| 00:09 | Hello. This is the lecture for 4th module of the cosh and you'll |
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| 00:18 | that this lecture is not totally aligned the content in the book. Um |
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| 00:26 | complement each other. So the same is being motivated different ways in the |
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| 00:33 | . And this lecture. Uh So do need to pay attention to |
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| 00:40 | Alright. Back to the basics. we have seen that the computer program |
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| 00:50 | the following major components and that's And the input, output, math |
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| 00:57 | logic that we have already seen in previous lecture. So far, the |
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| 01:04 | components that we have not got into the conditional execution and reputation. So |
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| 01:13 | is what we will focus on Right. So the program so far |
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| 01:26 | have seen just run from top to and not repeat the same statements |
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| 01:35 | But that of course is not what happens. So let's keep moving. |
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| 01:43 | so the main topic today is loops alterations. Loops allow us to specify |
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| 01:50 | set of statements, some things that to be repeated multiple times, one |
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| 01:56 | , two times 10 times 1000 And another way to think of it |
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| 02:03 | that computers are really good at automating and they're really good at repeating things |
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| 02:11 | and again, they don't get And so most programs will require us |
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| 02:18 | do something many many times. And examples you can think of say |
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| 02:24 | You need to do it for the class, not just for one student |
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| 02:28 | parole for employees, voice recognition, might be the same. Basic |
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| 02:34 | but it has to be applied over over again and that's where iterations and |
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| 02:39 | come in. So python, of provides features to support repeating things and |
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| 02:48 | is a four loop. The other the range function. And there's something |
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| 02:52 | the wild loop today. We'll go the four loop and range function. |
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| 02:58 | and in a couple of weeks we'll to the white loops. So let's |
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| 03:05 | at something very simple to get a for uh, for iteration or for |
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| 03:13 | things. Here is a program that a poster for a fruit shop. |
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| 03:19 | it says we have apples, we bananas, we have grapes, we |
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| 03:22 | oranges. This is actually a python here, which not surprisingly is going |
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| 03:30 | print those things because those are four statements. So now you can see |
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| 03:36 | could have done this more efficiently. if they were like instead of |
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| 03:40 | there were 20 fruits. This is the best way to be organizing this |
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| 03:50 | . So we would use something like four loop which will achieve this task |
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| 03:55 | little bit more efficiently. So here's few terms I'm going to throw in |
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| 04:01 | four loops. Iterate. Iterate means some lines of the program over each |
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| 04:10 | out of a collection of elements, at a time. And if you're |
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| 04:15 | quite clear on what what is? okay? We have to iterate over |
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| 04:20 | elements in a collection because we're gonna the next few minutes trying to get |
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| 04:26 | clear on what all that means. for luke illiterates executes for a |
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| 04:35 | So now think what do we need iterate over here? It's clear that |
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| 04:41 | is this line of code. It's same line we need to repeat several |
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| 04:48 | . And whatever the collection the collection the list of elements over which we |
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| 04:54 | . And you can see this is fruits. The list of fruits is |
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| 05:00 | collection we're talking about. So summarize . The collection in this particular example |
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| 05:06 | the list of fruits and iteration is print line. The code that we |
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| 05:10 | to repeat is a print line. how does this all look the equal |
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| 05:19 | program to achieve exactly this. Uh uh whatever is achieved by this chord |
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| 05:28 | here as a whole loop. It for fruit and apples, bananas, |
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| 05:36 | , oranges, print we have And the expectation is that the program |
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| 05:42 | will do exactly what the four lines the program above are doing. So |
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| 05:47 | dig in a little deeper so that make sure we understand this example well |
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| 05:54 | that we can apply this concept to other example too. So here is |
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| 06:01 | same program again um copied over from previous slide. So here fruit. |
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| 06:11 | fruit is the loop variable. So variable is the one that takes on |
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| 06:17 | values every time you go through the . And here is the list of |
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| 06:23 | collection of elements that we interpret or over. So we need a |
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| 06:28 | we need the this collection of And the loop body is what is |
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| 06:34 | every time the process goes forward and just the print statement. Okay. |
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| 06:48 | . So let's just go through this and see what is supposed to happen |
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| 06:58 | each iteration of the loop. When start executing this loop, there is |
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| 07:03 | variable fruit. The variable fruit is one of the values from the |
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| 07:09 | It can be apples, bananas, , oranges, one by one. |
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| 07:14 | will go over and take that value proceed. Then you go to the |
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| 07:22 | body here you have the loop We have and then the loop variable |
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| 07:28 | is being printed and this line is . So as you can imagine every |
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| 07:34 | you go through this section we will a different fruit over there. So |
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| 07:46 | try to make this even more clear happens when this this is executed and |
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| 07:54 | output. So first time you go have the loop variable fruit, it |
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| 08:01 | on the value apples and goes to statements and Prince, we have fruit |
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| 08:08 | apple. So we have apples. time it takes the next one bananas |
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| 08:15 | we have Prince the thing and it's and so on. So you have |
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| 08:22 | , bananas, grapes oranges and then done because there's nothing else in the |
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| 08:28 | the list that we are taking the variable is taking values from. |
