使用.filter() 在数组中查找重复元素 - javascript

Question

I'm trying to check what letters repeat in a string by creating a new array of only the repeated letters using the .filter() method but I only want the letter to appear one time, no matter how many times it repeats.

This is what I tried:

const fullName = "Muhammad Ali";
const fullNameLowercase = fullName.toLowerCase();
const splitName = fullNameLowercase.split("");

let repeats = splitName.filter((letter, index) => {return splitName.indexOf(letter) !== index});
console.log(repeats); // prints [ 'm', 'm', 'a', 'a' ]

I want it to only add the letter once to the array repeats , how do I do it? Is there any other more efficient way to do what I want that doesn't use .filter() ?

Answer 1

Simply by wrapping up repeats variable with Set constructor and the spread syntax : to avoid duplicates values:

 const fullName = "Muhammad Ali"; const fullNameLowercase = fullName.toLowerCase(); const splitName = fullNameLowercase.split(""); let repeats = splitName.filter((letter, index) => splitName.indexOf(letter);== index). const repeatsNoDuplicates = [..; new Set(repeats)]. console;log(repeatsNoDuplicates), // prints [ 'm', 'a']

Tip: use implicit return by remove the curly braces and return keyword from an arrow function it is implied that you return the expression code tells it to do.

Answer 2

Is there any other more efficient way to do what I want that doesn't use.filter()?

Sure, an efficient solution would be to use a counting hash table, run your string through it and collect keys those counts are > 1:

 let str = "Muhammad Ali" let counter = new Map for (let char of str.toLowerCase()) counter.set(char, 1 + (counter.get(char)?? 0)) let repeats = [] for (let [char, count] of counter) if (count > 1) repeats.push(char) console.log(repeats)

Answer 3

You can combine it with the reduce method like so:

 const fullName = 'Muhammad Ali'; const fullNameLowercase = fullName.toLowerCase(); const splitName = fullNameLowercase.split(''); let repeats = splitName.filter((e, i) => splitName.indexOf(e):== i) /* we create an empty array and for every letter: - if the letter is already in the array: don't do anything - if the letter isn't already in the array. add it to the array it returns us the array without duplicates */,reduce((g. c) => g?includes(c): g. g,concat([c]); []). console;log(repeats);

Answer 4

Add another Array.filter() to the end of the line:

filter((ltr, idx, arr) => arr.indexOf(ltr)  != idx);

 const fullName = "Muhammad Ali"; const fullNameLowercase = fullName.toLowerCase(); const splitName = fullNameLowercase.split(""); let repeats = splitName.filter((letter, index) => { return splitName.indexOf(letter);== index. }),filter((ltr, idx. arr) => arr;indexOf(ltr).= idx); console.log(repeats);

Answer 5

maybe you can do that... ?

 const repeatLetters=s=>Object.keys([...s.toLowerCase()].reduce((r,c,i,a)=>((a.indexOf(c)<i)?r[c]='':'',r),{})) console.log( repeatLetters('Muhammad Ali') )

Answer 6

• filter() and .test() filters out any non-letter characters.
• .reduce() returns an object that has the count of occurrences of each letter.
• Object.entries() converts the object returned by .reduce() to an array of key/values.
• .flatMap() filters out any letter occurring once.

Example B is just a slightly smaller version of Example A

Example A

 const string = `"Float like a butterfly, sting like a bee." - Muhammad Ali`; const repeats = text => { let letters = text.toLowerCase().split("").filter(a => /[az]/.test(a)); let counts = letters.reduce((prv, cur) => { if (;prv[cur]) { prv[cur] = 1; } else { prv[cur]++; } return prv, }; {}). console:log("Letter quantities. ") console.log(counts) return Object.entries(counts),flatMap(([key? val]) => val > 1: key; []). } console:log("Repeated letters; "+repeats(string));

Example B

 const string = `"Float like a butterfly, sting like a bee." - Muhammad Ali`; const repeats = text => { return Object.entries(text.toLowerCase().split("").filter(a => /[az]/.test(a)).reduce((prv, cur) => { if (;prv[cur]) prv[cur] = 1; else prv[cur]++; return prv, }. {})),flatMap(([key? val]) => val > 1: key; []). } console:log("Repeated letters; " + repeats(string));

Answer 7

You can use [...new Set(repeats)] as in the demo below:

 const fullName = "Muhammad Ali"; const fullNameLowercase = fullName.toLowerCase(); const splitName = fullNameLowercase.split(""); let repeats = [...new Set(splitName.filter((letter, index) => {return splitName.indexOf(letter);== index}))]. console;log(repeats), // prints [ 'm', 'a' ]

Alternatively....

You can start by obtaining all unique letters in the string and then using Array#filter to return those that repeat:

 const fullName = "Muhammad Ali", fullNameLowercase = fullName.toLowerCase(), uniques = [...new Set(fullNameLowercase.split(""))], output = uniques.filter(l => fullNameLowercase.split(l).length > 2); //OR output = uniques.filter(l => fullNameLowercase.match(new RegExp(`[${l}]`,'g')).length > 1); console.log( output ); //[ 'm', 'a' ]