WEBVTT 00:00.490 --> 00:06.050 In this lecture I want to formalize how we make decisions in C++. 00:06.120 --> 00:12.690 We talked before about if statements in control flow and that's what we're going to talk more about 00:12.690 --> 00:13.420 today. 00:14.190 --> 00:23.290 Need some way of making decisions based on a condition and we use the if in otherwise for that in the 00:23.290 --> 00:24.750 first section. 00:24.760 --> 00:28.600 Now we want to go over how we do that in C++. 00:28.630 --> 00:32.640 So let's start with a an example here. 00:32.650 --> 00:37.310 So index sequel 0 y with zero 00:40.190 --> 00:46.790 in say you want to get two numbers from the user. 00:46.800 --> 00:49.760 So please enter two numbers. 00:50.970 --> 00:53.950 This is piece to me. 00:58.260 --> 01:00.840 And we'll get those two numbers. 01:00.980 --> 01:01.360 In 01:05.200 --> 01:14.400 and now you know what we want to do is see which which number is bigger than the other. 01:14.430 --> 01:23.560 So how we do that is we say if x is greater than Y 01:27.750 --> 01:33.330 then and notice the traces here see. 01:33.360 --> 01:34.220 Ouch. 01:37.280 --> 01:50.110 Do something strange in this a case one bigger number is Ingrid's X 01:53.860 --> 01:57.230 so knows how we construct an if statement. 01:57.340 --> 02:07.640 It always starts with if the left keyword so is parentheses around this what's called the condition 02:09.820 --> 02:12.890 and the in parentheses. 02:13.220 --> 02:14.670 So this is how you start. 02:14.710 --> 02:18.970 If statement say say F for the C and then you write the condition. 02:18.980 --> 02:22.700 In this case is X greater than Y. 02:22.850 --> 02:30.320 And this condition has to evaluate to TRUE or FALSE and that's why we've been learning the boolean logic 02:31.340 --> 02:35.430 and notice here we have the statement Plock. 02:35.450 --> 02:42.520 So remember in the first lecture of this section we went over these curly braces and what they mean. 02:42.770 --> 02:51.460 Well this means that if this is true of this value it's true then it will do this statement. 02:51.470 --> 02:58.680 BLOCK They all can have as many statements here as we want. 02:58.810 --> 03:02.040 OK we only have one in this case. 03:02.190 --> 03:07.950 So if this is true if x is greater than Y it'll do this case. 03:07.950 --> 03:12.500 If it's not and we used both before we used otherwise. 03:12.510 --> 03:21.760 But really in C++ it's it's else use keyword else. 03:21.860 --> 03:24.730 And again notice these braces. 03:24.800 --> 03:45.520 So Otherwise if it's not greater then why you do case to bigger number is why. 03:45.600 --> 03:54.300 So if this evaluates to false then it'll go here into this else. 03:54.310 --> 04:00.280 Case in the else case is really a catch all case. 04:01.080 --> 04:05.380 And you'll see why I say that in a minute. 04:05.620 --> 04:14.060 But if this is a false then it'll always just do this last case this L's case case to hear. 04:14.170 --> 04:23.990 So let's run this and see say 10 in three. 04:24.070 --> 04:24.980 So it does. 04:24.980 --> 04:27.940 Case 1 the bigger number is 10. 04:28.250 --> 04:30.040 So we got the two numbers. 04:30.110 --> 04:32.860 X was 10 in Y's three. 04:32.920 --> 04:35.730 Ten is of course bigger than three right. 04:35.760 --> 04:37.710 Ten is greater than three. 04:37.790 --> 04:45.770 So it did this first case and said see out Case 1 in the output she's won the bigger number is 10. 04:46.520 --> 04:47.130 OK. 04:47.410 --> 04:49.500 Let's run this again. 04:49.660 --> 04:59.220 Let's say three and ten so we can switch the order and it says case to the bigger number is 10. 04:59.360 --> 05:05.300 So that's why we have this case one case to tell which one of these cases it went to. 05:05.300 --> 05:09.830 So this is really what I mean by control flow it depends. 05:10.380 --> 05:23.890 Well different executions will get will get executed depending on the initial condition as the initial 05:23.890 --> 05:26.360 condition x is greater than Y. 05:26.590 --> 05:30.880 So it will execute different code depending on that condition. 05:32.720 --> 05:37.110 So of course this is kind of still not right. 05:37.300 --> 05:44.720 What if the two numbers are the same size 10 and 10 here does. 05:44.770 --> 05:52.200 Case two the bigger number is 10 because 10 is not greater than 10 so that they evaluate to false. 05:52.220 --> 06:00.710 So it kind of skipped this first case and then went down to this this last case here and just outputted 06:00.710 --> 06:06.910 that it's why but really these are equal so we kind of want to fix that. 06:06.910 --> 06:11.470 So how do we fix that is will say if else if 06:14.190 --> 06:19.380 X is less than Y right. 06:19.400 --> 06:23.450 So why greater than x then what print out why. 06:23.570 --> 06:26.820 So notice how we have this else here. 06:27.050 --> 06:35.400 This is really a way of changing multiple conditions to a single control. 06:35.540 --> 06:40.100 In this case the state control flow structure. 06:40.280 --> 06:48.470 So how this works is if this was false then it all skipped. 06:48.470 --> 06:52.560 This can this case and then evaluate this case. 06:52.570 --> 07:01.060 But if this was true it would never execute this second case if X was greater than why it won't execute 07:02.240 --> 07:03.370 this at all. 07:04.460 --> 07:08.540 So this wouldn't be run at all. 07:10.300 --> 07:20.440 So if x is greater than Y is false and if Y where X is less than Y is false. 07:21.270 --> 07:24.020 Then Case 3 07:29.550 --> 07:33.550 they must be equal or 8. 07:33.670 --> 07:38.260 So they would have to be equal in this case. 07:38.510 --> 07:41.730 And this is a 10 and 10. 07:42.090 --> 07:49.700 They must be equal right because 10 is not greater than 10 and 10 is not less than 10. 07:49.710 --> 07:52.080 So these are false. 07:52.410 --> 07:54.420 And then it went OK. 07:54.420 --> 08:01.890 So this catch all caught it and said OK they must be equal which they are. 08:03.630 --> 08:04.590 So if I did 08:09.840 --> 08:16.410 I did 5 3 5 5 and 10. 08:16.890 --> 08:19.320 So it did case to rape. 08:19.530 --> 08:23.850 So it's it said is five greater than 10. 08:23.850 --> 08:25.540 No this is false. 08:25.530 --> 08:32.170 So skip this case and I'll do is five less than 10. 08:32.280 --> 08:33.360 This is true. 08:33.720 --> 08:34.610 And then the output. 08:34.620 --> 08:38.790 This case to note is nowhere in the output. 08:38.800 --> 08:43.110 It's a case one case three and this is a control flow. 08:43.740 --> 08:49.940 So you can have as many else if extra conditions as you want. 08:49.940 --> 08:51.400 You can chain them all together. 08:53.760 --> 09:00.700 But it is you know always evaluate each one until you find the first one that is true. 09:01.230 --> 09:03.090 If none are true none that they can. 09:03.090 --> 09:05.260 Else if or if conditions it's true. 09:05.280 --> 09:11.040 It always goes to this catch all else. 09:11.090 --> 09:18.950 So in the next lecture I'll show you a better way of changing many many commands are many many conditions 09:18.950 --> 09:22.670 sorry together in a more efficient way.