Would you like to inspect the original subtitles? These are the user uploaded subtitles that are being translated: 1 1 00:00:01,350 --> 00:00:03,340 In this video, we're gonna talk about 2 2 00:00:03,340 --> 00:00:06,230 the third data transformations method, 3 3 00:00:06,230 --> 00:00:08,363 which is the reduce method. 4 4 00:00:09,810 --> 00:00:11,480 And as you will remember, 5 5 00:00:11,480 --> 00:00:15,680 we use the reduce method to essentially boil down 6 6 00:00:15,680 --> 00:00:20,300 all the elements in an array to one single value. 7 7 00:00:20,300 --> 00:00:22,400 And we talked about the example of 8 8 00:00:22,400 --> 00:00:25,976 adding up all the numbers in one array, right? 9 9 00:00:25,976 --> 00:00:29,443 And so let's try that now with all movements array. 10 10 00:00:31,130 --> 00:00:35,493 And remember that array looks like this. Okay. 11 11 00:00:36,354 --> 00:00:39,190 And then by adding up all these numbers, 12 12 00:00:39,190 --> 00:00:41,682 so both the deposits and the withdrawals, 13 13 00:00:41,682 --> 00:00:46,682 we end up with the global balance of the account, basically. 14 14 00:00:46,850 --> 00:00:51,320 So let's start using the reduce method here. 15 15 00:00:51,320 --> 00:00:52,630 And so, as I just said, 16 16 00:00:52,630 --> 00:00:57,360 we're gonna call it on the movements array. 17 17 00:00:57,360 --> 00:01:00,590 And the results of the reduce method will be, 18 18 00:01:00,590 --> 00:01:04,223 as I just said, the global balance of the account. 19 19 00:01:06,260 --> 00:01:08,850 So let's call the result, balance. 20 20 00:01:08,850 --> 00:01:10,540 And remember that in this case, 21 21 00:01:10,540 --> 00:01:13,309 this balance will be simply one value 22 22 00:01:13,309 --> 00:01:15,383 and not an entire array. 23 23 00:01:16,928 --> 00:01:21,928 Okay. So the reduce function also gets a callback function, 24 24 00:01:22,610 --> 00:01:26,050 but this one is a little bit different from the other ones, 25 25 00:01:26,050 --> 00:01:28,825 like the one in map or for each. 26 26 00:01:28,825 --> 00:01:30,593 So in these other callbacks, 27 27 00:01:30,593 --> 00:01:32,860 the first parameter is always 28 28 00:01:32,860 --> 00:01:36,240 the current element of the array. 29 29 00:01:36,240 --> 00:01:37,500 Let's call it current. 30 30 00:01:37,500 --> 00:01:39,490 The second one is the current index 31 31 00:01:39,490 --> 00:01:42,630 and the third one is the entire array. 32 32 00:01:42,630 --> 00:01:45,915 But here in the callback function of the reduce method, 33 33 00:01:45,915 --> 00:01:48,410 the first parameter is actually 34 34 00:01:48,410 --> 00:01:50,950 something called the accumulator. 35 35 00:01:50,950 --> 00:01:55,030 So let's call it acc like this 36 36 00:01:55,030 --> 00:01:57,890 as an abbreviation of accumulator. 37 37 00:01:57,890 --> 00:02:02,030 And this accumulator is essentially like a snowball 38 38 00:02:02,030 --> 00:02:04,193 that keeps accumulating the value 39 39 00:02:04,193 --> 00:02:06,570 that we ultimately want to return. 40 40 00:02:06,570 --> 00:02:09,650 So in the case of adding all the elements 41 41 00:02:09,650 --> 00:02:12,445 or all the numbers of an array together, 42 42 00:02:12,445 --> 00:02:14,780 that will be the sum. 43 43 00:02:14,780 --> 00:02:16,580 So let me write that here. 44 44 00:02:16,580 --> 00:02:17,413 Oops. 45 45 00:02:18,410 --> 00:02:21,370 So let me write accumulator 46 46 00:02:22,260 --> 00:02:24,303 it's like a snowball. 47 47 00:02:25,779 --> 00:02:26,923 All right. 