1 00:00:04,939 --> 00:00:08,100 In this video, I want to talk about removing items backwards. 2 00:01:03,340 --> 00:01:07,430 Let's see two ways to do that. We need to create a new python 3 00:01:07,430 --> 00:01:18,860 file here. So I'm going to call this one gobackwards.py. 4 00:01:18,860 --> 00:01:25,600 Let's start by creating some data, so data equals, and in square brackets, because it's a list; 104 5 00:01:25,610 --> 00:01:35,930 comma 101 comma 4 comma 105 comma 308 comma 103 comma 5 comma, and I'm gonna 6 00:01:35,930 --> 00:01:42,900 advance to the next line, 107 comma 100 comma 306 comma 106 comma 7 00:01:42,900 --> 00:01:47,210 102 and the last one, 108. We've got our right square bracket to close off 8 00:01:47,210 --> 00:01:53,400 the list definition. On line three I'm going to type min_valid equals 100, 9 00:01:53,400 --> 00:01:59,680 line 5, line 4 rather, max_valid is equal to 200. So we've got 10 00:01:59,689 --> 00:02:03,890 similar data to our earlier example but it's not sorted. The values to remove 11 00:02:03,890 --> 00:02:11,030 appear randomly in the list, and it's 4 at position 2, 308 at position 4, 5 appears 12 00:02:11,030 --> 00:02:17,150 at position 6 and the last one is 306 at position 9. So we'll start off with a 13 00:02:17,150 --> 00:02:20,660 similar loop to the one we used in the last program, and we'll print out the 14 00:02:20,660 --> 00:02:25,790 index numbers to check that we're getting what we'd expect. So on line 6, for space 15 00:02:25,790 --> 00:02:33,900 index in range parenthesis len parentheses data and right parenthesis, 16 00:02:33,900 --> 00:02:42,340 minus 1 comma minus 1 comma minus 1, right parenthesis and a colon. And on line 17 00:02:42,349 --> 00:02:53,910 7, print parentheses index. So if we run this program now, you can 18 00:02:53,910 --> 00:02:57,930 see we get the values from 12 down to zero, which is what we want. We can safely 19 00:02:57,930 --> 00:03:01,709 remove the outlying values now, because the values that we've still got to 20 00:03:01,709 --> 00:03:06,140 process aren't affected, when the items shuffle down to fill in the gaps. 21 00:03:06,140 --> 00:03:09,680 And this'll be easier to understand if we print out the list each time, so let's go 22 00:03:09,680 --> 00:03:14,620 ahead add some code. So I'm going to add an if on line 7, 23 00:03:14,620 --> 00:03:21,360 if data left square bracket with index in it, if that's less than min_valid 24 00:03:21,360 --> 00:03:31,130 or data index then square brackets, is greater than max_valid: 25 00:03:31,130 --> 00:03:36,180 then we want print index to be indented. As I mentioned, it'll be easier if we print 26 00:03:36,180 --> 00:03:41,970 them out each time, and then we'll actually delete it, so del data left square 27 00:03:41,970 --> 00:03:45,420 brackets index. And what we might do, rather than just printing the index, 28 00:03:45,420 --> 00:03:55,540 let's also print out the data as well; comma data. Alright. Let's run the program. 29 00:03:55,540 --> 00:04:00,420 You can see that the first line of output has an index of 9. The value at 30 00:04:00,420 --> 00:04:06,000 position 9 is 306, which will be removed. When the loop goes around again, we'll be 31 00:04:06,000 --> 00:04:11,660 processing position 8. The items in the list before position 9 aren't affected 32 00:04:11,660 --> 00:04:16,579 when we delete 306. So that deletion will work fine and won't mess up our loop. 33 00:04:16,579 --> 00:04:21,279 Next we get to index position 6. The item there is 5 and that's correct - that's the 34 00:04:21,300 --> 00:04:28,240 next item to be deleted. 308 appears at position 4, so that's also correct. 35 00:04:28,240 --> 00:04:33,270 Finally, we get the value 4 at position 2. When that's deleted, we should have a 36 00:04:33,270 --> 00:04:36,660 list with all those outlying values removed. So I should have printed out a 37 00:04:36,660 --> 00:04:43,860 final list, so that we can check that. Let's add that to the code; print line 11 38 00:04:43,860 --> 00:04:50,970 parenthesis data. Run the code, and you can see that that works. The four rogue 39 00:04:50,970 --> 00:04:55,289 values have gone from the list. This code is a lot simpler than our previous 40 00:04:55,289 --> 00:04:59,920 example. If we ignore the diagnostic output on line 8, 41 00:04:59,920 --> 00:05:03,200 our loop is only three lines of code. Now this'll also work with a 42 00:05:03,210 --> 00:05:07,830 sorted list. Test it with all the test data from the previous example, to 43 00:05:07,830 --> 00:05:11,430 confirm that it works. I won't do that in the video, but remember to always 44 00:05:11,430 --> 00:05:15,380 consider edge and corner cases when testing your code. 45 00:05:15,380 --> 00:05:21,080 Alright, so if this short loop does the same job as all this, and I'll open up 46 00:05:21,080 --> 00:05:28,860 outliers.py again. So if it does the same as all this code, which you can see 47 00:05:28,860 --> 00:05:33,240 obviously here, contains two loops, why did I spend time writing that longer program. 48 00:05:33,240 --> 00:05:37,340 Surely shorter code is better. If there are fewer lines of code, there's less 49 00:05:37,350 --> 00:05:41,160 chance of having a bug in the code. Now there is a reason for this longer 50 00:05:41,160 --> 00:05:44,850 program. I'm going to keep you in suspense a bit longer. We'll see why I 51 00:05:44,850 --> 00:05:48,840 wrote all that, in the video after the next one. In the next video, however, 52 00:05:48,840 --> 00:05:54,560 I want to show you another way to iterate backwards. See you in the next video.