1 00:00:00,960 --> 00:00:04,800 In the last lesson we made it possible to reuse the result of the first 2 00:00:04,800 --> 00:00:05,633 calculation. 3 00:00:06,060 --> 00:00:10,470 The answer for the first calculation became our input for the next calculation, 4 00:00:10,830 --> 00:00:14,370 allowing us to chain the calculations together. However, 5 00:00:14,400 --> 00:00:16,920 this created quite a bit of repetition in our code 6 00:00:17,100 --> 00:00:20,520 and it limits our user to just two calculations. 7 00:00:21,690 --> 00:00:25,950 Let's allow the user to chain as many calculations as they want to. 8 00:00:26,760 --> 00:00:29,100 And I want to throw this over to you as a challenge, 9 00:00:29,100 --> 00:00:33,000 so can you use a while loop and the input function to achieve this? 10 00:00:33,360 --> 00:00:34,040 Ask the user 11 00:00:34,040 --> 00:00:39,040 this question, type 'y' to continue calculating with the previous result or type 'n' 12 00:00:39,630 --> 00:00:40,463 to exit. 13 00:00:41,700 --> 00:00:45,570 If the user types y then your calculator should allow them to continue chaining 14 00:00:45,570 --> 00:00:48,120 together calculations with the previous answer. 15 00:00:48,780 --> 00:00:52,020 If they type anything else the program should exit for now. 16 00:00:52,440 --> 00:00:55,500 I'll give you a few seconds to pause the video before I show you the result. 17 00:00:57,960 --> 00:01:02,520 All right. So the easiest way to solve this is through the use of a while loop. 18 00:01:02,940 --> 00:01:07,620 So let's say that after we've asked for the first number and we've shown them 19 00:01:07,620 --> 00:01:10,440 all of the different symbols that they could possibly pick from, 20 00:01:10,860 --> 00:01:14,130 lets set a flag called should_continue. 21 00:01:15,990 --> 00:01:18,510 And we'll set that to start out as true. 22 00:01:19,500 --> 00:01:24,120 Then we can create a while loop and while it should continue as true, 23 00:01:24,540 --> 00:01:26,490 then we're going to perform these things. 24 00:01:27,120 --> 00:01:31,500 Now we want to change some of this text so that it's a little bit more reusable. 25 00:01:31,950 --> 00:01:34,410 Let's say pick an operation, 26 00:01:35,460 --> 00:01:40,460 and what's the next number instead of the second number. 27 00:01:41,220 --> 00:01:45,750 So now we have a first number, we have a operation, 28 00:01:45,780 --> 00:01:50,610 we have a second number and then we pick out the function from that dictionary 29 00:01:50,610 --> 00:01:53,400 of operations and we get our answer. 30 00:01:53,670 --> 00:01:58,670 So I'm going to change this back to answer and we're passing in num1 31 00:01:59,550 --> 00:02:03,960 and num2 into the calculation function. Now, 32 00:02:03,960 --> 00:02:06,120 once this line has been printed, 33 00:02:06,840 --> 00:02:10,199 everything that occurs afterwards is pretty much repetition. 34 00:02:11,160 --> 00:02:16,160 And instead we're going to ask the user to type y-- type y to continue calculating 35 00:02:19,380 --> 00:02:21,840 with the answer from the previous step. 36 00:02:22,440 --> 00:02:27,440 Let's make that an fstring and let's check what this is actually equal to. 37 00:02:28,380 --> 00:02:33,180 So remember that the input function is also a function that has an output, 38 00:02:33,720 --> 00:02:35,040 which is a little bit then zen, 39 00:02:35,250 --> 00:02:39,240 but the output of this function is whatever the user typed in. 40 00:02:39,360 --> 00:02:44,360 So if they typed in y then this part is going to become y. That's where we're 41 00:02:44,880 --> 00:02:45,750 going to check. 42 00:02:45,840 --> 00:02:50,840 So we're going to check if the result of the input from the user is equal to y, 43 00:02:51,360 --> 00:02:55,650 then in that case that means we should continue and it should repeat back. 44 00:02:56,100 --> 00:02:57,930 But when it loops back, 45 00:02:57,990 --> 00:03:02,990 we want to make sure that num1 is equal to the answer from the previous step. 46 00:03:04,930 --> 00:03:07,540 So we can say if this is true, 47 00:03:07,690 --> 00:03:11,740 then num1 is going to be set to equal the answer. 48 00:03:12,460 --> 00:03:15,640 But on the other hand, else, if they typed anything else, 49 00:03:15,940 --> 00:03:20,940 so we could say maybe type y to continue calculating with answer or type n to 50 00:03:22,780 --> 00:03:23,613 exit. 51 00:03:24,760 --> 00:03:29,760 So now if they type y then the answer is going to be set as the num1 back at 52 00:03:30,940 --> 00:03:32,800 the start of this while loop 53 00:03:33,100 --> 00:03:36,550 so that this num1 becomes the answer from the previous step. 54 00:03:37,090 --> 00:03:40,510 But if they typed n then we want this while loop to the end. 55 00:03:40,780 --> 00:03:44,110 So we're going to change the flag should_continue to false. 56 00:03:44,830 --> 00:03:46,840 So now let's give that a run. 57 00:03:50,670 --> 00:03:51,000 Right? 