1 00:00:01,110 --> 00:00:02,130 In this exercise, 2 00:00:02,130 --> 00:00:05,010 you are going to be writing a function that checks 3 00:00:05,010 --> 00:00:10,010 whether if an input is a Prime Number. 4 00:00:10,410 --> 00:00:13,740 Prime numbers are numbers that can be cleanly divided 5 00:00:13,740 --> 00:00:17,730 by themselves and 1 and no other numbers. 6 00:00:17,730 --> 00:00:20,370 There are sort of mathematical peculiarity 7 00:00:20,370 --> 00:00:21,420 and there's a real beauty 8 00:00:21,420 --> 00:00:23,850 to these numbers if you read up on it. 9 00:00:23,850 --> 00:00:24,690 Now, in the Description, 10 00:00:24,690 --> 00:00:27,510 I've linked to the Wikipedia page on prime numbers. 11 00:00:27,510 --> 00:00:29,460 So if English is not your first language 12 00:00:29,460 --> 00:00:32,070 and you're not sure what it is I'm talking about 13 00:00:32,070 --> 00:00:33,810 take a look at Wikipedia 14 00:00:33,810 --> 00:00:37,923 and change the page to your own local language. 15 00:00:38,790 --> 00:00:40,616 The way that your function is going to work is 16 00:00:40,616 --> 00:00:42,370 it's going to be called prime_checker 17 00:00:44,190 --> 00:00:48,300 and it's going to use a logic to figure 18 00:00:48,300 --> 00:00:50,820 out whether if the input number that's put 19 00:00:50,820 --> 00:00:54,963 into the Input pane is a prime number or not. 20 00:00:55,830 --> 00:00:58,080 Now we're going limit the inputs to 21 00:00:58,080 --> 00:01:02,280 up to 100 because this program can be quite intensive 22 00:01:02,280 --> 00:01:04,050 and it can take quite long 23 00:01:04,050 --> 00:01:07,530 if you go up to a large number of numbers. 24 00:01:07,530 --> 00:01:10,650 So have a think about how you might achieve this 25 00:01:10,650 --> 00:01:14,910 by writing code and how you might achieve this 26 00:01:14,910 --> 00:01:17,250 by simply writing out the logic. 27 00:01:17,250 --> 00:01:18,210 If you get stuck, 28 00:01:18,210 --> 00:01:21,780 always try and figure out if you can think about it just 29 00:01:21,780 --> 00:01:26,280 from a logical point of view and then implement it in code. 30 00:01:26,280 --> 00:01:28,740 Either way, take a look at the description, 31 00:01:28,740 --> 00:01:30,630 take a look to see if you can figure it out 32 00:01:30,630 --> 00:01:32,130 and we'll go through the solution together 33 00:01:32,130 --> 00:01:33,120 on the next slide 34 00:01:33,120 --> 00:01:34,503 once you are done. 35 00:01:40,080 --> 00:01:43,560 The first thing we're going to do is to define our function. 36 00:01:43,560 --> 00:01:47,050 So on line one, I've got the definition for our function 37 00:01:47,995 --> 00:01:48,900 which is called prime_checker(), 38 00:01:48,900 --> 00:01:51,332 and it takes just one input, the number 39 00:01:51,332 --> 00:01:54,540 that we want to check. On the next line, 40 00:01:54,540 --> 00:01:58,410 I've defined a variable which is called is is_prime 41 00:01:58,410 --> 00:01:59,730 and I'm setting it to "True" 42 00:01:59,730 --> 00:02:04,050 by default and I'm going to change it to "False" 43 00:02:04,050 --> 00:02:07,080 if at some point my code detects that it's actually 44 00:02:07,080 --> 00:02:08,913 not a prime number. 45 00:02:10,199 --> 00:02:11,670 So how am I going check whether 46 00:02:11,670 --> 00:02:13,170 if it's a prime number or not? 47 00:02:13,170 --> 00:02:16,530 Well, I'm going to create a range and I'm going to start 48 00:02:16,530 --> 00:02:20,040 from the number 2 because we know 49 00:02:20,040 --> 00:02:24,570 that prime numbers are numbers that can only be divided 50 00:02:24,570 --> 00:02:27,780 by themselves and 1 cleanly, 51 00:02:27,780 --> 00:02:31,290 and that also means I don't need to increase the number 52 00:02:31,290 --> 00:02:34,263 by 1 at the end of the range either. 53 00:02:35,100 --> 00:02:38,880 We're going to loop through each of those numbers in this 54 00:02:38,880 --> 00:02:40,470 for loop, and we're going 55 00:02:40,470 --> 00:02:44,100 to check our potential prime number 56 00:02:44,100 --> 00:02:46,050 which is stored in the variable number 57 00:02:46,050 --> 00:02:47,970 as the input to our function, 58 00:02:47,970 --> 00:02:52,080 and we're going to check if we divide it by the current number 59 00:02:52,080 --> 00:02:55,320 in our range, does it divide cleanly? 60 00:02:55,320 --> 00:02:57,840 Because remember prime numbers can only be divided 61 00:02:57,840 --> 00:03:00,720 by 1 and itself cleanly. 62 00:03:00,720 --> 00:03:04,020 If any of the numbers in our range between 2 63 00:03:04,020 --> 00:03:08,520 and our number can substitute for i, and we end up 64 00:03:08,520 --> 00:03:09,660 with remainder 0, 65 00:03:09,660 --> 00:03:11,880 then it's clearly not a prime number. 66 00:03:11,880 --> 00:03:14,733 So we set is_prime to "False". 67 00:03:16,170 --> 00:03:18,570 Now the final part is the easiest. 68 00:03:18,570 --> 00:03:19,890 We just have an if and else statement. 69 00:03:19,890 --> 00:03:22,530 If at the end of our for loop, 70 00:03:22,530 --> 00:03:26,130 once we've checked through all of the numbers between 2 71 00:03:26,130 --> 00:03:30,210 up to the number that we're questioning and none 72 00:03:30,210 --> 00:03:34,050 of those happen to allow us to divide our number by it 73 00:03:34,050 --> 00:03:36,330 and have no remainder, 74 00:03:36,330 --> 00:03:41,330 then it's clearly a prime number and our is_prime variable 75 00:03:41,400 --> 00:03:43,830 is untouched, stays as "True". 76 00:03:43,830 --> 00:03:45,870 If anything else happens 77 00:03:45,870 --> 00:03:49,080 then it's obviously not a prime number 78 00:03:49,080 --> 00:03:52,274 and it is divisible by some number between 2 79 00:03:52,274 --> 00:03:53,643 and our number. 80 00:03:54,480 --> 00:03:57,150 The hardest part of this is to think 81 00:03:57,150 --> 00:04:00,150 and reason through the range that we create 82 00:04:00,150 --> 00:04:04,860 and using that range in our loop to check our number. 83 00:04:04,860 --> 00:04:07,770 See if you can understand how the logic works 84 00:04:07,770 --> 00:04:10,230 and if you had any issues passing the test 85 00:04:10,230 --> 00:04:11,640 in the previous lesson 86 00:04:11,640 --> 00:04:13,890 then go back there and try and fix your code.