1 00:00:01,290 --> 00:00:02,730 Instructor: All right, the first thing to notice 2 00:00:02,730 --> 00:00:06,780 about this exercise is that it is hard. 3 00:00:06,780 --> 00:00:08,340 So I just want to let you know 4 00:00:08,340 --> 00:00:10,110 that we're doing list comprehension 5 00:00:10,110 --> 00:00:13,200 but the previous two were a little bit on the easier side. 6 00:00:13,200 --> 00:00:14,940 And now that we've got our feet wet, 7 00:00:14,940 --> 00:00:17,880 I think we're ready for a real challenge. 8 00:00:17,880 --> 00:00:20,430 So just set your expectations. 9 00:00:20,430 --> 00:00:23,940 It's not gonna be something that you can breeze through. 10 00:00:23,940 --> 00:00:25,320 Now, I want you to take a look 11 00:00:25,320 --> 00:00:30,320 through the description and see what the goal is. 12 00:00:30,360 --> 00:00:35,360 We've got two external files, file1.txt and file2.txt. 13 00:00:36,510 --> 00:00:41,510 And your challenge is to create a list called result 14 00:00:42,000 --> 00:00:46,380 that contains the numbers that are common to both files. 15 00:00:46,380 --> 00:00:49,950 So this happens surprisingly often. 16 00:00:49,950 --> 00:00:53,430 I found myself using Python for this to check, for example, 17 00:00:53,430 --> 00:00:57,030 if my list of favorite movies is in the same list 18 00:00:57,030 --> 00:01:00,330 as my friend's list of favorite movies, or if my pick 19 00:01:00,330 --> 00:01:04,680 of fantasy football players is the same as my friend's. 20 00:01:04,680 --> 00:01:09,150 And this is going to be done using list comprehension. 21 00:01:09,150 --> 00:01:10,950 Now, if you look through the description, 22 00:01:10,950 --> 00:01:12,840 you'll see a number of hints 23 00:01:12,840 --> 00:01:15,960 and they are there to help you. 24 00:01:15,960 --> 00:01:18,960 As you get stuck, you should try to take a look 25 00:01:18,960 --> 00:01:21,060 at the hint and see incrementally 26 00:01:21,060 --> 00:01:24,780 if each of them can help you through the challenges. 27 00:01:24,780 --> 00:01:28,380 But just be aware, it is going to be a struggle. 28 00:01:28,380 --> 00:01:31,980 Now, what you're expecting is in the example output, 29 00:01:31,980 --> 00:01:34,620 this is the goal that we're trying to achieve. 30 00:01:34,620 --> 00:01:36,930 So the first step to prepare is read 31 00:01:36,930 --> 00:01:39,060 through the instructions. 32 00:01:39,060 --> 00:01:41,610 The next step is I want you to take a look 33 00:01:41,610 --> 00:01:45,270 at file1.txt and file2.txt. 34 00:01:45,270 --> 00:01:47,310 Notice, these are not editable. 35 00:01:47,310 --> 00:01:48,960 We've made them read only 36 00:01:48,960 --> 00:01:52,170 so that you don't accidentally change a number 37 00:01:52,170 --> 00:01:53,430 and something goes wrong 38 00:01:53,430 --> 00:01:56,250 or the result doesn't match, et cetera. 39 00:01:56,250 --> 00:01:58,260 File1.txt text contains some numbers, 40 00:01:58,260 --> 00:02:02,610 one on each line, and file2.txt contains more numbers, 41 00:02:02,610 --> 00:02:04,320 not the same number of numbers, 42 00:02:04,320 --> 00:02:07,050 and it's also one on each line. 43 00:02:07,050 --> 00:02:11,460 Your job is to import those numbers from those two files, 44 00:02:11,460 --> 00:02:14,790 use this comprehension to compare those two files 45 00:02:14,790 --> 00:02:18,570 and output something called result that contains the numbers 46 00:02:18,570 --> 00:02:20,640 which are common to both files. 47 00:02:20,640 --> 00:02:24,330 That is the goal, and I'll leave you to figure it out. 48 00:02:24,330 --> 00:02:25,263 So good luck. 49 00:02:30,000 --> 00:02:35,000 Now, first step is to use Python to open up our files. 50 00:02:35,070 --> 00:02:39,060 You can see on line one, we're using the with keyword 51 00:02:39,060 --> 00:02:42,903 to open file1.txt as file1. 