346 lines
13 KiB
Markdown
346 lines
13 KiB
Markdown
---
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id: 587d7da9367417b2b2512b68
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title: Usare il metodo reduce per analizzare i dati
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challengeType: 1
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forumTopicId: 301313
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dashedName: use-the-reduce-method-to-analyze-data
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---
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# --description--
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`Array.prototype.reduce()`, o semplicemente `reduce()`, è la più generale di tutte le operazioni sugli array in JavaScript. È possibile risolvere quasi tutti i problemi di elaborazione di array utilizzando il metodo `reduce`.
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Il metodo `reduce` consente forme più generali di elaborazione di array, ed è possibile mostrare che sia `filter` che `map` possono essere derivati come applicazioni speciali di `reduce`. Il metodo `reduce` iterate attraverso ogni elemento di un array e restituisce un singolo valore (cioè una stringa, un numero, un oggetto, un array...). Questo si ottiene tramite una funzione di callback che viene chiamata ad ogni iterazione.
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La funzione callback accetta quattro argomenti. Il primo argomento è conosciuto come l'accumulatore, al quale viene assegnato il valore di ritorno della funzione di callback dall'iterazione precedente, il secondo è l'elemento attuale in fase di elaborazione, il terzo è l'indice di quell'elemento e il quarto è l'array su cui `reduce` è chiamato.
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Oltre alla funzione di callback, `reduce` ha un parametro aggiuntivo che prende un valore iniziale per l'accumulatore. Se questo secondo parametro non è utilizzato, la prima iterazione è saltata e alla seconda iterazione viene passato il primo elemento dell'array come accumulatore.
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Qui sotto trovi un esempio che utilizza `reduce` sull'array `users` per restituire la somma di tutte le età degli utenti. Per semplicità, l'esempio utilizza solo il primo e il secondo argomento.
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```js
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const users = [
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{ name: 'John', age: 34 },
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{ name: 'Amy', age: 20 },
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{ name: 'camperCat', age: 10 }
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];
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const sumOfAges = users.reduce((sum, user) => sum + user.age, 0);
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console.log(sumOfAges);
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```
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La console visualizzerà il valore `64`.
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In quest'altro esempio, guarda come può essere restituito un oggetto contenente i nomi degli utenti come proprietà con le loro età come valori.
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```js
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const users = [
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{ name: 'John', age: 34 },
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{ name: 'Amy', age: 20 },
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{ name: 'camperCat', age: 10 }
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];
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const usersObj = users.reduce((obj, user) => {
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obj[user.name] = user.age;
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return obj;
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}, {});
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console.log(usersObj);
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```
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La console visualizzerà il valore `{ John: 34, Amy: 20, camperCat: 10 }`.
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# --instructions--
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La variabile `watchList` contiene una serie di oggetti con informazioni su diversi film. Usa `reduce` per trovare la valutazione IMDB media dei film diretti da `Christopher Nolan`. Ricorda dalle sfide precedenti come utilizzare `filter` sui dati e come utilizzare `map` su di essi per estrarre quello che ti serve. Potresti aver bisogno di creare altre variabili e restituire la valutazione media dalla funzione `getRating`. Nota che i valori delle valutazioni sono salvati come stringhe nell'oggetto e dovranno essere convertiti in numeri prima di essere utilizzati in qualsiasi operazione matematica.
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# --hints--
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La variabile `watchList` non dovrebbe cambiare.
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```js
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assert(
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watchList[0].Title === 'Inception' && watchList[4].Director == 'James Cameron'
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);
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```
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Il tuo codice dovrebbe usare il metodo `reduce`.
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```js
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assert(code.match(/\.reduce/g));
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```
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`getRating(watchList)` dovrebbe essere pari a 8.675.
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```js
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assert(getRating(watchList) === 8.675);
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```
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Il tuo codice non dovrebbe utilizzare un ciclo `for`.
