1 00:00:00,780 --> 00:00:05,110 All right. So now that we've managed to get our program to encode some text, 2 00:00:05,470 --> 00:00:07,960 it's time to do the opposite direction, 3 00:00:07,990 --> 00:00:12,990 which is how do we get our program to be able to decode a piece of text based on 4 00:00:13,210 --> 00:00:14,043 a shift number. 5 00:00:14,470 --> 00:00:19,470 So let's say that we try to decode the letter G and we had a shift number of 6 00:00:19,690 --> 00:00:21,760 three. Well, in this case, 7 00:00:21,790 --> 00:00:26,790 we actually have to work backward and shift backward in the alphabet in order 8 00:00:27,700 --> 00:00:31,960 to get the letter that's three spaces previously. 9 00:00:32,320 --> 00:00:33,070 So in this case, 10 00:00:33,070 --> 00:00:37,510 that would be D. Go ahead and head over to caesar- 11 00:00:37,510 --> 00:00:41,860 cipher-2-start and fork your own copy of it. 12 00:00:43,000 --> 00:00:46,720 Alright, so we've got all the code that we had at the end of part one, 13 00:00:47,170 --> 00:00:50,170 but now we have to create the decrypt function. 14 00:00:50,890 --> 00:00:52,960 And the decrypt function is again 15 00:00:52,960 --> 00:00:57,960 going to take the text that the user inputted and the shift number, 16 00:00:58,810 --> 00:01:02,050 and both of these are going to go into the function as inputs. 17 00:01:02,710 --> 00:01:04,599 And then inside the function, 18 00:01:04,660 --> 00:01:09,660 we're going to shift each letter of the text backward in the alphabet 19 00:01:10,000 --> 00:01:12,100 by the shift amount. For example, 20 00:01:12,220 --> 00:01:17,220 if we started out with this code and we needed to shift each letter by five, 21 00:01:17,980 --> 00:01:22,450 then we end up with the plain text, hello. And finally, 22 00:01:22,510 --> 00:01:24,430 our function needs to print 23 00:01:24,700 --> 00:01:29,700 the decoded text is, depending on what it is that we actually got. Finally, 24 00:01:30,760 --> 00:01:32,440 in to-do part three, 25 00:01:32,770 --> 00:01:35,830 we're going to use that direction 26 00:01:35,830 --> 00:01:40,660 variable that we ignored previously, where we ask the user to type encode 27 00:01:40,720 --> 00:01:44,050 if they want to encrypt and decode if they want to decrypt. 28 00:01:44,650 --> 00:01:49,450 And then we're going to use an if statement here to check which one they chose, 29 00:01:49,930 --> 00:01:54,280 and then we're going to call the corresponding function based on that direction. 30 00:01:54,760 --> 00:01:57,580 And once you've completed all three to-dos, 31 00:01:57,670 --> 00:02:02,050 you should be able to run your program and encode a message 32 00:02:03,340 --> 00:02:06,970 as well as decode a message when you hit run again, 33 00:02:09,460 --> 00:02:10,293 like that. 34 00:02:10,990 --> 00:02:15,340 Have a think about how you might tackle each of the three to-dos and then pause 35 00:02:15,340 --> 00:02:16,630 the video and give it a go. 36 00:02:21,380 --> 00:02:25,850 All right. So how did that go? The first part should be quite easy. 37 00:02:26,150 --> 00:02:29,870 All we have to do is define a new function called decrypt 38 00:02:30,410 --> 00:02:35,270 and this decrypt is going to take the same inputs as our encrypt, 39 00:02:35,450 --> 00:02:39,710 which is the text and the shift that the user entered. 40 00:02:40,220 --> 00:02:41,030 But again, 41 00:02:41,030 --> 00:02:45,020 I'm going to use some different parameter names to be able to differentiate the 42 00:02:45,020 --> 00:02:47,540 arguments and the parameter. So in this case, 43 00:02:47,540 --> 00:02:51,080 because the text that I'm getting is going to be scrambled, 44 00:02:51,380 --> 00:02:53,990 I'm going to call the first parameter cipher_text, 45 00:02:54,620 --> 00:02:58,520 and the second parameter I'll call it again, the shift amount. 46 00:02:59,170 --> 00:03:03,640 Now, after a colon, I'm done with to-do 1. Now let's move on to 47 00:03:03,640 --> 00:03:04,630 to-do 2. 