WEBVTT 00:00.410 --> 00:07.280 All right, so now that we've managed to get our program to encode some text, it's time to do the opposite 00:07.280 --> 00:13.520 direction, which is how do we get our program to be able to decode a piece of text based on a shift 00:13.520 --> 00:14.150 number? 00:14.180 --> 00:17.360 So let's say that we try to decode the letter G. 00:17.600 --> 00:20.390 And we had a shift number of three. 00:20.420 --> 00:27.740 Well in this case we actually have to work backwards and shift backwards in the alphabet in order to 00:27.770 --> 00:29.150 get the letter. 00:29.150 --> 00:32.180 That's three spaces previously. 00:32.210 --> 00:38.990 So go ahead and open up the step two of our Caesar cipher project. 00:39.020 --> 00:46.430 Now we've got all the code from the previous lesson, and we now have to complete three more to dos 00:46.430 --> 00:51.020 to complete the decryption part of our program. 00:51.200 --> 00:58.520 So the three to dos are you're going to create a function called decrypt that takes also two inputs. 00:58.550 --> 01:06.270 Then inside the decrypt function, you're going to write some code to shift each letter of the original 01:06.270 --> 01:13.560 text backwards in the alphabet, so that we decode it to the original text, and then we print out the 01:13.560 --> 01:15.030 decrypted text. 01:15.210 --> 01:23.430 Finally, we're going to figure out how to combine the encrypt and decrypt functions into a single function. 01:23.430 --> 01:28.890 And then within the function itself, write some conditional statements to figure out whether if the 01:28.890 --> 01:37.680 user wants to encrypt or decrypt using this direction variable, and then we end up with a single function 01:37.680 --> 01:41.190 rather than two functions encrypt and decrypt. 01:41.190 --> 01:48.150 So I recommend to do these to DOS in order and not to go straight towards to do number three, because 01:48.150 --> 01:54.120 it's quite important that you understand how the decryption works and how it differs from encryption 01:54.120 --> 01:56.730 before we move on to combining them. 01:56.730 --> 02:03.150 So there are some hints in the description box as well, and there's some more details in the description 02:03.150 --> 02:05.490 for you to read once you're ready. 02:05.520 --> 02:11.760 Pause the video and I'll go silent and wait for you to complete this exercise. 02:17.790 --> 02:22.140 All right, hopefully that went really well for you and you managed to figure it all out. 02:22.170 --> 02:24.360 It's really, really hard. 02:24.360 --> 02:31.050 So all of these projects are really hard because they don't just rely on what you've learned in class. 02:31.050 --> 02:37.200 They also rely a lot of thinking, a lot of thinking outside the box, a lot of googling, a lot of 02:37.200 --> 02:45.660 figuring things out, and also that key skill of engineering, which is breaking down problems to small 02:45.660 --> 02:47.550 enough, a problem that you can solve. 02:47.580 --> 02:54.090 So there is no way that you would be able to just breeze through any of these projects. 02:54.090 --> 02:57.110 So don't feel bad if that's not happening. 02:57.170 --> 03:02.990 But now I want to go through the solution with you in case you wanted to check your solution. 03:02.990 --> 03:07.400 Or you need a couple of more hints to figure out what's going on. 03:07.430 --> 03:11.900 So the first thing we're going to do is to create a function called decrypt. 03:11.930 --> 03:20.660 And this function is again going to take two inputs the original text and also the shift amount. 03:21.470 --> 03:30.170 So now that we're passing in these two variables inside the decrypt function we're going to shift each 03:30.170 --> 03:39.650 of these letters by the amount that is required based on whatever it is the user wanted. 03:39.680 --> 03:43.250 And then we're going to shift it backwards in the alphabet. 03:43.250 --> 03:49.610 So if the original letter was F and their shift amount is two becomes H. 03:49.640 --> 03:53.960 So now they're giving us h as the letter to decode. 03:53.960 --> 03:59.270 Then we take the shift amount and we subtract two and to get back to the original letter. 03:59.270 --> 04:05.990 So most of it is pretty much exactly the same as what happened in our encrypt. 04:05.990 --> 04:12.080 But the only difference between decrypt and encrypt is the shift direction. 04:12.080 --> 04:19.040 Because instead of taking the position of the current letter in the alphabet and shifting it up by adding 04:19.040 --> 04:25.460 the shift amount, what we want to do is we want to subtract it by the shift amount so that instead 04:25.460 --> 04:31.010 of going in this direction, we're now shifting it down this direction. 04:31.010 --> 04:35.570 And the rest of the code is pretty much exactly the same. 04:35.570 --> 04:39.650 Let's walk through them line by line, just so that we revise what's happening. 04:39.680 --> 04:44.390 Firstly, we're looping through each letter in the text to decode. 04:44.390 --> 04:47.060 We're working out the shifted position. 