1 00:00:00,520 --> 00:00:04,480 All right. It's finally time to tackle our final project, 2 00:00:04,540 --> 00:00:07,930 the Caesar Cipher. As I mentioned before, 3 00:00:08,109 --> 00:00:13,060 Caesar Cipher is this way of encoding text that, 4 00:00:13,090 --> 00:00:16,870 um, was seen as early as during the times of Julius Caesar. 5 00:00:17,110 --> 00:00:20,170 So when he had these top secret military messages, 6 00:00:20,560 --> 00:00:25,560 what he would do is he would shift each letter of the alphabet by a certain 7 00:00:26,830 --> 00:00:30,790 predetermined amount. I want to show you quickly how this works. 8 00:00:31,270 --> 00:00:36,270 So we've got the alphabet here and let's say we wanted to encode the letter E. So 9 00:00:36,970 --> 00:00:40,750 now we can line up the alphabet with a new set of alphabet. 10 00:00:41,260 --> 00:00:42,430 And at the moment, 11 00:00:42,490 --> 00:00:47,490 the shift is zero because the top alphabet and the bottom alphabet are lined up 12 00:00:47,860 --> 00:00:52,840 with a zero difference. Now, let's say that we had a shift of one. Well, 13 00:00:52,840 --> 00:00:57,700 then the alphabet moves to the left and A becomes B, B become C et cetera. 14 00:00:58,090 --> 00:01:02,710 And we can keep going until we get to the amount of shift that we wanted. 15 00:01:03,010 --> 00:01:07,060 So let's say we're going to encode all of our text with the shift of three. 16 00:01:07,480 --> 00:01:11,980 Well, then E becomes H, F becomes I, G becomes J et cetera, 17 00:01:12,250 --> 00:01:13,660 and so on and so forth. 18 00:01:14,020 --> 00:01:17,920 And they've actually discovered these artifacts from ages ago 19 00:01:17,920 --> 00:01:21,580 where people have created these sort of dials. 20 00:01:22,060 --> 00:01:27,060 And by simply rotating the dial to a certain amount of shift, 21 00:01:27,400 --> 00:01:29,620 then you can line up the letters with each other. 22 00:01:29,950 --> 00:01:34,510 So A becomes N and Z becomes O and so on and so forth. 23 00:01:34,750 --> 00:01:38,050 And this is a way that people actually encoded top secret messages. 24 00:01:38,830 --> 00:01:40,750 By the end of this day, 25 00:01:40,960 --> 00:01:45,070 you will have also built a digital form of the Caesar cipher. 26 00:01:45,730 --> 00:01:50,440 And all you have to do is to type encode to start encoding a message. 27 00:01:50,830 --> 00:01:53,380 Let's say something like Hello World. 28 00:01:54,130 --> 00:01:57,580 And once we hit enter, we get to type the shift number, 29 00:01:57,910 --> 00:01:59,710 which I'm just gonna choose a random one, 30 00:02:00,100 --> 00:02:02,980 and it gives us the encoded result. 31 00:02:03,460 --> 00:02:08,460 So now if I take this encoded results and I go ahead and type yes, 32 00:02:08,800 --> 00:02:10,570 to restart my program, 33 00:02:10,870 --> 00:02:15,340 and I'm going to use the decode function to decode my message. 34 00:02:16,180 --> 00:02:18,820 Now, when I use the same shift number, 35 00:02:19,450 --> 00:02:22,600 then I should be able to get back the decoded result. 36 00:02:23,050 --> 00:02:26,470 So if there's people monitoring your phone, or you're trying to throw a message to 37 00:02:26,470 --> 00:02:28,000 your friend in a paper ball, 38 00:02:28,330 --> 00:02:33,330 then this is an easy way to ensure that if your message was intercepted then it 39 00:02:33,430 --> 00:02:37,480 won't be understood by the other person. Once you're ready, 40 00:02:37,510 --> 00:02:42,510 let's get started by heading over to repl.it/@appbrewery/caesar 41 00:02:42,940 --> 00:02:47,320 -cipher-1-start. And this contains the starting code for this project 42 00:02:47,710 --> 00:02:49,480 and this is part one of four. 43 00:02:49,960 --> 00:02:54,520 So here I've already saved you the laborious task of typing out all the letters 44 00:02:54,520 --> 00:02:57,790 in the alphabet and they're stores in a single list. 45 00:02:58,390 --> 00:03:01,600 Now I've also got three inputs. 