WEBVTT 1 00:00:01.350 --> 00:00:03.340 In this video, we're gonna talk about 2 00:00:03.340 --> 00:00:06.230 the third data transformations method, 3 00:00:06.230 --> 00:00:08.363 which is the reduce method. 4 00:00:09.810 --> 00:00:11.480 And as you will remember, 5 00:00:11.480 --> 00:00:15.680 we use the reduce method to essentially boil down 6 00:00:15.680 --> 00:00:20.300 all the elements in an array to one single value. 7 00:00:20.300 --> 00:00:22.400 And we talked about the example of 8 00:00:22.400 --> 00:00:25.976 adding up all the numbers in one array, right? 9 00:00:25.976 --> 00:00:29.443 And so let's try that now with all movements array. 10 00:00:31.130 --> 00:00:35.493 And remember that array looks like this. Okay. 11 00:00:36.354 --> 00:00:39.190 And then by adding up all these numbers, 12 00:00:39.190 --> 00:00:41.682 so both the deposits and the withdrawals, 13 00:00:41.682 --> 00:00:46.682 we end up with the global balance of the account, basically. 14 00:00:46.850 --> 00:00:51.320 So let's start using the reduce method here. 15 00:00:51.320 --> 00:00:52.630 And so, as I just said, 16 00:00:52.630 --> 00:00:57.360 we're gonna call it on the movements array. 17 00:00:57.360 --> 00:01:00.590 And the results of the reduce method will be, 18 00:01:00.590 --> 00:01:04.223 as I just said, the global balance of the account. 19 00:01:06.260 --> 00:01:08.850 So let's call the result, balance. 20 00:01:08.850 --> 00:01:10.540 And remember that in this case, 21 00:01:10.540 --> 00:01:13.309 this balance will be simply one value 22 00:01:13.309 --> 00:01:15.383 and not an entire array. 23 00:01:16.928 --> 00:01:21.928 Okay. So the reduce function also gets a callback function, 24 00:01:22.610 --> 00:01:26.050 but this one is a little bit different from the other ones, 25 00:01:26.050 --> 00:01:28.825 like the one in map or for each. 26 00:01:28.825 --> 00:01:30.593 So in these other callbacks, 27 00:01:30.593 --> 00:01:32.860 the first parameter is always 28 00:01:32.860 --> 00:01:36.240 the current element of the array. 29 00:01:36.240 --> 00:01:37.500 Let's call it current. 30 00:01:37.500 --> 00:01:39.490 The second one is the current index 31 00:01:39.490 --> 00:01:42.630 and the third one is the entire array. 32 00:01:42.630 --> 00:01:45.915 But here in the callback function of the reduce method, 33 00:01:45.915 --> 00:01:48.410 the first parameter is actually 34 00:01:48.410 --> 00:01:50.950 something called the accumulator. 35 00:01:50.950 --> 00:01:55.030 So let's call it acc like this 36 00:01:55.030 --> 00:01:57.890 as an abbreviation of accumulator. 37 00:01:57.890 --> 00:02:02.030 And this accumulator is essentially like a snowball 38 00:02:02.030 --> 00:02:04.193 that keeps accumulating the value 39 00:02:04.193 --> 00:02:06.570 that we ultimately want to return. 40 00:02:06.570 --> 00:02:09.650 So in the case of adding all the elements 41 00:02:09.650 --> 00:02:12.445 or all the numbers of an array together, 42 00:02:12.445 --> 00:02:14.780 that will be the sum. 43 00:02:14.780 --> 00:02:16.580 So let me write that here. 44 00:02:16.580 --> 00:02:17.413 Oops. 45 00:02:18.410 --> 00:02:21.370 So let me write accumulator 46 00:02:22.260 --> 00:02:24.303 it's like a snowball. 47 00:02:25.779 --> 00:02:26.923 All right. 48 00:02:29.200 --> 00:02:33.520 But anyway, let's not write the actual function block here. 49 00:02:33.520 --> 00:02:36.710 So as always, this callback function here 50 00:02:36.710 --> 00:02:38.940 will be called in each iteration 51 00:02:38.940 --> 00:02:41.460 of a looping over the array. 