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```js
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assert(!code.match(/for\s*?\([\s\S]*?\)/g));
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```
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Il tuo codice dovrebbe restituire l'output corretto dopo aver valutato l'oggetto `watchList`.
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```js
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assert(getRating(watchList.filter((_, i) => i < 1 || i > 2)) === 8.55);
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```
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# --seed--
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## --seed-contents--
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```js
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// The global variable
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const watchList = [
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{
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"Title": "Inception",
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"Year": "2010",
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"Rated": "PG-13",
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"Released": "16 Jul 2010",
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"Runtime": "148 min",
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"Genre": "Action, Adventure, Crime",
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"Director": "Christopher Nolan",
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"Writer": "Christopher Nolan",
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"Actors": "Leonardo DiCaprio, Joseph Gordon-Levitt, Elliot Page, Tom Hardy",
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"Plot": "A thief, who steals corporate secrets through use of dream-sharing technology, is given the inverse task of planting an idea into the mind of a CEO.",
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"Language": "English, Japanese, French",
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"Country": "USA, UK",
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"Awards": "Won 4 Oscars. Another 143 wins & 198 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BMjAxMzY3NjcxNF5BMl5BanBnXkFtZTcwNTI5OTM0Mw@@._V1_SX300.jpg",
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"Metascore": "74",
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"imdbRating": "8.8",
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"imdbVotes": "1,446,708",
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"imdbID": "tt1375666",
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"Type": "movie",
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"Response": "True"
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},
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{
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"Title": "Interstellar",
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"Year": "2014",
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"Rated": "PG-13",
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"Released": "07 Nov 2014",
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"Runtime": "169 min",
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"Genre": "Adventure, Drama, Sci-Fi",
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"Director": "Christopher Nolan",
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"Writer": "Jonathan Nolan, Christopher Nolan",
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"Actors": "Ellen Burstyn, Matthew McConaughey, Mackenzie Foy, John Lithgow",
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"Plot": "A team of explorers travel through a wormhole in space in an attempt to ensure humanity's survival.",
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"Language": "English",
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"Country": "USA, UK",
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"Awards": "Won 1 Oscar. Another 39 wins & 132 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BMjIxNTU4MzY4MF5BMl5BanBnXkFtZTgwMzM4ODI3MjE@._V1_SX300.jpg",
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"Metascore": "74",
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"imdbRating": "8.6",
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"imdbVotes": "910,366",
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"imdbID": "tt0816692",
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"Type": "movie",
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"Response": "True"
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},
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{
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"Title": "The Dark Knight",
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"Year": "2008",
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"Rated": "PG-13",
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"Released": "18 Jul 2008",
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"Runtime": "152 min",
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"Genre": "Action, Adventure, Crime",
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"Director": "Christopher Nolan",
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"Writer": "Jonathan Nolan (screenplay), Christopher Nolan (screenplay), Christopher Nolan (story), David S. Goyer (story), Bob Kane (characters)",
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"Actors": "Christian Bale, Heath Ledger, Aaron Eckhart, Michael Caine",
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"Plot": "When the menace known as the Joker wreaks havoc and chaos on the people of Gotham, the caped crusader must come to terms with one of the greatest psychological tests of his ability to fight injustice.",
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"Language": "English, Mandarin",
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"Country": "USA, UK",
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"Awards": "Won 2 Oscars. Another 146 wins & 142 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BMTMxNTMwODM0NF5BMl5BanBnXkFtZTcwODAyMTk2Mw@@._V1_SX300.jpg",
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"Metascore": "82",
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"imdbRating": "9.0",
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"imdbVotes": "1,652,832",
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"imdbID": "tt0468569",
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"Type": "movie",
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"Response": "True"
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},
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{
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"Title": "Batman Begins",
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"Year": "2005",
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"Rated": "PG-13",
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"Released": "15 Jun 2005",
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"Runtime": "140 min",
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"Genre": "Action, Adventure",
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"Director": "Christopher Nolan",