48 00:03:05,950 --> 00:03:09,520 So here I have to be inside the decrypt function, 49 00:03:09,580 --> 00:03:11,950 so indented inside of the function, 50 00:03:12,550 --> 00:03:17,230 and then I'm going to shift each letter of the ciphertext backward in the 51 00:03:17,230 --> 00:03:19,390 alphabet by the shift amount 52 00:03:19,630 --> 00:03:23,110 and then print out the decrypted text. To do that I'm again 53 00:03:23,110 --> 00:03:24,310 going to need a for loop, 54 00:03:24,880 --> 00:03:28,990 and I'm going to loop through each letter in the cipher text. 55 00:03:29,710 --> 00:03:31,810 And for each of those letters, 56 00:03:31,870 --> 00:03:36,870 what I want to do is I want to get hold of the position that they are in the 57 00:03:37,390 --> 00:03:41,530 alphabet. So I'm gonna check the alphabet and then use again, 58 00:03:41,530 --> 00:03:43,210 that index function 59 00:03:43,630 --> 00:03:46,570 and then tap into the position of the letter. 60 00:03:47,950 --> 00:03:51,940 Once I've got the position, I'm going to define the new position 61 00:03:52,360 --> 00:03:55,570 which is going to be the current position. Now, 62 00:03:55,600 --> 00:03:58,420 remember, we're moving backwards in the alphabet now. 63 00:03:58,450 --> 00:04:02,680 So I'm going to subtract the shift amount from the position. 64 00:04:03,520 --> 00:04:04,720 Once that's done, 65 00:04:04,780 --> 00:04:09,220 I can define a blank variable called plain text, 66 00:04:09,640 --> 00:04:11,770 which is just going to be an empty string. 67 00:04:12,400 --> 00:04:15,790 And once I've gotten hold of each of the new positions, 68 00:04:16,149 --> 00:04:21,149 I can add to that plain text by tapping into the alphabet and then getting hold 69 00:04:22,780 --> 00:04:27,400 of the letter at the new position. So now finally, 70 00:04:27,400 --> 00:04:29,230 once that for loop's completed, 71 00:04:29,470 --> 00:04:34,470 then I can print out the results and I'm going to use the same sort of output 72 00:04:34,660 --> 00:04:37,150 structure as I've got in the example. 73 00:04:37,780 --> 00:04:42,780 And we're going to replace that hardcoded text with the plain text that we've 74 00:04:43,420 --> 00:04:45,280 decrypted. And finally, 75 00:04:45,280 --> 00:04:49,420 I just have to turn this into an fstring so that it actually gets inserted. 76 00:04:49,990 --> 00:04:54,130 Now, the last thing I have to do is the to-do number three. 77 00:04:54,640 --> 00:04:58,300 So here we're actually gonna check the direction variable. 78 00:04:58,930 --> 00:05:02,890 And we're going to see that if it's equal to encode, well 79 00:05:02,890 --> 00:05:07,180 then we're going to call that encrypt method that we had previously, 80 00:05:07,570 --> 00:05:11,410 so indented inside the if statement. But else 81 00:05:11,440 --> 00:05:16,180 if the direction was actually equal to decode, well 82 00:05:16,180 --> 00:05:20,740 in this case, we're going to call our newly created decrypt function. 83 00:05:21,280 --> 00:05:26,280 And we're going to pass in the cipher text as the text and the shift amount is 84 00:05:27,970 --> 00:05:31,360 going to be equal to the shift. So once again, 85 00:05:31,480 --> 00:05:34,390 these arguments come from up here 86 00:05:34,450 --> 00:05:37,180 when we ask the user to input those values. 87 00:05:37,780 --> 00:05:41,890 And now it should go over to the decrypt function and do everything that's 88 00:05:41,890 --> 00:05:43,630 inside to decrypt the text. 89 00:05:44,110 --> 00:05:47,980 So now let's give our program a run and just make sure that it works as 90 00:05:47,980 --> 00:05:52,240 expected. So I'm going to encode our message hello, 91 00:05:53,020 --> 00:05:56,530 shift it by five, and then I'm going to run it again. 92 00:05:56,930 --> 00:05:59,540 And I'm going to try and decode this message now. 93 00:06:00,950 --> 00:06:05,450 So I'm going to type decode and I'm going to paste in that message we had before 94 00:06:06,140 --> 00:06:08,600 and type the same shift number 95 00:06:09,020 --> 00:06:11,810 and now we've got our text decoded. 96 00:06:12,590 --> 00:06:16,310 So that's how you solve step 2 of our project. 97 00:06:16,970 --> 00:06:20,690 Have a review of the code here and once you're ready, 98 00:06:20,810 --> 00:06:25,040 head over to the next lesson and we're going to cover part 3 of the Caeser 99 00:06:25,040 --> 00:06:29,060 cipher. So for all of that, and more, I'll see you on the next lesson.