52 00:02:41.460 --> 00:02:44.100 So reduce also loops over the array 53 00:02:44.100 --> 00:02:47.480 and calls this callback in each iteration, 54 00:02:47.480 --> 00:02:50.050 but now what will we actually do 55 00:02:50.050 --> 00:02:51.998 in each of these iterations? 56 00:02:51.998 --> 00:02:55.333 Well, since the accumulator is the value 57 00:02:55.333 --> 00:02:58.890 that we will keep adding to what we're gonna do here 58 00:02:58.890 --> 00:03:02.063 is to add the current value to the accumulator. 59 00:03:03.360 --> 00:03:08.360 So the accumulator plus the current value. Okay. 60 00:03:09.510 --> 00:03:12.060 And this works because in each call of 61 00:03:12.060 --> 00:03:13.310 the callback function, 62 00:03:13.310 --> 00:03:15.840 the accumulator will be the current sum 63 00:03:15.840 --> 00:03:18.190 of all the previous values. 64 00:03:18.190 --> 00:03:21.430 And so we will really keep adding to this accumulator 65 00:03:21.430 --> 00:03:23.673 in each iteration of the loop. 66 00:03:25.010 --> 00:03:28.660 Finally, we also need to return this value here 67 00:03:28.660 --> 00:03:30.000 from the callback. 68 00:03:30.000 --> 00:03:32.240 And so this is how the new accumulator 69 00:03:32.240 --> 00:03:35.992 can then be used in the next iteration of the loop. 70 00:03:35.992 --> 00:03:38.770 So basically in each loop iteration, 71 00:03:38.770 --> 00:03:41.325 we return the updated accumulator 72 00:03:41.325 --> 00:03:44.465 so the current one, plus the new current value. 73 00:03:44.465 --> 00:03:48.010 And so like this, we can then keep adding to it 74 00:03:48.010 --> 00:03:49.863 in the next iteration. 75 00:03:49.863 --> 00:03:50.763 Okay? 76 00:03:51.610 --> 00:03:55.810 So this callback function is the first argument 77 00:03:55.810 --> 00:03:57.600 of the reduce method, 78 00:03:57.600 --> 00:04:01.010 but the reduce method actually has a another, 79 00:04:01.010 --> 00:04:03.020 so a second parameter, 80 00:04:03.020 --> 00:04:06.640 and that is the initial value of the accumulator. 81 00:04:06.640 --> 00:04:08.794 So the value that we specify here, 82 00:04:08.794 --> 00:04:12.763 which in this case is gonna be zero is the initial value 83 00:04:12.763 --> 00:04:16.300 of the accumulator in the first loop iteration. 84 00:04:16.300 --> 00:04:20.047 And so in this example, we want to start counting 85 00:04:20.047 --> 00:04:23.420 or we want to start adding at zero. 86 00:04:23.420 --> 00:04:26.893 And so therefore we simply specify zero here. 87 00:04:30.290 --> 00:04:32.626 Okay, and with this we should be able 88 00:04:32.626 --> 00:04:35.441 to already take a look at our balance. 89 00:04:35.441 --> 00:04:39.450 And yeah, so we have one single number. 90 00:04:39.450 --> 00:04:44.450 So indeed everything was boiled down to this one number, 91 00:04:45.250 --> 00:04:48.580 which we can suppose is all of these values here 92 00:04:48.580 --> 00:04:51.339 added together, all right. 93 00:04:51.339 --> 00:04:54.179 And to make this even more clear to see, 94 00:04:54.179 --> 00:04:58.920 that's actually always log the accumulator to the console. 95 00:04:58.920 --> 00:05:02.877 So let's say iteration number I 96 00:05:06.430 --> 00:05:08.233 and enter the accumulator in there. 