48 48 00:02:29,200 --> 00:02:33,520 But anyway, let's not write the actual function block here. 49 49 00:02:33,520 --> 00:02:36,710 So as always, this callback function here 50 50 00:02:36,710 --> 00:02:38,940 will be called in each iteration 51 51 00:02:38,940 --> 00:02:41,460 of a looping over the array. 52 52 00:02:41,460 --> 00:02:44,100 So reduce also loops over the array 53 53 00:02:44,100 --> 00:02:47,480 and calls this callback in each iteration, 54 54 00:02:47,480 --> 00:02:50,050 but now what will we actually do 55 55 00:02:50,050 --> 00:02:51,998 in each of these iterations? 56 56 00:02:51,998 --> 00:02:55,333 Well, since the accumulator is the value 57 57 00:02:55,333 --> 00:02:58,890 that we will keep adding to what we're gonna do here 58 58 00:02:58,890 --> 00:03:02,063 is to add the current value to the accumulator. 59 59 00:03:03,360 --> 00:03:08,360 So the accumulator plus the current value. Okay. 60 60 00:03:09,510 --> 00:03:12,060 And this works because in each call of 61 61 00:03:12,060 --> 00:03:13,310 the callback function, 62 62 00:03:13,310 --> 00:03:15,840 the accumulator will be the current sum 63 63 00:03:15,840 --> 00:03:18,190 of all the previous values. 64 64 00:03:18,190 --> 00:03:21,430 And so we will really keep adding to this accumulator 65 65 00:03:21,430 --> 00:03:23,673 in each iteration of the loop. 66 66 00:03:25,010 --> 00:03:28,660 Finally, we also need to return this value here 67 67 00:03:28,660 --> 00:03:30,000 from the callback. 68 68 00:03:30,000 --> 00:03:32,240 And so this is how the new accumulator 69 69 00:03:32,240 --> 00:03:35,992 can then be used in the next iteration of the loop. 70 70 00:03:35,992 --> 00:03:38,770 So basically in each loop iteration, 71 71 00:03:38,770 --> 00:03:41,325 we return the updated accumulator 72 72 00:03:41,325 --> 00:03:44,465 so the current one, plus the new current value. 73 73 00:03:44,465 --> 00:03:48,010 And so like this, we can then keep adding to it 74 74 00:03:48,010 --> 00:03:49,863 in the next iteration. 75 75 00:03:49,863 --> 00:03:50,763 Okay? 76 76 00:03:51,610 --> 00:03:55,810 So this callback function is the first argument 77 77 00:03:55,810 --> 00:03:57,600 of the reduce method, 78 78 00:03:57,600 --> 00:04:01,010 but the reduce method actually has a another, 79 79 00:04:01,010 --> 00:04:03,020 so a second parameter, 80 80 00:04:03,020 --> 00:04:06,640 and that is the initial value of the accumulator. 81 81 00:04:06,640 --> 00:04:08,794 So the value that we specify here, 82 82 00:04:08,794 --> 00:04:12,763 which in this case is gonna be zero is the initial value 83 83 00:04:12,763 --> 00:04:16,300 of the accumulator in the first loop iteration. 84 84 00:04:16,300 --> 00:04:20,047 And so in this example, we want to start counting 85 85 00:04:20,047 --> 00:04:23,420 or we want to start adding at zero. 86 86 00:04:23,420 --> 00:04:26,893 And so therefore we simply specify zero here. 87 87 00:04:30,290 --> 00:04:32,626 Okay, and with this we should be able 88 88 00:04:32,626 --> 00:04:35,441 to already take a look at our balance. 89 89 00:04:35,441 --> 00:04:39,450 And yeah, so we have one single number. 90 90 00:04:39,450 --> 00:04:44,450 So indeed everything was boiled down to this one number, 91 91 00:04:45,250 --> 00:04:48,580 which we can suppose is all of these values here 92 92 00:04:48,580 --> 00:04:51,339 added together, all right. 93 93 00:04:51,339 --> 00:04:54,179 And to make this even more clear to see, 94 94 00:04:54,179 --> 00:04:58,920 that's actually always log the accumulator to the console. 95 95 00:04:58,920 --> 00:05:02,877 So let's say iteration number I 96 96 00:05:06,430 --> 00:05:08,233 and enter the accumulator in there. 