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"Writer": "Bob Kane (characters), David S. Goyer (story), Christopher Nolan (screenplay), David S. Goyer (screenplay)",
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"Actors": "Christian Bale, Michael Caine, Liam Neeson, Katie Holmes",
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"Plot": "After training with his mentor, Batman begins his fight to free crime-ridden Gotham City from the corruption that Scarecrow and the League of Shadows have cast upon it.",
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"Language": "English, Urdu, Mandarin",
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"Country": "USA, UK",
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"Awards": "Nominated for 1 Oscar. Another 15 wins & 66 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BNTM3OTc0MzM2OV5BMl5BanBnXkFtZTYwNzUwMTI3._V1_SX300.jpg",
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"Metascore": "70",
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"imdbRating": "8.3",
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"imdbVotes": "972,584",
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"imdbID": "tt0372784",
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"Type": "movie",
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"Response": "True"
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},
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{
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"Title": "Avatar",
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"Year": "2009",
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"Rated": "PG-13",
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"Released": "18 Dec 2009",
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"Runtime": "162 min",
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"Genre": "Action, Adventure, Fantasy",
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"Director": "James Cameron",
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"Writer": "James Cameron",
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"Actors": "Sam Worthington, Zoe Saldana, Sigourney Weaver, Stephen Lang",
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"Plot": "A paraplegic marine dispatched to the moon Pandora on a unique mission becomes torn between following his orders and protecting the world he feels is his home.",
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"Language": "English, Spanish",
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"Country": "USA, UK",
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"Awards": "Won 3 Oscars. Another 80 wins & 121 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BMTYwOTEwNjAzMl5BMl5BanBnXkFtZTcwODc5MTUwMw@@._V1_SX300.jpg",
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"Metascore": "83",
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"imdbRating": "7.9",
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"imdbVotes": "876,575",
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"imdbID": "tt0499549",
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"Type": "movie",
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"Response": "True"
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}
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];
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function getRating(watchList) {
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// Only change code below this line
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let averageRating;
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// Only change code above this line
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return averageRating;
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}
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console.log(getRating(watchList));
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```
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# --solutions--
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```js
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const watchList = [
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{
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"Title": "Inception",
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"Year": "2010",
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"Rated": "PG-13",
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"Released": "16 Jul 2010",
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"Runtime": "148 min",
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"Genre": "Action, Adventure, Crime",
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"Director": "Christopher Nolan",
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"Writer": "Christopher Nolan",
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"Actors": "Leonardo DiCaprio, Joseph Gordon-Levitt, Elliot Page, Tom Hardy",
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"Plot": "A thief, who steals corporate secrets through use of dream-sharing technology, is given the inverse task of planting an idea into the mind of a CEO.",
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"Language": "English, Japanese, French",
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"Country": "USA, UK",
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"Awards": "Won 4 Oscars. Another 143 wins & 198 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BMjAxMzY3NjcxNF5BMl5BanBnXkFtZTcwNTI5OTM0Mw@@._V1_SX300.jpg",
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"Metascore": "74",
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"imdbRating": "8.8",
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"imdbVotes": "1,446,708",
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"imdbID": "tt1375666",
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"Type": "movie",
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"Response": "True"
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},
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{
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"Title": "Interstellar",
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"Year": "2014",
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"Rated": "PG-13",
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"Released": "07 Nov 2014",
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"Runtime": "169 min",
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"Genre": "Adventure, Drama, Sci-Fi",
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"Director": "Christopher Nolan",
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"Writer": "Jonathan Nolan, Christopher Nolan",
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"Actors": "Ellen Burstyn, Matthew McConaughey, Mackenzie Foy, John Lithgow",
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"Plot": "A team of explorers travel through a wormhole in space in an attempt to ensure humanity's survival.",
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"Language": "English",
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"Country": "USA, UK",
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"Awards": "Won 1 Oscar. Another 39 wins & 132 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BMjIxNTU4MzY4MF5BMl5BanBnXkFtZTgwMzM4ODI3MjE@._V1_SX300.jpg",