97 00:05:12.180 --> 00:05:15.380 Okay, so we see that in iteration zero, 98 00:05:15.380 --> 00:05:17.970 the accumulator is zero. 99 00:05:17.970 --> 00:05:20.710 And so this is the value that we specified here 100 00:05:20.710 --> 00:05:23.800 as the starter so as the initial value. 101 00:05:23.800 --> 00:05:25.170 Then in the second iteration, 102 00:05:25.170 --> 00:05:28.090 the accumulator is already at 200 103 00:05:28.090 --> 00:05:30.589 and that's because that is the initial value 104 00:05:30.589 --> 00:05:33.870 plus the current value in the previous iteration. 105 00:05:33.870 --> 00:05:35.283 So that was 200 there. 106 00:05:37.091 --> 00:05:41.220 And so this zero plus the 200 creates the new accumulator, 107 00:05:41.220 --> 00:05:44.460 which is done in the next iteration, 200. 108 00:05:44.460 --> 00:05:48.000 Then in this iteration, we add the current value again, 109 00:05:48.000 --> 00:05:49.840 which is 450. 110 00:05:49.840 --> 00:05:52.190 So that result is 650. 111 00:05:52.190 --> 00:05:53.888 And therefore in the next iteration, 112 00:05:53.888 --> 00:05:56.263 that's the value of the accumulator. 113 00:05:57.250 --> 00:05:59.532 Okay. And so here you can really see 114 00:05:59.532 --> 00:06:02.970 like the snowball effect of all of these values, 115 00:06:02.970 --> 00:06:05.530 adding up to one final value. 116 00:06:05.530 --> 00:06:09.279 And in the end, that value is essentially the accumulator. 117 00:06:09.279 --> 00:06:11.770 Now here we see the value of going down 118 00:06:11.770 --> 00:06:13.200 because in this iteration, 119 00:06:13.200 --> 00:06:15.340 we are adding minus 400 120 00:06:15.340 --> 00:06:19.533 and so 250 is 650 minus 400. 121 00:06:21.330 --> 00:06:25.283 Okay, so in the last iteration the value is 2,540, 122 00:06:27.520 --> 00:06:31.080 and then adding 1,300 will eventually result 123 00:06:31.080 --> 00:06:32.943 in this value here. 124 00:06:33.970 --> 00:06:38.420 Great. So this is really how the reduce method works. 125 00:06:38.420 --> 00:06:40.260 And I think this a log here 126 00:06:40.260 --> 00:06:42.333 is a great visualization of that. 127 00:06:43.230 --> 00:06:45.833 Let's give it all the space we need. 128 00:06:45.833 --> 00:06:48.986 And let me just change this one here to 100, 129 00:06:48.986 --> 00:06:52.870 so that you see the effect of changing this. 130 00:06:52.870 --> 00:06:56.220 And so now we can expect this accumulator 131 00:06:56.220 --> 00:06:59.320 in the beginning being 100. 132 00:06:59.320 --> 00:07:02.660 And so we already start with a small snowball here, 133 00:07:02.660 --> 00:07:04.981 which then we keep adding to, 134 00:07:04.981 --> 00:07:09.347 and therefore our final balance is now 100 euros 135 00:07:09.347 --> 00:07:12.793 or whatever it is larger. 136 00:07:13.950 --> 00:07:18.300 Okay. And yeah, 137 00:07:18.300 --> 00:07:21.270 let's one more time do the same thing manually, 138 00:07:21.270 --> 00:07:22.903 basically with a four loop. 139 00:07:26.140 --> 00:07:29.480 So of movements. 140 00:07:29.480 --> 00:07:33.380 And so we need one more time, an external variable here, 141 00:07:33.380 --> 00:07:35.907 which is gonna be the sum. 142 00:07:35.907 --> 00:07:39.290 Well, let's call it balance two. 143 00:07:39.290 --> 00:07:41.770 So balance two starts at zero. 144 00:07:41.770 --> 00:07:42.920 And so essentially this is, 145 00:07:42.920 --> 00:07:46.630 or initial accumulator value just like this zero 146 00:07:47.800 --> 00:07:52.800 and then sum plus equal the current movement. 