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| 08:37 | Um So here is we'll just spend minute go over this one more |
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| 08:44 | Now, that was one example. try to think how that fits in |
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| 08:51 | generally what uh four loop is supposed do as you enter the four |
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| 08:58 | The first thing that the python logic look at is are you done with |
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| 09:05 | items in the sequence? If you're then you're done. The loop is |
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| 09:10 | . If you're not it's saying assigned element to the loop variable and then |
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| 09:16 | all the statements in the loop. , so let's look at this uh |
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| 09:27 | , in terms of how this uh general logic works for our particular |
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| 09:35 | So when you start you have your value that the variable fruit texas, |
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| 09:45 | . So once you go in you see, are you done with |
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| 09:48 | items in the sequence? No, number of elements left. So it |
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| 09:53 | no. And then assigned next item the loop variable. Actually, that's |
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| 09:58 | point at which fruit is assigned And then it says execute all statements |
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| 10:04 | the body. The only statement here this so you execute, we have |
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| 10:10 | and then when you're done, you back here again, it asks, |
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| 10:14 | you done with everything? You're done apples, but you're not done with |
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| 10:18 | here. So you said then take next item. So you take |
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| 10:24 | and then you print we have bananas then you go back repeat that for |
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| 10:30 | , repeat that for oranges. And after that you go 1234. And |
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| 10:36 | that point this logic asks, are done with all elements in the |
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| 10:41 | The answer is yes and you're So hopefully this is super clear that |
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| 10:49 | particular way of execution is not just , you know, apples, |
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| 10:55 | It is for any sequence of elements might be there in the uh in |
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| 11:02 | list of elements. Okay, so look at one more example of this |
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| 11:14 | , very general construct. The example have in front of us is summing |
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| 11:21 | bunch of numbers. So look at program some equal zero sum equals sum |
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| 11:27 | one, sum equals sum plus two three plus four plus five. And |
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| 11:31 | you print the sum. Now. , this could be more concise. |
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| 11:37 | ? So you're just going adding one adding to than adding three, then |
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| 11:42 | four and five. There's a clear here, that's one thing. And |
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| 11:47 | any case, even if there wasn't structure, you're executing a very similar |
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| 11:53 | many times. And that's what the four construct does for you. It |
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| 12:01 | you to express this kind of a in a much more compact fashion. |
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| 12:08 | let's see what we can do. here is here is our same old |
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| 12:15 | and here is how it looks with for loop, whether with this edition |
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| 12:24 | one after the other is also something the accumulator pattern that will get back |
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| 12:30 | in this course um at multiple So to look at this for a |
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| 12:38 | , um now you have four Again, the loop variable is number |
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| 12:43 | the list of elements is 12345, that's 12345. So if you um |
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| 12:54 | here then um it's just summing this inside the loop is essentially doing exactly |
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| 13:04 | same thing as each of these statements that instead of one or two or |
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| 13:09 | , four or five, it has variable number and when we execute the |
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| 13:15 | will first are the value one, the value two, then the value |
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| 13:19 | , then the value for then the of five. So you should get |
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| 13:22 | the same results in both these so hopefully this should be pretty |
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| 13:35 | Okay, now this is the same here, nothing has been changed, |
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| 13:40 | been copied over and as I this kind of computation is fairly common |
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| 13:52 | , um this was only tell from to 5? Uh it's more compact |
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| 13:59 | the previous code you have that had edition statements, but uh it's still |
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| 14:07 | . So what if this edition or of this series wasn't from 1 to |
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| 14:14 | , it was from 1 to 100 1 to 1000. So at some |
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| 14:19 | this becomes like you keep writing And it gets longer and longer. |
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| 14:26 | ? So that is problematic. You have to type 12345, 600 102 |
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| 14:32 | 100 so on. So it stands reason that there should be a more |
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| 14:39 | way of expressing what you'd like get . And in fact there is just |
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| 14:46 | patterns like this that's called the range , another very important component of python |
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| 14:56 | . So next few minutes we're going just see what the range function can |
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| 15:01 | for us. So back to that that we had before. Let's the |
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| 15:13 | that we had before was here with difference. It was 12345. We've |
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| 15:21 | a zero here, that was not . But as you can tell, |
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| 15:27 | a zero in this program is not make any difference. So we'll temporarily |
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| 15:34 | make a life easier. Put a there and you'll see in a second |
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| 15:38 | ? So the computation is sum equals plus number. And first time you |
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| 15:43 | zero, which is meaning less. then you continue to add +12345. |
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| 15:49 | the reason we put a zero, is uh the to introduce the range |
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| 15:57 | , this program here can be replaced the program here with this entire sequence |
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| 16:05 | by a range of six. So of sex means the sequence a set |
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| 16:15 | elements 012345. Notice that it starts zero, not one and it ends |
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| 16:25 | 55 is six minus one, not . So it does have six elements |
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| 16:31 | the first element is 012345. If count it has six elements. So |
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| 16:37 | rain six. The six elements with with zero and each subsequent element is |
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| 16:44 | more than the previous element. So find this by the way in general |
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| 16:49 | computer signs that counting often starts with , not one, so might as |