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"Metascore": "74",
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"imdbRating": "8.6",
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"imdbVotes": "910,366",
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"imdbID": "tt0816692",
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"Type": "movie",
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"Response": "True"
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},
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{
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"Title": "The Dark Knight",
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"Year": "2008",
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"Rated": "PG-13",
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"Released": "18 Jul 2008",
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"Runtime": "152 min",
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"Genre": "Action, Adventure, Crime",
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"Director": "Christopher Nolan",
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"Writer": "Jonathan Nolan (screenplay), Christopher Nolan (screenplay), Christopher Nolan (story), David S. Goyer (story), Bob Kane (characters)",
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"Actors": "Christian Bale, Heath Ledger, Aaron Eckhart, Michael Caine",
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"Plot": "When the menace known as the Joker wreaks havoc and chaos on the people of Gotham, the caped crusader must come to terms with one of the greatest psychological tests of his ability to fight injustice.",
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"Language": "English, Mandarin",
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"Country": "USA, UK",
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"Awards": "Won 2 Oscars. Another 146 wins & 142 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BMTMxNTMwODM0NF5BMl5BanBnXkFtZTcwODAyMTk2Mw@@._V1_SX300.jpg",
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"Metascore": "82",
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"imdbRating": "9.0",
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"imdbVotes": "1,652,832",
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"imdbID": "tt0468569",
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"Type": "movie",
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"Response": "True"
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},
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{
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"Title": "Batman Begins",
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"Year": "2005",
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"Rated": "PG-13",
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"Released": "15 Jun 2005",
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"Runtime": "140 min",
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"Genre": "Action, Adventure",
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"Director": "Christopher Nolan",
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"Writer": "Bob Kane (characters), David S. Goyer (story), Christopher Nolan (screenplay), David S. Goyer (screenplay)",
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"Actors": "Christian Bale, Michael Caine, Liam Neeson, Katie Holmes",
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"Plot": "After training with his mentor, Batman begins his fight to free crime-ridden Gotham City from the corruption that Scarecrow and the League of Shadows have cast upon it.",
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"Language": "English, Urdu, Mandarin",
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"Country": "USA, UK",
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"Awards": "Nominated for 1 Oscar. Another 15 wins & 66 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BNTM3OTc0MzM2OV5BMl5BanBnXkFtZTYwNzUwMTI3._V1_SX300.jpg",
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"Metascore": "70",
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"imdbRating": "8.3",
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"imdbVotes": "972,584",
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"imdbID": "tt0372784",
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"Type": "movie",
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"Response": "True"
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},
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{
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"Title": "Avatar",
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"Year": "2009",
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"Rated": "PG-13",
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"Released": "18 Dec 2009",
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"Runtime": "162 min",
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"Genre": "Action, Adventure, Fantasy",
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"Director": "James Cameron",
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"Writer": "James Cameron",
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"Actors": "Sam Worthington, Zoe Saldana, Sigourney Weaver, Stephen Lang",
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"Plot": "A paraplegic marine dispatched to the moon Pandora on a unique mission becomes torn between following his orders and protecting the world he feels is his home.",
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"Language": "English, Spanish",
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"Country": "USA, UK",
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"Awards": "Won 3 Oscars. Another 80 wins & 121 nominations.",
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"Poster": "http://ia.media-imdb.com/images/M/MV5BMTYwOTEwNjAzMl5BMl5BanBnXkFtZTcwODc5MTUwMw@@._V1_SX300.jpg",
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"Metascore": "83",
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"imdbRating": "7.9",
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"imdbVotes": "876,575",
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"imdbID": "tt0499549",
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"Type": "movie",
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"Response": "True"
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}
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];
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function getRating(watchList) {
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let averageRating;
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const rating = watchList
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.filter(obj => obj.Director === "Christopher Nolan")
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.map(obj => Number(obj.imdbRating));
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averageRating = rating.reduce((accum, curr) => accum + curr)/rating.length;
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return averageRating;
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}
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```
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