147 00:07:54.750 --> 00:07:58.623 And if we lock balance two, we will get, okay. 148 00:08:00.330 --> 00:08:03.903 Now of course this has to be balance two, 149 00:08:03.903 --> 00:08:07.320 but now we get indeed the same result. 150 00:08:07.320 --> 00:08:10.800 And so here you can see this common pattern 151 00:08:10.800 --> 00:08:13.090 that we always need an external variable 152 00:08:13.090 --> 00:08:16.160 whenever we want to use a for loop. 153 00:08:16.160 --> 00:08:18.840 And that's fine if you only need one loop, 154 00:08:18.840 --> 00:08:21.870 but it starts to become really cumbersome 155 00:08:21.870 --> 00:08:24.710 and unpractical when we use many loops 156 00:08:24.710 --> 00:08:26.588 for doing many operations. 157 00:08:26.588 --> 00:08:29.350 So these methods that we've been studying, 158 00:08:29.350 --> 00:08:32.340 they completely avoid this extra variable 159 00:08:32.340 --> 00:08:34.570 and they simply return the variable 160 00:08:34.570 --> 00:08:37.238 or the value actually right away. 161 00:08:37.238 --> 00:08:38.153 All right. 162 00:08:39.278 --> 00:08:44.278 And of course we can write here in an even simpler way. 163 00:08:45.035 --> 00:08:47.127 So using an arrow function 164 00:08:47.127 --> 00:08:49.079 and I will still keep this here 165 00:08:49.079 --> 00:08:52.283 so that we can see the return that's happening. 166 00:08:53.670 --> 00:08:56.291 So first we can get rid of this. 167 00:08:56.291 --> 00:08:58.720 Then we get rid of this. 168 00:08:58.720 --> 00:09:00.633 We also don't need any of this, 169 00:09:01.490 --> 00:09:06.100 but here we always use or need the accumulator 170 00:09:06.100 --> 00:09:07.303 and the current value. 171 00:09:08.290 --> 00:09:09.870 So here we need parenthesis 172 00:09:10.800 --> 00:09:12.556 then the arrow. 173 00:09:12.556 --> 00:09:17.556 And then as always, we can get rid of all of this. 174 00:09:18.830 --> 00:09:23.710 And so now still, or balanced is the same. Awesome. 175 00:09:23.710 --> 00:09:26.700 So understanding how the reduce method works 176 00:09:26.700 --> 00:09:28.940 is something really important, 177 00:09:28.940 --> 00:09:32.220 but it's also way more confusing than the other ones, 178 00:09:32.220 --> 00:09:34.350 but I'm sure you're well underway 179 00:09:34.350 --> 00:09:36.366 of really understanding it. 180 00:09:36.366 --> 00:09:38.740 And now that we know how it works, 181 00:09:38.740 --> 00:09:42.600 we can actually now also calculate the balance 182 00:09:42.600 --> 00:09:46.460 of these movements here and then print that balance 183 00:09:46.460 --> 00:09:49.663 right here to our application and user interface. 184 00:09:50.560 --> 00:09:53.243 And so let's go ahead and do that here. 185 00:09:54.490 --> 00:09:57.155 So finally back to our application here. 186 00:09:57.155 --> 00:10:00.103 And so let's do that down here. 187 00:10:01.810 --> 00:10:06.363 So calc and print, balance. 188 00:10:09.880 --> 00:10:12.380 And so this function here will once again, 189 00:10:12.380 --> 00:10:15.273 receive the movements as an input. 190 00:10:16.110 --> 00:10:18.273 So any array of movements will work. 191 00:10:19.363 --> 00:10:24.363 And so now let's calculate the balance based on this array. 192 00:10:26.380 --> 00:10:29.943 And essentially this is exactly what we wrote to before. 193 00:10:31.220 --> 00:10:35.220 So that's movement dot reduce, 194 00:10:35.220 --> 00:10:38.360 but it doesn't hurt to write it here again. 