58 00:03:51,000 --> 00:03:52,890 Let's say the first number is five. 59 00:03:53,310 --> 00:03:57,570 Then we're going to add three to five and then we're going to type y to 60 00:03:57,570 --> 00:04:00,990 continue. So now we get to pick another operation, 61 00:04:01,020 --> 00:04:03,180 so let's multiply, um, 62 00:04:03,240 --> 00:04:08,240 eight by two and we get the result from the next step and we can actually keep 63 00:04:09,600 --> 00:04:12,360 going until we're basically done, right? 64 00:04:12,360 --> 00:04:16,980 So we can take the 16 and we can divide it by four. 65 00:04:17,610 --> 00:04:19,529 But if we type 'n' on the other hand, 66 00:04:19,800 --> 00:04:23,250 then our program ends and we see the prompt once more. 67 00:04:24,990 --> 00:04:26,790 And that's the solution to the challenge, 68 00:04:26,850 --> 00:04:30,540 but it's a little bit sad to just exit, right? 69 00:04:30,540 --> 00:04:33,270 Because with a calculator more often than not, 70 00:04:33,270 --> 00:04:37,950 what you want to do is to start a fresh calculation where you get to determine 71 00:04:37,980 --> 00:04:41,310 the first number and the second number once more. 72 00:04:42,540 --> 00:04:45,120 If the user didn't want to exit 73 00:04:45,570 --> 00:04:48,690 but if they wanted to start a new calculation, 74 00:04:49,230 --> 00:04:54,230 then how can we get them to go back all the way up here so that they provide the 75 00:04:54,720 --> 00:04:59,610 num1 as a fresh input? This is a little bit tricky 76 00:04:59,760 --> 00:05:03,750 and in programming, this concept is known as recursion. 77 00:05:04,260 --> 00:05:08,700 It's basically the idea that you could have a function that calls itself. 78 00:05:09,300 --> 00:05:14,300 So let's define a new function called calculator and this function takes no 79 00:05:14,820 --> 00:05:17,100 inputs and has no outputs. 80 00:05:17,640 --> 00:05:22,410 But all of this code that we've got here so far is inside the calculator 81 00:05:22,410 --> 00:05:25,230 function. To begin when we start, 82 00:05:25,350 --> 00:05:30,350 we have to call the calculator function in order for it to find the place where 83 00:05:30,450 --> 00:05:35,190 this function was defined and to actually carry out all of these instructions. 84 00:05:35,790 --> 00:05:36,180 Now, 85 00:05:36,180 --> 00:05:40,560 the next thing we get to do is a little bit interesting because when the user 86 00:05:40,560 --> 00:05:41,850 types no, 87 00:05:42,690 --> 00:05:47,280 and says that they don't want to continue calculating with the previous answer 88 00:05:47,640 --> 00:05:50,760 but instead they want to start a new calculation, 89 00:05:51,120 --> 00:05:54,270 what we want to do instead of just exiting the while loop, 90 00:05:54,630 --> 00:05:59,630 we want to call the calculator function because what this is going to do is it's 91 00:06:00,560 --> 00:06:05,030 basically going to take us all the way back up to the beginning where we get to 92 00:06:05,030 --> 00:06:07,430 enter a new input again. 93 00:06:08,510 --> 00:06:11,990 And remember that once you reach the end of a function, 94 00:06:12,350 --> 00:06:17,300 everything gets reset to the beginning. So should_continue becomes true again 95 00:06:17,570 --> 00:06:19,820 and this while loop will continue working. 96 00:06:20,600 --> 00:06:22,970 So let's try running this new version. 97 00:06:23,720 --> 00:06:28,720 We've performed a calculation and now we want to start a new calculation, 98 00:06:29,030 --> 00:06:33,710 so I'm going to type n. And now I get to again define the first number again. 99 00:06:37,100 --> 00:06:42,100 This recursion basically happens because we're calling this calculator function 100 00:06:42,890 --> 00:06:45,110 within the calculator function. 101 00:06:45,710 --> 00:06:49,490 Essentially the code runs and runs and runs until it reaches here 102 00:06:50,060 --> 00:06:55,010 and if these conditions are met and this calculator function is called, 103 00:06:55,250 --> 00:07:00,230 then it goes and finds the calculator function in order to call it once more. 104 00:07:01,310 --> 00:07:05,960 But now you have to be quite careful with while loops and with these recursive 105 00:07:05,960 --> 00:07:06,793 functions 106 00:07:06,920 --> 00:07:11,920 because let's say that instead of having all of this code and some checking code 107 00:07:12,500 --> 00:07:14,750 to determine when I should call calculator, 108 00:07:15,230 --> 00:07:18,830 if I actually just called calculator within calculator, 109 00:07:19,130 --> 00:07:21,590 then this is going to be a infinite loop. 110 00:07:21,620 --> 00:07:23,900 It's just going to keep going back and forth. 111 00:07:24,290 --> 00:07:27,410 The calculator function calls the calculator function, 112 00:07:27,440 --> 00:07:31,400 which goes back up and calls and it goes around and round around until forever. 113 00:07:32,090 --> 00:07:32,480 Again, 114 00:07:32,480 --> 00:07:36,620 be careful and make sure that there is some sort of condition that needs to be 115 00:07:36,620 --> 00:07:41,620 met in order for this function to call itself. In the next lesson, 116 00:07:41,810 --> 00:07:45,830 we're gonna add the finishing touches to our program and also fix a bug that you 117 00:07:45,830 --> 00:07:47,450 may have already spotted at this point.