1 00:00:05,100 --> 00:00:09,100 So how did you get on with the sentence challenge, that was in the last video. 2 00:00:09,100 --> 00:00:13,400 Did you manage to finish it? Well the first step is to create a new string, 3 00:00:13,400 --> 00:00:17,760 containing only the characters that are either letters or numbers. What I'm going 4 00:00:17,760 --> 00:00:24,300 to do is call up the Python string methods in the documentation. 5 00:00:24,330 --> 00:00:28,290 We've looked at this documentation back in the Program Flow Control in Python 6 00:00:28,290 --> 00:00:31,830 section. It's an alphabetical list of all the methods that the string class 7 00:00:31,830 --> 00:00:37,350 provides. So scrolling down, the two methods that we're looking are isalnum 8 00:00:37,350 --> 00:00:42,660 and isalpha - so these two here. If you want to allow 9 00:00:42,660 --> 00:00:47,550 numbers in your palindromes, you'd use the isalnum method. isalpha will only work 10 00:00:47,550 --> 00:00:52,710 with sentences that don't include digits. I'm going to use isalnum. It returns 11 00:00:52,710 --> 00:00:57,540 True if all characters are alphanumeric. We'll be calling this method on a single 12 00:00:57,540 --> 00:01:02,520 character, as we iterate over the string. The document mentions all characters, 13 00:01:02,520 --> 00:01:06,960 but the string we'll be checking will only contain one character. Alright so go back 14 00:01:06,960 --> 00:01:12,560 to the code. We're going to add the function after the is_palindrome function. 15 00:01:12,560 --> 00:01:15,680 Remember that we should put two break lines after each function, so 16 00:01:15,690 --> 00:01:21,470 therefore I'm going to begin on line 12. So I'm going to type def palindrome 17 00:01:21,470 --> 00:01:29,610 underscore sentence, in parentheses, sentence. Outside of the parentheses I'm 18 00:01:29,610 --> 00:01:34,800 going to colon. Put a couple of lines there, which we need to have between the end of the 19 00:01:34,800 --> 00:01:40,710 method and the start of the other code. I'm gonna start by typing string equals two 20 00:01:40,710 --> 00:01:50,080 double quotes - so it's an empty string. Next line, for char in sentence : 21 00:01:50,080 --> 00:02:01,040 I'm going to type if char.isalnum parentheses : string plus equals char. 22 00:02:01,040 --> 00:02:05,009 Let me call the function we iterate over the sentence, that's passed as its 23 00:02:05,009 --> 00:02:09,209 argument. If a character is an alphanumeric character - when isalnum 24 00:02:09,209 --> 00:02:13,280 turns True in other words, then we'll add that character to the string. 25 00:02:13,280 --> 00:02:17,400 Otherwise, we ignore it and move on to the next one. When the loop finishes, 26 00:02:17,400 --> 00:02:20,600 we'll have a string containing only the letters and digits from the original 27 00:02:20,610 --> 00:02:24,870 sentence. Alright, so that's the first step completed. We've produced a string 28 00:02:24,870 --> 00:02:28,960 that will only contain letters and digits. If you've used isalpha instead, 29 00:02:28,960 --> 00:02:33,020 then your string won't contain numbers. The next bit of code is easy because we've 30 00:02:33,030 --> 00:02:36,930 already written it. We want to check if this new string is a palindrome, and we've 31 00:02:36,930 --> 00:02:42,020 already got the code to do that, in our is_palindrome function on line six. 32 00:02:42,020 --> 00:02:46,560 Now what we should do, is just call is palindrome, but I haven't explained about 33 00:02:46,560 --> 00:02:48,480 calling one function from another one yet, 34 00:02:48,480 --> 00:02:52,019 so we'll do it the long way to start with. So we're going to copy the code 35 00:02:52,020 --> 00:02:56,560 from is_palindrome and paste it into our palindrome_sentence function. 36 00:02:56,560 --> 00:03:00,370 I'm gonna copy this one line here, and we're going to paste it down here 37 00:03:00,370 --> 00:03:05,349 on line 18, indented at the same level as before. 38 00:03:05,349 --> 00:03:10,400 Two line breaks there - two blank lines - and I'm also going to move it up a line 39 00:03:10,400 --> 00:03:13,080 because we only need the two blank lines after the end of the function 40 00:03:13,090 --> 00:03:17,349 there, before the start of the other code. And I am going to talk more about 41 00:03:17,349 --> 00:03:22,540 this in the next video. One reason we set this challenge, is to lead into functions 42 00:03:22,540 --> 00:03:26,290 that call other functions, but with what we've covered so far, this is probably 43 00:03:26,290 --> 00:03:30,099 how you'll have solved the challenge. Remember to have two blank lines after 44 00:03:30,100 --> 00:03:34,240 the function, which I've already done. Alright, so let's see if this works. 45 00:03:34,240 --> 00:03:38,200 We're going to change our main code now, to call this new function. At the moment, you 46 00:03:38,200 --> 00:03:43,510 can see down here, on line 22. So what I'm gonna do is change line 22. Instead of calling 47 00:03:43,510 --> 00:03:52,900 is_palindrome, we're going to call our palindrome_sentence function, like so. 48 00:03:52,900 --> 00:03:57,780 Alright, so at this point now, we've done that, let's run it. I'm going to 49 00:03:57,790 --> 00:04:04,060 paste in a sentence, Was it a car or a cat I saw, like so. Press ENTER. Was it a 50 00:04:04,060 --> 00:04:09,180 cat or a car I saw, is a palindrome. Now It should still work with single words. 51 00:04:09,180 --> 00:04:15,260 Let's run it again, and this time I'm going to enter Radar with a capital R. 52 00:04:15,260 --> 00:04:20,320 That still works as well, and just run a quick test - 53 00:04:20,320 --> 00:04:26,700 Python isn't a palindrome. And again, remember to test your code 54 00:04:26,710 --> 00:04:30,220 with words and sentences that aren't palindromes, to make sure you get the 55 00:04:30,220 --> 00:04:35,050 correct results, or rather, do further testing with multiple palindromes, non- 56 00:04:35,050 --> 00:04:39,670 palindrome_sentences, to make sure it works. Now a useful tip when writing more 57 00:04:39,670 --> 00:04:44,230 complex code, is to print things out. That's often quicker than using the 58 00:04:44,230 --> 00:04:48,610 debugger to find problems. In our palindrome_sentence function, you can 59 00:04:48,610 --> 00:04:52,780 check that the first step is working, by printing out the value of string. 60 00:04:52,780 --> 00:04:56,320 Let's have a go doing that, and we'll put the code right before the return 61 00:04:56,320 --> 00:05:01,020 statement. So I'm going to add some code down here. Over here, I'm going to type print 62 00:05:01,020 --> 00:05:06,960 string, in parentheses. Alright, so let's run it again. 63 00:05:08,420 --> 00:05:15,090 This time I'll enter do geese see god? 64 00:05:15,090 --> 00:05:18,930 You see we get dogeeseseegod printed out, which is a quick way to check that 65 00:05:18,930 --> 00:05:22,280 the first part of the function is working correctly. 66 00:05:22,280 --> 00:05:26,440 Alright, so test the function thoroughly, and we'll make some improvements to it, 67 00:05:26,440 --> 00:05:29,240 in the next video.