195 00:10:38.360 --> 00:10:42.680 So remember the first parameter is now the accumulator 196 00:10:42.680 --> 00:10:45.810 and only the second one is the current value. 197 00:10:45.810 --> 00:10:48.163 So let's call it the movement now. 198 00:10:49.915 --> 00:10:54.372 And so here simply we return the accumulator 199 00:10:54.372 --> 00:10:58.793 plus the current movement and we start at zero. 200 00:10:59.750 --> 00:11:03.330 Great. And now all we need to do is to display it 201 00:11:03.330 --> 00:11:05.210 here in this element. 202 00:11:05.210 --> 00:11:10.210 So let's check this out and it is called the balance value. 203 00:11:10.500 --> 00:11:13.593 And just like before I already have it selected here, 204 00:11:14.840 --> 00:11:19.300 and this is a label, so balance value is here. 205 00:11:19.300 --> 00:11:21.047 So I called it label balance 206 00:11:21.047 --> 00:11:24.110 and so label is basically all the things 207 00:11:24.110 --> 00:11:27.983 where we simply want to put some text, all right? 208 00:11:33.785 --> 00:11:36.607 So let's say label balance dot text content. 209 00:11:37.620 --> 00:11:42.060 And this one we already know equals the balance 210 00:11:43.000 --> 00:11:47.470 and let's actually do a template string here, 211 00:11:47.470 --> 00:11:51.926 or a template literal and also display the Euro sign. 212 00:11:51.926 --> 00:11:53.710 Well, I don't know where it is, 213 00:11:53.710 --> 00:11:57.419 so let's just write EUR, which also means EURO. 214 00:11:57.419 --> 00:12:02.419 And so now let's see of course 215 00:12:02.720 --> 00:12:04.363 we didn't call the function yet. 216 00:12:05.210 --> 00:12:07.360 So let's do that here as well. 217 00:12:07.360 --> 00:12:10.670 And actually let's put this function here 218 00:12:10.670 --> 00:12:12.690 where it makes a little bit more sense 219 00:12:13.800 --> 00:12:17.693 because it kind of belongs to this display movements here. 220 00:12:18.955 --> 00:12:22.080 Okay. So let's actually also call this one here, 221 00:12:22.080 --> 00:12:25.633 calc and display balance. 222 00:12:26.610 --> 00:12:30.548 So to give it a similar name as that one. 223 00:12:30.548 --> 00:12:34.430 And so now I'm gonna call it, calc display balance. 224 00:12:34.430 --> 00:12:38.532 And once again, I'm calling it with account one. 225 00:12:38.532 --> 00:12:42.063 So account one dot movements, 226 00:12:43.090 --> 00:12:46.590 and then later on, as we keep developing this application 227 00:12:46.590 --> 00:12:49.800 of course these movements here that we're gonna use 228 00:12:49.800 --> 00:12:53.490 will be from the account that locked into the application. 229 00:12:53.490 --> 00:12:56.270 So if Jonas locked into the application, 230 00:12:56.270 --> 00:12:59.554 then Jonas's movements will be shown and Jonas balance. 231 00:12:59.554 --> 00:13:02.560 But if Steven locks in then of course 232 00:13:02.560 --> 00:13:06.910 Steven's movements and Steven's balance will be shown here. 233 00:13:06.910 --> 00:13:10.648 And right now we already see our balance up here. 234 00:13:10.648 --> 00:13:13.903 And so this actually happens to be exactly the same 235 00:13:13.903 --> 00:13:16.610 we calculated before. 236 00:13:16.610 --> 00:13:19.490 So also 3,840, 237 00:13:19.490 --> 00:13:23.453 because the movements are indeed also exactly the same. 238 00:13:23.453 --> 00:13:28.453 Great. So that's beautiful with this, 239 00:13:28.460 --> 00:13:31.776 we actually already completed some parts here 240 00:13:31.776 --> 00:13:34.370 of our flow chart. 