97 97 00:05:12,180 --> 00:05:15,380 Okay, so we see that in iteration zero, 98 98 00:05:15,380 --> 00:05:17,970 the accumulator is zero. 99 99 00:05:17,970 --> 00:05:20,710 And so this is the value that we specified here 100 100 00:05:20,710 --> 00:05:23,800 as the starter so as the initial value. 101 101 00:05:23,800 --> 00:05:25,170 Then in the second iteration, 102 102 00:05:25,170 --> 00:05:28,090 the accumulator is already at 200 103 103 00:05:28,090 --> 00:05:30,589 and that's because that is the initial value 104 104 00:05:30,589 --> 00:05:33,870 plus the current value in the previous iteration. 105 105 00:05:33,870 --> 00:05:35,283 So that was 200 there. 106 106 00:05:37,091 --> 00:05:41,220 And so this zero plus the 200 creates the new accumulator, 107 107 00:05:41,220 --> 00:05:44,460 which is done in the next iteration, 200. 108 108 00:05:44,460 --> 00:05:48,000 Then in this iteration, we add the current value again, 109 109 00:05:48,000 --> 00:05:49,840 which is 450. 110 110 00:05:49,840 --> 00:05:52,190 So that result is 650. 111 111 00:05:52,190 --> 00:05:53,888 And therefore in the next iteration, 112 112 00:05:53,888 --> 00:05:56,263 that's the value of the accumulator. 113 113 00:05:57,250 --> 00:05:59,532 Okay. And so here you can really see 114 114 00:05:59,532 --> 00:06:02,970 like the snowball effect of all of these values, 115 115 00:06:02,970 --> 00:06:05,530 adding up to one final value. 116 116 00:06:05,530 --> 00:06:09,279 And in the end, that value is essentially the accumulator. 117 117 00:06:09,279 --> 00:06:11,770 Now here we see the value of going down 118 118 00:06:11,770 --> 00:06:13,200 because in this iteration, 119 119 00:06:13,200 --> 00:06:15,340 we are adding minus 400 120 120 00:06:15,340 --> 00:06:19,533 and so 250 is 650 minus 400. 121 121 00:06:21,330 --> 00:06:25,283 Okay, so in the last iteration the value is 2,540, 122 122 00:06:27,520 --> 00:06:31,080 and then adding 1,300 will eventually result 123 123 00:06:31,080 --> 00:06:32,943 in this value here. 124 124 00:06:33,970 --> 00:06:38,420 Great. So this is really how the reduce method works. 125 125 00:06:38,420 --> 00:06:40,260 And I think this a log here 126 126 00:06:40,260 --> 00:06:42,333 is a great visualization of that. 127 127 00:06:43,230 --> 00:06:45,833 Let's give it all the space we need. 128 128 00:06:45,833 --> 00:06:48,986 And let me just change this one here to 100, 129 129 00:06:48,986 --> 00:06:52,870 so that you see the effect of changing this. 130 130 00:06:52,870 --> 00:06:56,220 And so now we can expect this accumulator 131 131 00:06:56,220 --> 00:06:59,320 in the beginning being 100. 132 132 00:06:59,320 --> 00:07:02,660 And so we already start with a small snowball here, 133 133 00:07:02,660 --> 00:07:04,981 which then we keep adding to, 134 134 00:07:04,981 --> 00:07:09,347 and therefore our final balance is now 100 euros 135 135 00:07:09,347 --> 00:07:12,793 or whatever it is larger. 136 136 00:07:13,950 --> 00:07:18,300 Okay. And yeah, 137 137 00:07:18,300 --> 00:07:21,270 let's one more time do the same thing manually, 138 138 00:07:21,270 --> 00:07:22,903 basically with a four loop. 139 139 00:07:26,140 --> 00:07:29,480 So of movements. 140 140 00:07:29,480 --> 00:07:33,380 And so we need one more time, an external variable here, 141 141 00:07:33,380 --> 00:07:35,907 which is gonna be the sum. 142 142 00:07:35,907 --> 00:07:39,290 Well, let's call it balance two. 143 143 00:07:39,290 --> 00:07:41,770 So balance two starts at zero. 144 144 00:07:41,770 --> 00:07:42,920 And so essentially this is, 145 145 00:07:42,920 --> 00:07:46,630 or initial accumulator value just like this zero 146 146 00:07:47,800 --> 00:07:52,800 and then sum plus equal the current movement. 