WEBVTT 00:00.830 --> 00:06.950 I said before that variables are locations in memory of certain size because of that type and those 00:06.950 --> 00:12.960 variables can change using the assignment operator or addition or other operators. 00:13.370 --> 00:19.610 But a lot of the time we want something that's just completely constant all the time and we never want 00:19.610 --> 00:21.630 it to change. 00:21.680 --> 00:25.090 So let's see how we can do this. 00:25.130 --> 00:30.090 So there's something in C++ called the conc keyword. 00:30.170 --> 00:34.380 And let's make a conc constant for pi. 00:35.050 --> 00:35.770 Hi. 00:35.800 --> 00:42.430 So we say Konst float Pi three point 1 4 1 5 9. 00:42.470 --> 00:45.280 Say OK. 00:45.630 --> 00:53.560 And so our convention for constants naming the constants will always be uppercase letters and if we 00:53.560 --> 00:57.820 have multiple words we'll use underscores. 00:57.820 --> 01:09.230 So let's say two pi would be two times this say. 01:09.540 --> 01:16.350 For now we're CAN going to use pi so see a complete example with this. 01:16.540 --> 01:23.610 So let's say float radius in c c h. 01:25.740 --> 01:32.100 These enter the radius of a circle 01:37.160 --> 01:42.060 in radius C.. 01:47.130 --> 01:53.190 Come brunette's the circle is 01:56.370 --> 02:00.900 two times pi times the radius 02:07.500 --> 02:07.860 k. 02:07.980 --> 02:09.270 So run this 02:11.750 --> 02:14.260 say 10. 02:14.430 --> 02:19.030 So the circumference is 62 point three you on it. 02:19.470 --> 02:29.350 So notice how we can use pi after you'd find it in other places in say error arithmetic operations. 02:29.460 --> 02:31.820 And I said before that it's constant. 02:31.820 --> 02:38.770 So let's try to actually change PI rate here and see what happens. 02:38.780 --> 02:49.760 So we actually got an error so it says hey you can't just add 1 to pi because it's constant to read 02:49.760 --> 02:55.360 only variable and not assignable as it says in gcc. 02:55.430 --> 02:57.940 So it really is conc and we can't change it. 03:00.830 --> 03:04.400 So you may ask why would we ever need constants. 03:04.400 --> 03:10.580 Why can't we just put three point 1 4 1 5 here instead. 03:11.030 --> 03:18.710 Well say if we use pi in many places in the code but for some reason we had to change it to make it 03:18.890 --> 03:25.930 more or less precise let's say we would only need to change it here in this one place. 03:26.120 --> 03:31.130 And then all the other places that use pi would get that change. 03:31.160 --> 03:36.170 So that's one reason why we always want to use constants. 03:36.170 --> 03:44.690 If they apply to our situation I never want to see what's called Magic Numbers which is just say we 03:44.690 --> 03:52.880 had instead of Pi we had three point 1 4 here and this is this three point four is really just a magic 03:52.880 --> 03:53.240 number. 03:53.240 --> 04:01.550 We don't really know what it is or what it represents and really constants are a good way to make code 04:01.610 --> 04:04.000 more readable and understandable. 04:04.280 --> 04:05.750 So that's why we use it. 04:05.930 --> 04:11.600 So we're always going to use constants where appropriate unless something like zero we don't need a 04:11.600 --> 04:17.150 constant effort for this for example doesn't that doesn't really matter too much. 04:17.240 --> 04:27.210 So we'll be using the concepts he word in actually in a few different ways in the next sections.