241 00:13:34.370 --> 00:13:37.773 So let's also keep this one handy here 242 00:13:37.773 --> 00:13:40.163 so that we can keep looking at it. 243 00:13:40.163 --> 00:13:43.680 And so we already did calculating the balance 244 00:13:43.680 --> 00:13:45.380 and displaying the balance. 245 00:13:45.380 --> 00:13:47.726 So we did all this in just one function 246 00:13:47.726 --> 00:13:51.547 and we also already have this playing the movements. 247 00:13:51.547 --> 00:13:54.651 So indeed we are making some nice progress here. 248 00:13:54.651 --> 00:13:56.670 And the rest that you see here 249 00:13:56.670 --> 00:14:00.095 will be even easier as we go on. 250 00:14:00.095 --> 00:14:03.454 Now, just to finish this lecture here about reduce 251 00:14:03.454 --> 00:14:06.260 let me show you one final example 252 00:14:07.190 --> 00:14:09.422 because we can also do other stuff, 253 00:14:09.422 --> 00:14:12.090 than just adding up values 254 00:14:12.090 --> 00:14:15.220 that would become boring at some point, right? 255 00:14:15.220 --> 00:14:18.251 And so we can also do other stuff. 256 00:14:18.251 --> 00:14:22.780 So this time, what I want to do is to get the maximum value 257 00:14:24.750 --> 00:14:26.918 of the movements array here. 258 00:14:26.918 --> 00:14:29.880 Okay, so in this case, 259 00:14:29.880 --> 00:14:32.910 the result we're looking for is this 3,000. 260 00:14:32.910 --> 00:14:36.840 Okay. And so for that, we can also use reduce, 261 00:14:36.840 --> 00:14:40.688 because remember reduce is for boiling down the array 262 00:14:40.688 --> 00:14:43.410 into just one single value, 263 00:14:43.410 --> 00:14:46.100 but that value can be whatever we want. 264 00:14:46.100 --> 00:14:47.800 So it doesn't have to be a sum. 265 00:14:47.800 --> 00:14:49.654 It could be a multiplication 266 00:14:49.654 --> 00:14:52.160 or even something completely different, 267 00:14:52.160 --> 00:14:54.768 like a string or an object, 268 00:14:54.768 --> 00:14:57.930 but here we will keep working with numbers, 269 00:14:57.930 --> 00:15:00.113 but this time we want the maximum number. 270 00:15:01.170 --> 00:15:04.720 So let's kind of go manually through this array 271 00:15:04.720 --> 00:15:08.155 to determine what the reduce method should do. 272 00:15:08.155 --> 00:15:10.700 So let's start here at 200. 273 00:15:10.700 --> 00:15:13.580 Is this the maximum value at this point? 274 00:15:13.580 --> 00:15:16.870 Well, we don't know yet because we just started, 275 00:15:16.870 --> 00:15:21.520 so let's save 200 here mentally as our current maximum, 276 00:15:21.520 --> 00:15:25.930 then we move on to the next value and we see 450. 277 00:15:25.930 --> 00:15:28.800 So is this greater than our current maximum, 278 00:15:28.800 --> 00:15:30.860 which was 200? 279 00:15:30.860 --> 00:15:33.200 Well, yes it is, it is greater. 280 00:15:33.200 --> 00:15:37.640 And so 450 is now our current maximum 281 00:15:37.640 --> 00:15:41.433 then minus 400 is of course less than our current maximum. 282 00:15:41.433 --> 00:15:43.602 Then we move on even further. 283 00:15:43.602 --> 00:15:48.060 So 3,000 is a greater than our current maximum? 284 00:15:48.060 --> 00:15:49.260 Yes it is. 285 00:15:49.260 --> 00:15:52.200 And so that becomes our current maximum. 286 00:15:52.200 --> 00:15:54.520 So we store this 3,000 287 00:15:54.520 --> 00:15:56.867 and then we keep going through the array. 288 00:15:56.867 --> 00:16:01.867 And as you see, there is no other value greater than 3,000. 