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| 16:55 | get used to it. So this program is exactly the same as |
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| 17:00 | it will compute the sum with some is some plus zero plus one plus |
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| 17:06 | plus three plus four plus five. this program here, we produce the |
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| 17:11 | here should also produce exactly the same . Next couple of minutes we focus |
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| 17:18 | on what else can we do with range function general thinking of it in |
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| 17:25 | general terms than this specific case. the first issue is what if it |
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| 17:36 | important not to have zero. Now can imagine that in this series in |
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| 17:43 | series it did not matter. But if instead of plus you had multiplication |
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| 17:49 | ? Your goal was to some to multiplied by one, then add multiplied |
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| 17:55 | two and so on. And in case if you multiplied by zero, |
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| 18:03 | the answer will be very very So sometimes you don't want that zero |
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| 18:08 | we just added for convenience in this . So there is something we can |
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| 18:14 | about it. Two more um accurately this list as a range function you |
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| 18:25 | to this example. So the only the court here is different from the |
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| 18:31 | one. In the previous page we range six and here we have range |
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| 18:39 | comma six. So range one comma is exactly this. So one the |
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| 18:46 | parameter says you start counting, you the series at one then you keep |
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| 18:53 | and your last in the series will one less than six. That would |
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| 18:57 | fine. So let me give you . This program will be exactly the |
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| 19:02 | sum equals equals sum plus one sum sum plus two until some plus |
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| 19:09 | Okay. All right Now what if wanted to go from 1-1000? No |
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| 19:26 | deal. The only thing that has is instead of five, you have |
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| 19:31 | the previous example you have 1000. for the five, the corresponding um |
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| 19:38 | here was 600 range function. There's one more than five. So from |
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| 19:44 | we have 1,001. So um this be a large number. It doesn't |
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| 19:50 | . The court is just as Now what let's make it a little |
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| 19:57 | more complex. What if you wanted total even numbers from 1 to |
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| 20:04 | So here we are totaling all numbers 1 to 1000. Now this is |
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| 20:09 | little bit more complex. You only to total 2468, 10 up to |
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| 20:17 | the last one would be 1000. actually the range function does have the |
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| 20:24 | to do that to that we have looked at yet. So this is |
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| 20:35 | you do that. So basically the element of this range function is to |
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| 20:45 | then it is showing that the last should be one less than 1001. |
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| 20:52 | from 1000 you have 1,001 and here last one is the step. So |
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| 21:01 | all previous examples, this step was that means you started the to the |
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| 21:07 | will be 345678. But you can the step here. The step is |
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| 21:14 | as to that means every time you your uh this the item or the |
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| 21:23 | that the loop variable will take by . So you will have 2468 and |
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| 21:30 | will go all the way until So again, if this was not |
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| 21:38 | , the step parameter was not The step is assumed to be one |
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| 21:43 | you can give any step parameter that that you want. You could give |
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| 21:48 | , any individuals here. Okay, there is a general counting kind of |
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| 21:58 | sequence of numbers or a series. range function can capture more similar simple |
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| 22:07 | . So here is a general definition the rain function range and produces integers |
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| 22:17 | zero to n minus one. This basically a review of everything we've seen |
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| 22:22 | the last few minutes range. Start producing sequence of integers. First one |
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| 22:28 | start and the last one is stop one. Okay. And then we |
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| 22:37 | range, start, stop and So this produces a sequence starting with |
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| 22:46 | going up to stop but not including , whatever it is, that is |
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| 22:52 | than stop. It could be stopped or it could be something else but |
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| 22:56 | the counting reaches stock or exceeds that we don't uh that will not be |
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| 23:06 | . So the number before that would the last one. And step is |
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| 23:10 | integer. So here is where we'll with start and start the steps, |
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| 23:15 | this two step and so on until reach or exceed. Stop. And |
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| 23:20 | when that's when we end. So should be fairly straightforward I hope. |
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| 23:32 | one last thing. Um what if wanted to count backwards. Okay, |
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| 23:41 | the trick here is that all this we assume that step is a positive |
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| 23:48 | but we never explicitly said that it to be a positive integer. So |
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| 23:55 | can um count backwards by just making step negative. So the simplest case |
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| 24:04 | -1 for step for a negative and is an example of something that will |
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| 24:14 | backwards until from 10 to 1. , so basically it the the variable |
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| 24:26 | increased by one every time. So will go from 10 nine, all |
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| 24:34 | way until it reaches the other. the uh with the detriments of of |
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| 24:42 | and uh it will actually stop at because The it um the two would |
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| 24:55 | one. Um Yeah, the sense limit is is one here. All |
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| 25:09 | , we're going to stop this lecture . There are some interesting cases with |
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| 25:17 | range function and and the step. and the best way to figure |
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| 25:24 | you know, you know what what if, what if and the |
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| 25:27 | way to figure that out would be just, you know, run the |
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| 25:32 | function on your interface with various combinations see if your intuition with what it |
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| 25:39 | do matches with what python does. if there are any questions then we |
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| 25:45 | discuss, I'll stop here and we see you at the next in class |
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| 25:52 | |
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