46 00:03:01,750 --> 00:03:03,820 So this is what the user is going to type in. 47 00:03:03,850 --> 00:03:06,610 They're gonna type in encode or decode, 48 00:03:06,760 --> 00:03:10,630 and that's gonna be saved to the direction. Tey're going to type them message 49 00:03:10,660 --> 00:03:12,730 which is going to be changed to lower case 50 00:03:13,090 --> 00:03:15,850 and this is going to be saved in text, and finally, 51 00:03:15,850 --> 00:03:17,800 they're going to input a shift number 52 00:03:18,130 --> 00:03:22,180 which is going to be converted to an integer and saved inside this variable 53 00:03:22,180 --> 00:03:27,040 called shift. So if you want to, have a run of the program as it is right now, 54 00:03:27,280 --> 00:03:30,970 so that you fully understand what's actually going on before you get started. 55 00:03:31,390 --> 00:03:33,880 And remember, don't change any of the code above, 56 00:03:34,210 --> 00:03:37,150 but instead tackle the to-dos one by one. 57 00:03:37,750 --> 00:03:40,330 The first step is to create a function called encrypt 58 00:03:40,570 --> 00:03:45,430 that's going to take this text that the user typed in and the shift that they 59 00:03:45,430 --> 00:03:47,260 typed in as inputs. 60 00:03:48,100 --> 00:03:51,310 And this is going to use what you learned about functions with inputs, 61 00:03:51,340 --> 00:03:52,780 from today's lessons. 62 00:03:53,530 --> 00:03:58,000 And then once you're done with that, step 2 is to go inside the encrypt 63 00:03:58,000 --> 00:04:03,000 function that you just created and shift each letter of the text forwards by the 64 00:04:05,650 --> 00:04:09,870 shift amount. So, again, as we saw before a shift of one, 65 00:04:09,880 --> 00:04:11,860 a shifter of two, a shift of three, 66 00:04:12,100 --> 00:04:15,310 basically just moves the characters forward 67 00:04:15,340 --> 00:04:19,060 so that each letter of the alphabet becomes a letter of the alphabet much 68 00:04:19,089 --> 00:04:23,830 further down the line. Let's say that the user entered the text 69 00:04:23,860 --> 00:04:27,370 Hello and if we tried a shift of five, 70 00:04:27,640 --> 00:04:31,000 then the end result should be mjqqt 71 00:04:31,450 --> 00:04:35,530 and the printed output from this function should say something like the encoded 72 00:04:35,530 --> 00:04:38,770 text is this. And finally, 73 00:04:38,770 --> 00:04:42,880 you're going to call the encrypt function and then pass in the user inputs from 74 00:04:42,880 --> 00:04:43,713 over here 75 00:04:43,900 --> 00:04:48,190 and you should be able to test the code and encrypt a message and see something 76 00:04:48,190 --> 00:04:52,480 like this happen. Pause the video now, have a look through the starting code, 77 00:04:52,720 --> 00:04:53,830 and once you're ready, 78 00:04:53,980 --> 00:04:57,250 go ahead and tackle it and then come back and we'll go through the solution 79 00:04:57,250 --> 00:05:02,200 together. Good luck. All right. 80 00:05:02,200 --> 00:05:07,200 So I'm gonna fork a copy of the starting file and tackle to-do number one, 81 00:05:07,930 --> 00:05:12,930 which is to create a function which is done using Def and it's called encrypt. 82 00:05:14,020 --> 00:05:18,250 And this function is going to take two inputs. So inside the parentheses, 83 00:05:18,580 --> 00:05:22,300 I'm going to put two parameters. Now you can, 84 00:05:22,300 --> 00:05:26,110 of course use the same word as the inputs that you're going to put into it. 85 00:05:26,380 --> 00:05:28,000 But in an ideal world, 86 00:05:28,000 --> 00:05:31,600 you try to not confuse yourself between the arguments and the parameters. 