147 147 00:07:54,750 --> 00:07:58,623 And if we lock balance two, we will get, okay. 148 148 00:08:00,330 --> 00:08:03,903 Now of course this has to be balance two, 149 149 00:08:03,903 --> 00:08:07,320 but now we get indeed the same result. 150 150 00:08:07,320 --> 00:08:10,800 And so here you can see this common pattern 151 151 00:08:10,800 --> 00:08:13,090 that we always need an external variable 152 152 00:08:13,090 --> 00:08:16,160 whenever we want to use a for loop. 153 153 00:08:16,160 --> 00:08:18,840 And that's fine if you only need one loop, 154 154 00:08:18,840 --> 00:08:21,870 but it starts to become really cumbersome 155 155 00:08:21,870 --> 00:08:24,710 and unpractical when we use many loops 156 156 00:08:24,710 --> 00:08:26,588 for doing many operations. 157 157 00:08:26,588 --> 00:08:29,350 So these methods that we've been studying, 158 158 00:08:29,350 --> 00:08:32,340 they completely avoid this extra variable 159 159 00:08:32,340 --> 00:08:34,570 and they simply return the variable 160 160 00:08:34,570 --> 00:08:37,238 or the value actually right away. 161 161 00:08:37,238 --> 00:08:38,153 All right. 162 162 00:08:39,278 --> 00:08:44,278 And of course we can write here in an even simpler way. 163 163 00:08:45,035 --> 00:08:47,127 So using an arrow function 164 164 00:08:47,127 --> 00:08:49,079 and I will still keep this here 165 165 00:08:49,079 --> 00:08:52,283 so that we can see the return that's happening. 166 166 00:08:53,670 --> 00:08:56,291 So first we can get rid of this. 167 167 00:08:56,291 --> 00:08:58,720 Then we get rid of this. 168 168 00:08:58,720 --> 00:09:00,633 We also don't need any of this, 169 169 00:09:01,490 --> 00:09:06,100 but here we always use or need the accumulator 170 170 00:09:06,100 --> 00:09:07,303 and the current value. 171 171 00:09:08,290 --> 00:09:09,870 So here we need parenthesis 172 172 00:09:10,800 --> 00:09:12,556 then the arrow. 173 173 00:09:12,556 --> 00:09:17,556 And then as always, we can get rid of all of this. 174 174 00:09:18,830 --> 00:09:23,710 And so now still, or balanced is the same. Awesome. 175 175 00:09:23,710 --> 00:09:26,700 So understanding how the reduce method works 176 176 00:09:26,700 --> 00:09:28,940 is something really important, 177 177 00:09:28,940 --> 00:09:32,220 but it's also way more confusing than the other ones, 178 178 00:09:32,220 --> 00:09:34,350 but I'm sure you're well underway 179 179 00:09:34,350 --> 00:09:36,366 of really understanding it. 180 180 00:09:36,366 --> 00:09:38,740 And now that we know how it works, 181 181 00:09:38,740 --> 00:09:42,600 we can actually now also calculate the balance 182 182 00:09:42,600 --> 00:09:46,460 of these movements here and then print that balance 183 183 00:09:46,460 --> 00:09:49,663 right here to our application and user interface. 184 184 00:09:50,560 --> 00:09:53,243 And so let's go ahead and do that here. 185 185 00:09:54,490 --> 00:09:57,155 So finally back to our application here. 186 186 00:09:57,155 --> 00:10:00,103 And so let's do that down here. 187 187 00:10:01,810 --> 00:10:06,363 So calc and print, balance. 188 188 00:10:09,880 --> 00:10:12,380 And so this function here will once again, 189 189 00:10:12,380 --> 00:10:15,273 receive the movements as an input. 190 190 00:10:16,110 --> 00:10:18,273 So any array of movements will work. 191 191 00:10:19,363 --> 00:10:24,363 And so now let's calculate the balance based on this array. 192 192 00:10:26,380 --> 00:10:29,943 And essentially this is exactly what we wrote to before. 193 193 00:10:31,220 --> 00:10:35,220 So that's movement dot reduce, 194 194 00:10:35,220 --> 00:10:38,360 but it doesn't hurt to write it here again. 