04:47.060 --> 04:52.970 So the position of the new letter, by taking the current letter's position and subtracting it by the 04:52.970 --> 04:53.900 shift amount. 04:53.930 --> 05:01.070 Once we've got that shift to position, we're using the modulo to divide that position by the length 05:01.070 --> 05:03.830 of the alphabet, which is 26. 05:03.860 --> 05:05.060 26 letters. 05:05.060 --> 05:11.480 And what that means is we're always going to be within that range of 0 to 25. 05:11.480 --> 05:16.220 And we're never going to go out of range of our list, which is called alphabet. 05:16.280 --> 05:23.180 Finally, we take our empty ciphertext and we add each of these letters as we loop through each of the 05:23.180 --> 05:28.100 letters in the original text, and we end up with the final ciphertext. 05:29.000 --> 05:35.630 Now, even though ciphertext is what worked for us previously in encrypt, it's probably not the best 05:35.630 --> 05:44.510 wording in this case, because the ciphertext is the result that we get after we encrypt some plaintext. 05:44.510 --> 05:48.230 So let's just change this to output text. 05:50.360 --> 05:56.490 And we need to change it all the places that we use it, and now we're ready to go ahead and test our 05:56.490 --> 05:57.870 decrypt function. 05:57.870 --> 06:06.240 So let's go ahead and call decrypt and pass in the original text, which is going to be whatever the 06:06.240 --> 06:09.420 user added to this input called text. 06:09.450 --> 06:16.710 And then we also add the shift amount based on what the user typed for the shift. 06:17.310 --> 06:20.940 So now let's go ahead and run our code. 06:20.970 --> 06:24.300 And this part is still not considered. 06:24.300 --> 06:26.310 So it doesn't actually matter what you type. 06:26.310 --> 06:29.280 And then we're going to check just one letter. 06:29.280 --> 06:37.680 So let's say that I have the letter A and I want to shift it down by two. 06:37.710 --> 06:43.620 So remember even though we can type, encode or decode, the only function that's being called right 06:43.620 --> 06:45.180 now in our code is decrypt. 06:45.180 --> 06:50.280 So it should be taking my letter and shifting it down by two. 06:50.310 --> 06:56.980 So if A is right here, then shifting it backwards will take it to Z and then to Y. 06:57.010 --> 06:59.590 So now let's go ahead and hit enter. 06:59.590 --> 07:04.810 And you can see that my decryption is working as expected. 07:05.410 --> 07:13.180 So now all that we have left to do is to combine the encrypt and decrypt functions together into a single 07:13.180 --> 07:13.690 one. 07:13.690 --> 07:18.010 Because as you can see, they are extremely repetitive. 07:18.040 --> 07:24.100 They're so repetitive that I was able to copy and paste the encrypt function into the decrypt function 07:24.100 --> 07:26.500 to get the same functionality. 07:26.530 --> 07:31.870 So let's go ahead and create this new function called Caesar. 07:32.530 --> 07:40.450 And this function is going to take the original text as an input and also the shift amount as the input. 07:40.450 --> 07:41.920 So same as before. 07:41.920 --> 07:48.790 But in addition I'm also going to add the directionality into this function. 07:48.820 --> 07:54.760 Now instead of calling it direction, which will be a little bit confusing because am I referring to 07:54.790 --> 08:00.130 the direction variable here, or am I referring to the direction variable here? 08:00.130 --> 08:05.680 And you can see as soon as I write that PyCharm is already giving me a warning, telling me that it 08:05.680 --> 08:12.340 shadows the name direction from the outer scope, which means you're trying to create a new variable 08:12.340 --> 08:18.670 name that's only going to be used inside your function, but you've already got this variable name declared 08:18.670 --> 08:19.600 outside. 08:19.630 --> 08:21.430 Now this will still work. 08:21.430 --> 08:25.180 By the way, this is valid code, but it's just confusing code. 08:25.180 --> 08:26.380 So let's change it. 08:26.380 --> 08:30.190 Let's call this variable encode or decode. 08:30.220 --> 08:32.380 It's pretty easy to understand right? 08:32.410 --> 08:36.190 We're trying to figure out do we encode or do we decode in this function. 08:36.190 --> 08:43.540 So I'm going to copy everything I had from the decrypt function and paste it inside my Caesar function. 08:43.540 --> 08:53.390 And now what I want to do is to change it so that whether if the user wanted encode or decode, I will 08:53.390 --> 09:00.920 make this function do the exact thing that they want and encode and decode the text. 09:00.950 --> 09:03.410 So what is the difference? 09:03.410 --> 09:06.770 Well, the only difference really is this line right here. 09:06.800 --> 09:14.060 Either we subtract the shift amount when we're decoding or we add the shift amount when we're encoding. 