that means that runs perfectly 20:11 imagine if okay so if I said one plus 20:13 two plus three should not give you a six 20:15 let's say I said give it it should give 20:17 me a seven you'll see that it gives me 20:19 an assertion error it says hey calling 20:21 some list on one two three does not give 20:23 me a seven so it should give you back a 20:26 six okay that's good 20:28 and just for sanity check let's run it 20:31 and print something let's say one two 20:33 what should that give us that should 20:35 give us a four right or sorry three one 20:38 plus two