195 195 00:10:38,360 --> 00:10:42,680 So remember the first parameter is now the accumulator 196 196 00:10:42,680 --> 00:10:45,810 and only the second one is the current value. 197 197 00:10:45,810 --> 00:10:48,163 So let's call it the movement now. 198 198 00:10:49,915 --> 00:10:54,372 And so here simply we return the accumulator 199 199 00:10:54,372 --> 00:10:58,793 plus the current movement and we start at zero. 200 200 00:10:59,750 --> 00:11:03,330 Great. And now all we need to do is to display it 201 201 00:11:03,330 --> 00:11:05,210 here in this element. 202 202 00:11:05,210 --> 00:11:10,210 So let's check this out and it is called the balance value. 203 203 00:11:10,500 --> 00:11:13,593 And just like before I already have it selected here, 204 204 00:11:14,840 --> 00:11:19,300 and this is a label, so balance value is here. 205 205 00:11:19,300 --> 00:11:21,047 So I called it label balance 206 206 00:11:21,047 --> 00:11:24,110 and so label is basically all the things 207 207 00:11:24,110 --> 00:11:27,983 where we simply want to put some text, all right? 208 208 00:11:33,785 --> 00:11:36,607 So let's say label balance dot text content. 209 209 00:11:37,620 --> 00:11:42,060 And this one we already know equals the balance 210 210 00:11:43,000 --> 00:11:47,470 and let's actually do a template string here, 211 211 00:11:47,470 --> 00:11:51,926 or a template literal and also display the Euro sign. 212 212 00:11:51,926 --> 00:11:53,710 Well, I don't know where it is, 213 213 00:11:53,710 --> 00:11:57,419 so let's just write EUR, which also means EURO. 214 214 00:11:57,419 --> 00:12:02,419 And so now let's see of course 215 215 00:12:02,720 --> 00:12:04,363 we didn't call the function yet. 216 216 00:12:05,210 --> 00:12:07,360 So let's do that here as well. 217 217 00:12:07,360 --> 00:12:10,670 And actually let's put this function here 218 218 00:12:10,670 --> 00:12:12,690 where it makes a little bit more sense 219 219 00:12:13,800 --> 00:12:17,693 because it kind of belongs to this display movements here. 220 220 00:12:18,955 --> 00:12:22,080 Okay. So let's actually also call this one here, 221 221 00:12:22,080 --> 00:12:25,633 calc and display balance. 222 222 00:12:26,610 --> 00:12:30,548 So to give it a similar name as that one. 223 223 00:12:30,548 --> 00:12:34,430 And so now I'm gonna call it, calc display balance. 224 224 00:12:34,430 --> 00:12:38,532 And once again, I'm calling it with account one. 225 225 00:12:38,532 --> 00:12:42,063 So account one dot movements, 226 226 00:12:43,090 --> 00:12:46,590 and then later on, as we keep developing this application 227 227 00:12:46,590 --> 00:12:49,800 of course these movements here that we're gonna use 228 228 00:12:49,800 --> 00:12:53,490 will be from the account that locked into the application. 229 229 00:12:53,490 --> 00:12:56,270 So if Jonas locked into the application, 230 230 00:12:56,270 --> 00:12:59,554 then Jonas's movements will be shown and Jonas balance. 231 231 00:12:59,554 --> 00:13:02,560 But if Steven locks in then of course 232 232 00:13:02,560 --> 00:13:06,910 Steven's movements and Steven's balance will be shown here. 233 233 00:13:06,910 --> 00:13:10,648 And right now we already see our balance up here. 234 234 00:13:10,648 --> 00:13:13,903 And so this actually happens to be exactly the same 235 235 00:13:13,903 --> 00:13:16,610 we calculated before. 236 236 00:13:16,610 --> 00:13:19,490 So also 3,840, 237 237 00:13:19,490 --> 00:13:23,453 because the movements are indeed also exactly the same. 238 238 00:13:23,453 --> 00:13:28,453 Great. So that's beautiful with this, 239 239 00:13:28,460 --> 00:13:31,776 we actually already completed some parts here 240 240 00:13:31,776 --> 00:13:34,370 of our flow chart. 