52 00:02:43,740 --> 00:02:48,330 And then we are using file1.readlines 53 00:02:48,330 --> 00:02:53,283 in order to create a list from all the lines in that file. 54 00:02:55,050 --> 00:03:00,000 Step two is to perform the same action on file2 55 00:03:00,000 --> 00:03:03,420 and get a list that contains a number 56 00:03:03,420 --> 00:03:05,790 on each line of that file. 57 00:03:05,790 --> 00:03:08,940 Readlines is a method that we needed 58 00:03:08,940 --> 00:03:11,730 in order to complete this action, and I left it 59 00:03:11,730 --> 00:03:14,520 in the hints, but hopefully at this point, you're familiar 60 00:03:14,520 --> 00:03:17,190 with googling for what you need on the internet as well 61 00:03:17,190 --> 00:03:19,410 and you might have found it that way. 62 00:03:19,410 --> 00:03:21,510 Now, the next step is the hard step. 63 00:03:21,510 --> 00:03:24,600 This is our actual list comprehension. 64 00:03:24,600 --> 00:03:26,730 Let's start off with the for loop. 65 00:03:26,730 --> 00:03:30,550 So on line seven, you can see we're looping through list1 66 00:03:31,620 --> 00:03:34,410 and we are getting hold of each number 67 00:03:34,410 --> 00:03:36,630 using the variable num. 68 00:03:36,630 --> 00:03:41,630 So for num in list1, if num is in list2, 69 00:03:42,980 --> 00:03:46,680 so the in keyword here, you see in show up twice 70 00:03:46,680 --> 00:03:48,870 in this line, but they actually mean different things 71 00:03:48,870 --> 00:03:50,550 and they do different things. 72 00:03:50,550 --> 00:03:53,910 The first one is a for in loop, which means 73 00:03:53,910 --> 00:03:57,990 in a particular list, loop through each one of the items. 74 00:03:57,990 --> 00:04:02,910 But if numb in list2 is actually a condition. 75 00:04:02,910 --> 00:04:04,950 It's a check to see 76 00:04:04,950 --> 00:04:08,880 if the number that we're currently looping in through list1 77 00:04:08,880 --> 00:04:11,100 exists in list2. 78 00:04:11,100 --> 00:04:14,070 And this is a very short and succinct way 79 00:04:14,070 --> 00:04:15,990 of achieving our goals, 80 00:04:15,990 --> 00:04:19,290 but it is not the most straightforward to read 81 00:04:19,290 --> 00:04:20,123 and to understand. 82 00:04:20,123 --> 00:04:23,670 You might have to think about it a few times. 83 00:04:23,670 --> 00:04:25,260 You can try expanding it out 84 00:04:25,260 --> 00:04:29,790 into a full for loop and a full if statement. 85 00:04:29,790 --> 00:04:32,100 But essentially, the first step is to loop 86 00:04:32,100 --> 00:04:33,660 through each number in list1. 87 00:04:33,660 --> 00:04:35,280 Second step is to check each 88 00:04:35,280 --> 00:04:38,310 of those numbers against every item in list2. 89 00:04:38,310 --> 00:04:40,830 And then we end up with the values that match 90 00:04:40,830 --> 00:04:45,830 that criteria and we add it into our result 91 00:04:46,170 --> 00:04:50,403 and we create a new list using those results. 92 00:04:52,320 --> 00:04:54,900 Now, the final thing to remember is that in order 93 00:04:54,900 --> 00:04:59,760 for the result that's printed to match the tests, 94 00:04:59,760 --> 00:05:03,360 the result has to be a list of integers. 95 00:05:03,360 --> 00:05:07,170 So the final step before adding our num 96 00:05:07,170 --> 00:05:10,980 into the list is to convert it into an integer. 97 00:05:10,980 --> 00:05:15,300 And we do that at the first part of our list comprehension. 98 00:05:15,300 --> 00:05:17,940 And now with these few lines of code, 99 00:05:17,940 --> 00:05:20,970 we would've achieved the goals of the challenge 100 00:05:20,970 --> 00:05:25,970 and we would end up with a result that passes all the tests. 101 00:05:26,280 --> 00:05:27,113 So this is the point, 102 00:05:27,113 --> 00:05:29,460 if you need to go back to fix your solution, 103 00:05:29,460 --> 00:05:31,470 then it's time to go and do that. 104 00:05:31,470 --> 00:05:33,030 If you manage to achieve everything, 105 00:05:33,030 --> 00:05:35,760 then congratulations, a huge pat on the back. 106 00:05:35,760 --> 00:05:38,400 This is not easy. I know. 107 00:05:38,400 --> 00:05:40,080 And once you're happy with the code, 108 00:05:40,080 --> 00:05:42,453 then proceed on to the next lesson.