87 00:05:31,900 --> 00:05:34,690 So I'm going to call this plain_text, 88 00:05:34,840 --> 00:05:37,570 this first parameter and the second parameter 89 00:05:37,600 --> 00:05:41,230 I'm going to call it shift_amount. Now, of course, at a later date, 90 00:05:41,230 --> 00:05:43,330 when we're sending the input to this function, 91 00:05:43,570 --> 00:05:47,860 we're going to be sending the text to plain_text and shift to shift_amount. 92 00:05:48,460 --> 00:05:53,460 Keeping the names of the argument and the parameter different will help us 93 00:05:54,430 --> 00:05:59,430 later on to see which one is which. The next step is to go inside the encrypt 94 00:06:00,200 --> 00:06:03,410 function, which of course means indenting by one, 95 00:06:03,710 --> 00:06:08,710 we're going to shift each letter of the text forwards in the alphabet by the 96 00:06:08,930 --> 00:06:11,450 shift amount, and then print the encrypted text. 97 00:06:12,020 --> 00:06:15,230 This is going to be the challenging part of this whole exercise. 98 00:06:15,740 --> 00:06:18,020 But essentially if we think about the problem, 99 00:06:18,050 --> 00:06:22,940 what we want to do is we want to take each letter in the plain text. 100 00:06:23,000 --> 00:06:26,060 So let's say all plain text is equal to hello. 101 00:06:26,540 --> 00:06:31,540 Then we're going to take each of these letters in turn and then shift it up in 102 00:06:31,790 --> 00:06:36,290 the alphabet by the shift amount. So let's say for example, 103 00:06:36,290 --> 00:06:39,800 our shift amount was five and our plain text was hello, 104 00:06:40,340 --> 00:06:43,670 the first thing we're going to do is we're going to take the H which is over 105 00:06:43,670 --> 00:06:48,440 here, and then we're going to shift it up by five. So one, two, three, four, 106 00:06:48,680 --> 00:06:49,220 five, 107 00:06:49,220 --> 00:06:54,220 and we're going to turn it into an M. We can start off by writing a for loop that 108 00:06:54,380 --> 00:06:58,520 loops through each of the letters in the plain text input. 109 00:06:59,270 --> 00:07:04,270 And then we can take each of these letters and work out its position in the 110 00:07:05,390 --> 00:07:06,620 alphabet list here. 111 00:07:07,340 --> 00:07:10,820 And we can do that by using the index method. 112 00:07:11,030 --> 00:07:15,140 So we can tap into a list, write .index, 113 00:07:15,470 --> 00:07:18,470 and then put in the value that we want to get the index for 114 00:07:18,800 --> 00:07:23,720 and we should be able to get that as the result. So back over here, 115 00:07:23,750 --> 00:07:24,890 we're going to say, 116 00:07:25,010 --> 00:07:29,030 alphabet, is the list, .index 117 00:07:29,360 --> 00:07:33,560 and then inside the parentheses, we can put in our letter. 118 00:07:34,070 --> 00:07:39,070 So we want to find out the index of the letter that we're looping through in 119 00:07:39,410 --> 00:07:43,430 this alphabet up here. And once we've gotten a hold of that, 120 00:07:43,460 --> 00:07:47,180 we're gonna save it into a variable called position. 121 00:07:48,410 --> 00:07:52,790 Now, once we've got that, then we're going to calculate the new position. 122 00:07:54,050 --> 00:07:57,320 Now the new position is just going to be the previous position, 123 00:07:57,350 --> 00:08:02,270 which is going to be a number, and then plus the shift amount. 124 00:08:03,230 --> 00:08:06,170 So while we're going through this code, let's look at this word 125 00:08:06,260 --> 00:08:11,260 H. So the first time the loop runs letter's going to be equal to H, position is 126 00:08:11,330 --> 00:08:15,920 going to be equal to zero one, two, three, four, five, six, seven. 127 00:08:16,610 --> 00:08:18,620 And then the shift amount is five, 128 00:08:18,650 --> 00:08:21,620 so seven plus five is going to be 12. 129 00:08:21,950 --> 00:08:24,680 So now the new position is going to be 12, 130 00:08:25,070 --> 00:08:29,150 and we can tap into the alphabet list at position 12, 131 00:08:30,410 --> 00:08:33,320 so seven, eight, nine, ten, eleven, twelve, 132 00:08:33,409 --> 00:08:36,289 and get hold of the value at that position. 