241 241 00:13:34,370 --> 00:13:37,773 So let's also keep this one handy here 242 242 00:13:37,773 --> 00:13:40,163 so that we can keep looking at it. 243 243 00:13:40,163 --> 00:13:43,680 And so we already did calculating the balance 244 244 00:13:43,680 --> 00:13:45,380 and displaying the balance. 245 245 00:13:45,380 --> 00:13:47,726 So we did all this in just one function 246 246 00:13:47,726 --> 00:13:51,547 and we also already have this playing the movements. 247 247 00:13:51,547 --> 00:13:54,651 So indeed we are making some nice progress here. 248 248 00:13:54,651 --> 00:13:56,670 And the rest that you see here 249 249 00:13:56,670 --> 00:14:00,095 will be even easier as we go on. 250 250 00:14:00,095 --> 00:14:03,454 Now, just to finish this lecture here about reduce 251 251 00:14:03,454 --> 00:14:06,260 let me show you one final example 252 252 00:14:07,190 --> 00:14:09,422 because we can also do other stuff, 253 253 00:14:09,422 --> 00:14:12,090 than just adding up values 254 254 00:14:12,090 --> 00:14:15,220 that would become boring at some point, right? 255 255 00:14:15,220 --> 00:14:18,251 And so we can also do other stuff. 256 256 00:14:18,251 --> 00:14:22,780 So this time, what I want to do is to get the maximum value 257 257 00:14:24,750 --> 00:14:26,918 of the movements array here. 258 258 00:14:26,918 --> 00:14:29,880 Okay, so in this case, 259 259 00:14:29,880 --> 00:14:32,910 the result we're looking for is this 3,000. 260 260 00:14:32,910 --> 00:14:36,840 Okay. And so for that, we can also use reduce, 261 261 00:14:36,840 --> 00:14:40,688 because remember reduce is for boiling down the array 262 262 00:14:40,688 --> 00:14:43,410 into just one single value, 263 263 00:14:43,410 --> 00:14:46,100 but that value can be whatever we want. 264 264 00:14:46,100 --> 00:14:47,800 So it doesn't have to be a sum. 265 265 00:14:47,800 --> 00:14:49,654 It could be a multiplication 266 266 00:14:49,654 --> 00:14:52,160 or even something completely different, 267 267 00:14:52,160 --> 00:14:54,768 like a string or an object, 268 268 00:14:54,768 --> 00:14:57,930 but here we will keep working with numbers, 269 269 00:14:57,930 --> 00:15:00,113 but this time we want the maximum number. 270 270 00:15:01,170 --> 00:15:04,720 So let's kind of go manually through this array 271 271 00:15:04,720 --> 00:15:08,155 to determine what the reduce method should do. 272 272 00:15:08,155 --> 00:15:10,700 So let's start here at 200. 273 273 00:15:10,700 --> 00:15:13,580 Is this the maximum value at this point? 274 274 00:15:13,580 --> 00:15:16,870 Well, we don't know yet because we just started, 275 275 00:15:16,870 --> 00:15:21,520 so let's save 200 here mentally as our current maximum, 276 276 00:15:21,520 --> 00:15:25,930 then we move on to the next value and we see 450. 277 277 00:15:25,930 --> 00:15:28,800 So is this greater than our current maximum, 278 278 00:15:28,800 --> 00:15:30,860 which was 200? 279 279 00:15:30,860 --> 00:15:33,200 Well, yes it is, it is greater. 280 280 00:15:33,200 --> 00:15:37,640 And so 450 is now our current maximum 281 281 00:15:37,640 --> 00:15:41,433 then minus 400 is of course less than our current maximum. 282 282 00:15:41,433 --> 00:15:43,602 Then we move on even further. 283 283 00:15:43,602 --> 00:15:48,060 So 3,000 is a greater than our current maximum? 284 284 00:15:48,060 --> 00:15:49,260 Yes it is. 285 285 00:15:49,260 --> 00:15:52,200 And so that becomes our current maximum. 286 286 00:15:52,200 --> 00:15:54,520 So we store this 3,000 287 287 00:15:54,520 --> 00:15:56,867 and then we keep going through the array. 288 288 00:15:56,867 --> 00:16:01,867 And as you see, there is no other value greater than 3,000. 