09:14.060 --> 09:22.430 So in maths, as you might have seen in the little hint I had here in maths, when you multiply a number 09:22.430 --> 09:26.030 by minus one you will reverse its sign. 09:26.030 --> 09:32.720 So positive five can become negative five by simply multiplying it by minus one. 09:32.750 --> 09:38.930 And then if we add a negative number, that's the same as subtracting it. 09:38.930 --> 09:45.470 So this might be a little bit of a mathematical knowledge that you may or may not know or may or may 09:45.470 --> 09:52.950 not remember, as in my case, but hopefully with that hint, you could work out that in order to have 09:52.950 --> 10:01.410 this line of code work, both during encoding and decoding, all we need to do is to change that shift 10:01.410 --> 10:06.690 amount based on the encode or decode variable. 10:06.720 --> 10:17.130 So if encode or decode is equal to decode well in this case we're going to take the shift amount and 10:17.130 --> 10:22.110 we're going to multiply equals it by minus one. 10:22.110 --> 10:28.290 So this is going to change this value to the opposite sign. 10:28.290 --> 10:32.310 So if it was positive it becomes negative. 10:32.310 --> 10:35.340 And that's what we want when we're decoding. 10:35.580 --> 10:40.290 Now in this case instead of subtracting we can simply add. 10:40.320 --> 10:48.300 So whatever it is when we add a negative number such as in the case of decoding, it's the same UVs 10:48.330 --> 10:57.810 as subtraction, and when it is in code, then this if statement is not going to trigger and we have 10:57.810 --> 11:03.300 the original code that we have for the encrypt function. 11:03.300 --> 11:07.500 So this is a really neat way that you can get around this problem. 11:10.440 --> 11:18.930 So now all that's left to do is to actually call this Caesar function instead of encrypt or decrypt. 11:18.930 --> 11:29.040 So let's clear the code a little bit to make this file a little less confusing, so that we only have 11:29.040 --> 11:31.560 our Caesar function. 11:31.560 --> 11:39.990 And now I'm going to call this function Caesar and pass in the three variables that it expects original 11:40.020 --> 11:42.600 text shift amount, encode or decode. 11:42.600 --> 11:53.320 So text is the name that we had for the input of the message, and then the shift is the variable we 11:53.320 --> 11:57.130 had for the shift amount input. 11:57.220 --> 12:01.300 And finally we also have the direction. 12:01.300 --> 12:07.330 So this is of course a positional argument way of calling this function. 12:07.330 --> 12:17.260 But if you want it to be more clear then you can of course add in the keyword arguments rthy. 12:19.060 --> 12:22.570 So now let's go ahead and run our code to see if it works. 12:22.570 --> 12:26.470 So I'm first going to type encode to encode my message. 12:26.470 --> 12:31.720 And I'm going to encode just a single letter to make our lives a little bit more easy. 12:31.780 --> 12:35.770 So now I want to encode which means shifting it up. 12:35.770 --> 12:37.360 And I want to shift it up by two. 12:37.390 --> 12:39.340 So a then b, then C. 12:39.340 --> 12:42.910 So hopefully I get the result c. 12:42.920 --> 12:48.080 Now let's go ahead and rerun this, um, code. 12:48.080 --> 12:50.660 So go ahead and run it again. 12:50.660 --> 12:53.360 And this time I'm going to say decode. 12:53.360 --> 12:55.880 So again I'm going to use the same letter A. 12:55.880 --> 12:57.740 And I want to shift it by two. 12:57.770 --> 13:06.380 And this time instead of getting C, I now have y because our code is working as expected, and we only 13:06.380 --> 13:12.110 have a single function that achieves all of that functionality. 13:12.110 --> 13:19.460 Now there's just one last thing that you might be bothered by if you have a little bit of OCD like me, 13:19.490 --> 13:25.940 I don't like the fact that this prints out here is the encoded result, whether if it's decode or encode. 13:25.940 --> 13:32.030 So how can we get it to say here is the encoded result when it is encoded. 13:32.030 --> 13:35.840 And here is the decoded result when it is decode. 13:35.840 --> 13:43.550 Well there's a really neat trick here because notice that the word encoded and decode it in English. 13:43.580 --> 13:51.830 Actually, both have the word encode and decode, and the only thing that they have in common is the 13:51.860 --> 13:54.980 D to represent the fact that it happened in the past. 13:54.980 --> 14:05.810 So for us, it's as simple as simply taking this part, adding in the encode or decode variable, adding 14:05.840 --> 14:08.420 D at the end no matter which one it is. 14:08.450 --> 14:19.070 And this f string will now print out encoded result if it is encoded, and it will print out decoded 14:19.070 --> 14:23.000 result when it is decoded. 14:24.470 --> 14:30.980 So hopefully you manage to understand what's going on in the explanation or you already got it by yourself 14:30.980 --> 14:33.260 anyways and you're just watching to check. 14:33.260 --> 14:39.470 But once you're ready, we're going to move on to the next step and complete our Caesar cipher program. 14:39.470 --> 14:41.750 So for all of that and more, I'll see you there.