133 00:08:37,520 --> 00:08:42,520 So we can say that new_letter is equal to the alphabet at the new_position. 134 00:08:47,240 --> 00:08:50,120 And now once we've got this new_letter, 135 00:08:50,210 --> 00:08:55,040 we can create a empty string here which we'll call the ciphertext 136 00:08:56,160 --> 00:09:00,330 which is going to be the encoded text. And it starts out being an empty string 137 00:09:00,810 --> 00:09:03,600 and then once we've gotten each of the new letters, 138 00:09:03,870 --> 00:09:08,160 we're going to add it to the cipher text. Now, 139 00:09:08,160 --> 00:09:12,750 the final thing we have to do inside this encrypt function is once the 140 00:09:12,750 --> 00:09:14,340 for loop has run its course, 141 00:09:14,730 --> 00:09:19,260 and we've added all of the new letters into the cipher text, 142 00:09:19,530 --> 00:09:24,180 then at this point, it's a good idea to print out the output. 143 00:09:24,600 --> 00:09:27,570 And it tells us that we should say something like this, 144 00:09:27,870 --> 00:09:32,340 where we replace this final thing with the actual encoded text, 145 00:09:32,400 --> 00:09:35,250 which has of course, called a cipher text in our case. 146 00:09:35,910 --> 00:09:38,250 Now let's just make that into an fstring 147 00:09:38,250 --> 00:09:43,080 so that it'll insert it when it runs and we're ready to tackle the last to-do. So 148 00:09:43,080 --> 00:09:43,620 here, 149 00:09:43,620 --> 00:09:47,850 we're going to call the encrypt function and we're going to pass in all of the 150 00:09:47,850 --> 00:09:48,750 user inputs. 151 00:09:49,140 --> 00:09:53,850 So it remember that our parameter names are plain text and shift amount. 152 00:09:54,270 --> 00:09:57,840 So I'm actually going to use the keyword arguments, so I'm going to say 153 00:09:57,870 --> 00:10:02,870 plain text is equal to the text that the user has typed in. 154 00:10:04,650 --> 00:10:07,380 And then on the next argument, 155 00:10:07,530 --> 00:10:12,530 I'm going to say the shift amount is equal to the shift that the user has typed 156 00:10:13,440 --> 00:10:14,273 in. 157 00:10:17,100 --> 00:10:21,870 So now we're ready to test our code and let's take a look at what happens. 158 00:10:22,290 --> 00:10:27,290 So I'm going to use this example that we've done here by trying to encode hello 159 00:10:27,870 --> 00:10:31,980 with the shift of five. Now the first thing that we type, the direction, 160 00:10:32,010 --> 00:10:35,820 doesn't actually matter because it's not being used anywhere in our code, 161 00:10:36,000 --> 00:10:39,060 at least not yet. So you can type anything you want, 162 00:10:39,060 --> 00:10:43,380 I'm just going to type encode. And then the message is going to be hello. 163 00:10:44,310 --> 00:10:47,790 And the shift number is going to be five. 164 00:10:48,390 --> 00:10:52,500 And now it tells us that the encoded text is mjqqt, 165 00:10:52,830 --> 00:10:55,410 which matches with the expected output. 166 00:10:55,990 --> 00:11:00,430 You might think that we're done, right? This is perfect. This is great 167 00:11:00,430 --> 00:11:03,850 and we've completed this entire exercise. 168 00:11:04,390 --> 00:11:07,600 But lets consider a different scenario. 169 00:11:08,200 --> 00:11:13,200 What happens if we tried to encode a word that had letters that were really 170 00:11:15,190 --> 00:11:17,200 close to the end of the alphabet? 171 00:11:17,770 --> 00:11:22,360 Let's say that we wanted to encode the message zulu. 172 00:11:23,320 --> 00:11:26,590 Now I'm only doing that because if there's a z in that word, of course. 173 00:11:27,100 --> 00:11:29,380 And we wanted to shift it by five. 174 00:11:29,560 --> 00:11:32,620 Now let's think of what's going to happen right? In our code, 175 00:11:32,620 --> 00:11:36,130 we're going to take each of the letters, we're going to take z first 176 00:11:36,490 --> 00:11:41,490 and then we're going to get its position in the alphabet and add the shift 177 00:11:41,530 --> 00:11:45,430 amount to the position to get the new position. 