289 289 00:16:02,120 --> 00:16:06,943 And so in the end or maximum was this one, so 3,000. 290 290 00:16:08,950 --> 00:16:11,900 Okay. And now all we need to do is to 291 291 00:16:11,900 --> 00:16:14,760 essentially translate this into code, 292 292 00:16:14,760 --> 00:16:16,303 using the reduce method. 293 293 00:16:18,660 --> 00:16:23,363 So let's call it max movements to reduce. 294 294 00:16:24,670 --> 00:16:27,360 And as always, we need the accumulator 295 295 00:16:27,360 --> 00:16:29,120 and the current value. 296 296 00:16:29,120 --> 00:16:31,020 So I'm calling it movement here again. 297 297 00:16:32,320 --> 00:16:33,300 All right. 298 298 00:16:33,300 --> 00:16:36,010 And now this time, what should be the purpose 299 299 00:16:36,010 --> 00:16:38,190 of this accumulator value? 300 300 00:16:38,190 --> 00:16:40,639 So that's always the big question that we have to ask 301 301 00:16:40,639 --> 00:16:42,640 when we use reduce. 302 302 00:16:42,640 --> 00:16:46,257 So up here, when we wanted to add all the numbers together, 303 303 00:16:46,257 --> 00:16:48,600 the purpose of the accumulator 304 304 00:16:48,600 --> 00:16:51,600 was to keep track of the current sum. 305 305 00:16:51,600 --> 00:16:54,233 And so here, the accumulator will be the one 306 306 00:16:54,233 --> 00:16:57,393 that will keep track of the current maximum value. 307 307 00:16:58,270 --> 00:17:02,090 So let's just start by writing the logic here 308 308 00:17:02,090 --> 00:17:05,387 in a bigger way so we can understand what's happening. 309 309 00:17:05,387 --> 00:17:07,080 And then I will simplify it 310 310 00:17:07,080 --> 00:17:10,432 to make this a one-liner function one more time. 311 311 00:17:10,432 --> 00:17:15,288 So translating the logic that we just went over here 312 312 00:17:15,288 --> 00:17:19,588 is to say, if the accumulator is greater 313 313 00:17:19,588 --> 00:17:21,660 than the current value, 314 314 00:17:21,660 --> 00:17:26,660 which is the movement, then return the accumulator, right? 315 315 00:17:28,640 --> 00:17:30,350 Because in the reduce method, 316 316 00:17:30,350 --> 00:17:34,010 we always have to somehow return the accumulator 317 317 00:17:34,010 --> 00:17:35,860 to the next iteration. 318 318 00:17:35,860 --> 00:17:38,940 And in this case, we simply want to keep the accumulator 319 319 00:17:38,940 --> 00:17:42,332 at the value that it already is and not change it. 320 320 00:17:42,332 --> 00:17:47,332 Okay. We want to change it in the other scenario, basically. 321 321 00:17:48,310 --> 00:17:52,100 So when the current value is greater than the accumulator, 322 322 00:17:52,100 --> 00:17:54,050 so that would be this case here. 323 323 00:17:54,050 --> 00:17:57,230 So here, remember the current value is 450, 324 324 00:17:57,230 --> 00:17:59,594 but the accumulator is 200. 325 325 00:17:59,594 --> 00:18:03,529 And so now the movement is greater than accumulator. 326 326 00:18:03,529 --> 00:18:06,990 And so what we want to return here as the accumulator 327 327 00:18:06,990 --> 00:18:09,883 in the next iteration is the current movement. 328 328 00:18:12,944 --> 00:18:15,813 And so that's it, okay. 329 329 00:18:17,040 --> 00:18:19,390 And now we also need the initial value. 330 330 00:18:19,390 --> 00:18:23,480 And so the initial value make sense to be 331 331 00:18:23,480 --> 00:18:26,230 this first value here of the array. 332 332 00:18:26,230 --> 00:18:31,128 Okay. And so that's movements at position zero. 333 333 00:18:31,128 --> 00:18:33,400 Now we could have used zero here, 334 334 00:18:33,400 --> 00:18:35,540 but that would not be correct 335 335 00:18:35,540 --> 00:18:37,770 because imagine that the first value 336 336 00:18:37,770 --> 00:18:39,783 would be like a negative, 337 337 00:18:39,783 --> 00:18:42,810 then this might not work as expected. 