289 00:16:02.120 --> 00:16:06.943 And so in the end or maximum was this one, so 3,000. 290 00:16:08.950 --> 00:16:11.900 Okay. And now all we need to do is to 291 00:16:11.900 --> 00:16:14.760 essentially translate this into code, 292 00:16:14.760 --> 00:16:16.303 using the reduce method. 293 00:16:18.660 --> 00:16:23.363 So let's call it max movements to reduce. 294 00:16:24.670 --> 00:16:27.360 And as always, we need the accumulator 295 00:16:27.360 --> 00:16:29.120 and the current value. 296 00:16:29.120 --> 00:16:31.020 So I'm calling it movement here again. 297 00:16:32.320 --> 00:16:33.300 All right. 298 00:16:33.300 --> 00:16:36.010 And now this time, what should be the purpose 299 00:16:36.010 --> 00:16:38.190 of this accumulator value? 300 00:16:38.190 --> 00:16:40.639 So that's always the big question that we have to ask 301 00:16:40.639 --> 00:16:42.640 when we use reduce. 302 00:16:42.640 --> 00:16:46.257 So up here, when we wanted to add all the numbers together, 303 00:16:46.257 --> 00:16:48.600 the purpose of the accumulator 304 00:16:48.600 --> 00:16:51.600 was to keep track of the current sum. 305 00:16:51.600 --> 00:16:54.233 And so here, the accumulator will be the one 306 00:16:54.233 --> 00:16:57.393 that will keep track of the current maximum value. 307 00:16:58.270 --> 00:17:02.090 So let's just start by writing the logic here 308 00:17:02.090 --> 00:17:05.387 in a bigger way so we can understand what's happening. 309 00:17:05.387 --> 00:17:07.080 And then I will simplify it 310 00:17:07.080 --> 00:17:10.432 to make this a one-liner function one more time. 311 00:17:10.432 --> 00:17:15.288 So translating the logic that we just went over here 312 00:17:15.288 --> 00:17:19.588 is to say, if the accumulator is greater 313 00:17:19.588 --> 00:17:21.660 than the current value, 314 00:17:21.660 --> 00:17:26.660 which is the movement, then return the accumulator, right? 315 00:17:28.640 --> 00:17:30.350 Because in the reduce method, 316 00:17:30.350 --> 00:17:34.010 we always have to somehow return the accumulator 317 00:17:34.010 --> 00:17:35.860 to the next iteration. 318 00:17:35.860 --> 00:17:38.940 And in this case, we simply want to keep the accumulator 319 00:17:38.940 --> 00:17:42.332 at the value that it already is and not change it. 320 00:17:42.332 --> 00:17:47.332 Okay. We want to change it in the other scenario, basically. 321 00:17:48.310 --> 00:17:52.100 So when the current value is greater than the accumulator, 322 00:17:52.100 --> 00:17:54.050 so that would be this case here. 323 00:17:54.050 --> 00:17:57.230 So here, remember the current value is 450, 324 00:17:57.230 --> 00:17:59.594 but the accumulator is 200. 325 00:17:59.594 --> 00:18:03.529 And so now the movement is greater than accumulator. 326 00:18:03.529 --> 00:18:06.990 And so what we want to return here as the accumulator 327 00:18:06.990 --> 00:18:09.883 in the next iteration is the current movement. 328 00:18:12.944 --> 00:18:15.813 And so that's it, okay. 329 00:18:17.040 --> 00:18:19.390 And now we also need the initial value. 330 00:18:19.390 --> 00:18:23.480 And so the initial value make sense to be 331 00:18:23.480 --> 00:18:26.230 this first value here of the array. 332 00:18:26.230 --> 00:18:31.128 Okay. And so that's movements at position zero. 333 00:18:31.128 --> 00:18:33.400 Now we could have used zero here, 334 00:18:33.400 --> 00:18:35.540 but that would not be correct 335 00:18:35.540 --> 00:18:37.770 because imagine that the first value 336 00:18:37.770 --> 00:18:39.783 would be like a negative, 337 00:18:39.783 --> 00:18:42.810 then this might not work as expected. 