178 00:11:45,910 --> 00:11:50,620 Now, this is already at the end of the alphabet. So if this number 179 00:11:50,620 --> 00:11:52,150 which is going to be 25, 180 00:11:52,180 --> 00:11:57,180 because there's 26 letters in the alphabet and we start the position at zero. 181 00:11:57,820 --> 00:12:01,210 So this plus five is going to be 30. 182 00:12:01,750 --> 00:12:06,580 And what happens when we try to get the 30th item from the alphabet? Well, 183 00:12:06,610 --> 00:12:07,900 you'll find out when I hit enter. 184 00:12:08,350 --> 00:12:13,350 We get an index error and it tells us that list index out of range, and it 185 00:12:13,690 --> 00:12:16,240 occurred on line 15. 186 00:12:16,660 --> 00:12:20,710 So just as we predicted, this does not exist. 187 00:12:21,100 --> 00:12:25,510 We have to try and tackle this right? So if you haven't thought about this, 188 00:12:25,630 --> 00:12:29,440 this is the time to think about how can you solve this problem. 189 00:12:30,160 --> 00:12:35,160 I want you to pause for a moment and try to see if you can fix it. 190 00:12:39,480 --> 00:12:39,870 All right. 191 00:12:39,870 --> 00:12:44,870 So we know that this has a fixed length and what we want to happen when we have 192 00:12:46,470 --> 00:12:51,470 a letter towards the end that we need to encode is to loop back to the beginning 193 00:12:51,720 --> 00:12:54,360 of the alphabet. So to solve this, 194 00:12:54,600 --> 00:12:59,600 all we have to do is just to copy the existing letters in the alphabet and then 195 00:13:00,450 --> 00:13:02,610 just duplicate it once more. 196 00:13:03,120 --> 00:13:08,120 So this means that if we find a letter that's z and it needs to be shifted up 197 00:13:08,190 --> 00:13:13,020 by five letters, it's going to continue until it gets to e. 198 00:13:13,650 --> 00:13:14,010 Now, 199 00:13:14,010 --> 00:13:19,010 the really wonderful thing about this index function is it's going to give you the 200 00:13:19,080 --> 00:13:21,510 first index that it finds. 201 00:13:21,960 --> 00:13:26,370 So if we're looking for the letter a, it's going to give you the index zero 202 00:13:26,670 --> 00:13:28,080 and then it's going to stop. 203 00:13:28,170 --> 00:13:33,170 It's not going to look for all the indices and find this one at position 26. 204 00:13:33,690 --> 00:13:36,690 So it means the our code will now work 205 00:13:37,020 --> 00:13:41,670 even if we have a word that have a lot of letters towards the end of the 206 00:13:41,670 --> 00:13:44,970 alphabet. Let's go ahead and give it a spin, 207 00:13:46,100 --> 00:13:46,890 Right? 208 00:13:46,890 --> 00:13:51,240 Let's type encode. And our message is again, 209 00:13:51,510 --> 00:13:55,140 zulu. And we hit enter. 210 00:13:55,800 --> 00:14:00,300 And then we have a shift number of five and we get our encoded text. 211 00:14:00,540 --> 00:14:03,810 z becomes one, two, three, four, 212 00:14:03,810 --> 00:14:08,250 five, it becomes e, and u, one, two, three, four, 213 00:14:08,250 --> 00:14:12,870 five, becomes z. So now we've solved that small bug. 214 00:14:14,520 --> 00:14:17,280 Now there's a lot of other ways that you could have solved this bug, 215 00:14:17,310 --> 00:14:21,960 but I think this is the easiest way without disturbing our logic here and not 216 00:14:21,960 --> 00:14:24,330 making our function even longer. 217 00:14:25,110 --> 00:14:29,310 If you had any problems with your code, then this is the time to go back and fix 218 00:14:29,310 --> 00:14:33,630 it and make sure that you really understand everything that's going on here 219 00:14:33,930 --> 00:14:37,530 before you move on to the next lesson. But once you're ready, 220 00:14:37,770 --> 00:14:41,160 I'll see you on the next lesson where we're going to decrypt our code.