338 338 00:18:42,810 --> 00:18:45,020 Maybe it might work with the maximum, 339 339 00:18:45,020 --> 00:18:47,334 but not with a minimum, for example. 340 340 00:18:47,334 --> 00:18:50,010 So don't just put zero here 341 341 00:18:50,010 --> 00:18:53,410 when you're trying to find a maximum or a minimum value, 342 342 00:18:53,410 --> 00:18:56,800 always just go with the first value of the array. 343 343 00:18:56,800 --> 00:18:59,480 Okay. And that's it. 344 344 00:18:59,480 --> 00:19:00,960 Let's see if it works 345 345 00:19:00,960 --> 00:19:03,393 and then we can maybe go over this again. 346 346 00:19:06,040 --> 00:19:09,670 So indeed we get 3,000. 347 347 00:19:09,670 --> 00:19:12,420 So we started at 200. 348 348 00:19:12,420 --> 00:19:13,950 So in this iteration, 349 349 00:19:13,950 --> 00:19:17,040 is the accumulator greater than the movement? 350 350 00:19:17,040 --> 00:19:18,690 Well, it is not. 351 351 00:19:18,690 --> 00:19:23,300 And so return the current movement. Okay. 352 352 00:19:23,300 --> 00:19:25,245 But the first iteration is not that important. 353 353 00:19:25,245 --> 00:19:28,570 It gets interesting here in the second one. 354 354 00:19:28,570 --> 00:19:31,090 So now our accumulator is at 200 355 355 00:19:31,090 --> 00:19:34,360 and the current movement is at 450. 356 356 00:19:34,360 --> 00:19:36,970 So this one here is false again. 357 357 00:19:36,970 --> 00:19:39,190 And so now we return to movement 358 358 00:19:39,190 --> 00:19:42,220 as the new accumulator in the next iteration. 359 359 00:19:42,220 --> 00:19:44,710 All right. And so in the next iteration, 360 360 00:19:44,710 --> 00:19:47,289 that accumulator is 450 now, 361 361 00:19:47,289 --> 00:19:52,289 then here, the current movement is minus 400. 362 362 00:19:52,540 --> 00:19:57,540 And so here is 450 greater than minus 400. 363 363 00:19:57,830 --> 00:19:59,530 Yes, of course it is. 364 364 00:19:59,530 --> 00:20:01,840 And so let's simply return the accumulator 365 365 00:20:01,840 --> 00:20:04,970 so that it stays the same in the next iteration. 366 366 00:20:04,970 --> 00:20:06,910 And so that's the logic all the way 367 367 00:20:06,910 --> 00:20:08,800 until the end of the array. 368 368 00:20:08,800 --> 00:20:12,820 And, by this, we then end up with this maximum value. 369 369 00:20:12,820 --> 00:20:16,320 All right. So I hope that made sense. 370 370 00:20:16,320 --> 00:20:18,610 And in fact, there is a ton of things 371 371 00:20:18,610 --> 00:20:21,550 that we can do with this reduce method. 372 372 00:20:21,550 --> 00:20:24,863 It is by far the most powerful array method there is. 373 373 00:20:26,380 --> 00:20:27,490 And because of that, 374 374 00:20:27,490 --> 00:20:30,180 it can also be the hardest one to use. 375 375 00:20:30,180 --> 00:20:33,070 So we always need to think exactly what we want 376 376 00:20:33,070 --> 00:20:36,170 the accumulator and the core value to be 377 377 00:20:36,170 --> 00:20:37,765 and how they should interact. 378 378 00:20:37,765 --> 00:20:40,807 But I hope that with this exercise here 379 379 00:20:40,807 --> 00:20:44,210 I could demonstrate the thought process a little bit 380 380 00:20:44,210 --> 00:20:47,290 and throughout the section and the rest of the course, 381 381 00:20:47,290 --> 00:20:50,890 there will be some more exercises of the reduce method 382 382 00:20:50,890 --> 00:20:53,250 so that you can also learn better and better 383 383 00:20:53,250 --> 00:20:55,263 how to use it yourself one day. 33224