338 00:18:42.810 --> 00:18:45.020 Maybe it might work with the maximum, 339 00:18:45.020 --> 00:18:47.334 but not with a minimum, for example. 340 00:18:47.334 --> 00:18:50.010 So don't just put zero here 341 00:18:50.010 --> 00:18:53.410 when you're trying to find a maximum or a minimum value, 342 00:18:53.410 --> 00:18:56.800 always just go with the first value of the array. 343 00:18:56.800 --> 00:18:59.480 Okay. And that's it. 344 00:18:59.480 --> 00:19:00.960 Let's see if it works 345 00:19:00.960 --> 00:19:03.393 and then we can maybe go over this again. 346 00:19:06.040 --> 00:19:09.670 So indeed we get 3,000. 347 00:19:09.670 --> 00:19:12.420 So we started at 200. 348 00:19:12.420 --> 00:19:13.950 So in this iteration, 349 00:19:13.950 --> 00:19:17.040 is the accumulator greater than the movement? 350 00:19:17.040 --> 00:19:18.690 Well, it is not. 351 00:19:18.690 --> 00:19:23.300 And so return the current movement. Okay. 352 00:19:23.300 --> 00:19:25.245 But the first iteration is not that important. 353 00:19:25.245 --> 00:19:28.570 It gets interesting here in the second one. 354 00:19:28.570 --> 00:19:31.090 So now our accumulator is at 200 355 00:19:31.090 --> 00:19:34.360 and the current movement is at 450. 356 00:19:34.360 --> 00:19:36.970 So this one here is false again. 357 00:19:36.970 --> 00:19:39.190 And so now we return to movement 358 00:19:39.190 --> 00:19:42.220 as the new accumulator in the next iteration. 359 00:19:42.220 --> 00:19:44.710 All right. And so in the next iteration, 360 00:19:44.710 --> 00:19:47.289 that accumulator is 450 now, 361 00:19:47.289 --> 00:19:52.289 then here, the current movement is minus 400. 362 00:19:52.540 --> 00:19:57.540 And so here is 450 greater than minus 400. 363 00:19:57.830 --> 00:19:59.530 Yes, of course it is. 364 00:19:59.530 --> 00:20:01.840 And so let's simply return the accumulator 365 00:20:01.840 --> 00:20:04.970 so that it stays the same in the next iteration. 366 00:20:04.970 --> 00:20:06.910 And so that's the logic all the way 367 00:20:06.910 --> 00:20:08.800 until the end of the array. 368 00:20:08.800 --> 00:20:12.820 And, by this, we then end up with this maximum value. 369 00:20:12.820 --> 00:20:16.320 All right. So I hope that made sense. 370 00:20:16.320 --> 00:20:18.610 And in fact, there is a ton of things 371 00:20:18.610 --> 00:20:21.550 that we can do with this reduce method. 372 00:20:21.550 --> 00:20:24.863 It is by far the most powerful array method there is. 373 00:20:26.380 --> 00:20:27.490 And because of that, 374 00:20:27.490 --> 00:20:30.180 it can also be the hardest one to use. 375 00:20:30.180 --> 00:20:33.070 So we always need to think exactly what we want 376 00:20:33.070 --> 00:20:36.170 the accumulator and the core value to be 377 00:20:36.170 --> 00:20:37.765 and how they should interact. 378 00:20:37.765 --> 00:20:40.807 But I hope that with this exercise here 379 00:20:40.807 --> 00:20:44.210 I could demonstrate the thought process a little bit 380 00:20:44.210 --> 00:20:47.290 and throughout the section and the rest of the course, 381 00:20:47.290 --> 00:20:50.890 there will be some more exercises of the reduce method 382 00:20:50.890 --> 00:20:53.250 so that you can also learn better and better 383 00:20:53.250 --> 00:20:55.263 how to use it yourself one day.