WEBVTT 00:00.460 --> 00:07.740 The saw the last section the pattern for control flow where it took different actions based on a condition. 00:07.800 --> 00:09.460 So look something like this. 00:09.480 --> 00:19.850 We had the word in the parentheses which contained the condition that condition of course evaluates 00:19.850 --> 00:21.980 to TRUE or FALSE. 00:22.210 --> 00:28.350 And then you had the statement block where you had all your statements 00:30.880 --> 00:32.040 in the last section. 00:32.040 --> 00:36.210 This was the if statement and the switch statement. 00:36.480 --> 00:43.650 But in this section we want to take a look at how we can do statements multiple times. 00:43.650 --> 00:48.150 And if you remember in the first section we call this a loop. 00:48.180 --> 00:58.710 So in this lecture I went to go over what's called the while loop and the while loop has this syntax. 00:58.980 --> 01:09.390 So the key word you just have the key word while while here and the while has a condition just like 01:09.390 --> 01:10.120 an IF statement. 01:10.140 --> 01:17.070 But this condition is really what I talked about in the first section which is the stopping condition 01:17.670 --> 01:22.320 or the condition which is long as this is true. 01:22.550 --> 01:26.710 It will do the statements essentially. 01:26.880 --> 01:34.530 So remember that the we have to have we have to have this stopping condition otherwise the loop will 01:34.620 --> 01:37.020 go infinitely. 01:37.050 --> 01:44.970 So the dition is really any boolean condition you want and here's how I kind of think about it. 01:44.970 --> 01:52.590 So while the condition is true let me just put true here. 01:52.860 --> 02:03.940 It will do one iteration that is one set of all of these statements so do these statements one time 02:04.810 --> 02:12.080 and then it'll go back up and check the condition again to see if it's still true. 02:12.820 --> 02:17.920 And if it is it will again do one more iteration of all these statements. 02:17.920 --> 02:21.610 But if it's false it'll sort of exit this while loop. 02:21.610 --> 02:29.890 It won't do any of the statements in this statement block and it'll just continue on doing the rest 02:29.890 --> 02:32.510 of the program. 02:32.510 --> 02:37.420 So let's take a look at an example. 02:37.560 --> 02:49.990 Let's say we had a variable I would call or set it to be 1 in one of the constants and we'll call this 02:50.000 --> 02:52.410 maximal set to 10. 02:52.870 --> 03:04.500 And the condition is while it is less than or equal to 10 or Max don't use magic numbers. 03:05.410 --> 03:20.290 So it will just say seek out the current iteration is an analyst say I. 03:20.820 --> 03:24.090 So really just printing out 1 to 10. 03:24.650 --> 03:33.080 And then here we need the we need to increment I because otherwise if he didn't i will always have the 03:33.080 --> 03:41.670 value 1 and since one is less than or equal to Max this will always be true. 03:41.750 --> 03:53.180 So we loop infinitely if we didn't have this I so this really controls when the loop stops. 03:53.210 --> 03:59.640 Plus plus in its run it and see what happens. 03:59.640 --> 04:00.800 So here we go. 04:01.050 --> 04:08.250 So says the current iteration is 1 and then 2 and so on up to 10. 04:08.250 --> 04:22.870 So one thing to note here is when just after Princess this last line here and the I variable is 10 so 04:22.870 --> 04:25.790 when it's 10 it'll print this out. 04:25.810 --> 04:30.380 And then it will increment the variable which will be 11. 04:30.590 --> 04:39.230 Now it'll go back up to the while condition and it will actually check the condition one last time. 04:39.230 --> 04:42.750 So 11 is not less than or equal to 10. 04:42.770 --> 04:51.280 Therefore this condition becomes false and then it will break out into the rest of the program. 04:52.050 --> 04:59.420 And so it always does it one more time to check to see if it's false or not. 05:02.460 --> 05:04.050 So here 05:06.600 --> 05:13.740 the the while loop is really just another control flow structure but instead of making one decision 05:14.310 --> 05:21.030 it really does make the same decision multiple times until it's false. 05:21.030 --> 05:23.930 That's kind of the gist of the While loop. 05:23.950 --> 05:26.720 So let's see another example. 05:26.780 --> 05:29.490 So let's say we had some 05:33.530 --> 05:38.860 in it to be zero for now and we have Max to be 100. 05:38.950 --> 05:46.450 And what we're going to do is we're going to sum up all the even numbers from 1 to Max. 05:47.050 --> 05:48.580 So do this 05:52.100 --> 05:59.600 say if it is modulus called zero. 05:59.610 --> 06:13.820 So this means if it's even if it is even then right some are going to say plus equals. 06:13.980 --> 06:26.720 So this going to sum all the even eyes and then we're going to increment K and we're on to see out some 06:28.700 --> 06:38.050 of all even numbers from 1 to and really see Max here. 06:42.160 --> 06:42.880 Is 06:46.190 --> 06:46.590 some 06:51.080 --> 06:53.380 in is going to run this. 06:53.390 --> 07:04.700 So the sum of all even numbers from 1 to 100 is 2500 and 50 and notice we're using a constant. 07:04.710 --> 07:14.570 So you change this in you one to 1000 in there you go see. 07:14.580 --> 07:16.320 See the power of Constance. 07:16.330 --> 07:20.260 We to change one thing and then it changed the rest. 07:20.660 --> 07:29.310 That's why we introduced Constance her useful so the while loop really is the mother of all loops. 07:29.310 --> 07:33.060 And in fact we don't really need any other loops at all. 07:33.060 --> 07:43.560 However there are some that are make things a bit more convenient and Runcie to other loops at have 07:43.560 --> 07:48.160 some special purpose or make things a little bit different than the wild. 07:48.580 --> 07:50.520 And we're going to see that in the next lecture.