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I have an array of numbers like A = ['1', '34', '23', '55'] and want to use .includes() to find true or false. However, my tested values are in array like B = ['3', '1', '543', '33']
I'd tried to do A.includes(B) but it seems it is not working. A.includes('1', '123') returns true. How can I use my Array B to do the same?
I wanted to Check if the array A has at least one of the array B’s value then return true. Apology for missing this part!
If I understand you correctly, you're looking to do A.includes(B), but your inputs are stored in arrays. In this case, simply loop through the values and call includes() on the elements:
const A = ['1', '34', '23', '55'];
const B = ['3', '1', '543', '33'];
for (var i = 0; i < A.length; ++i)
console.log(A[i].includes(B[i]));
If you need to check if all values in B are in A, you can do it like below:
B.every(item => A.includes(item)) // true or false
includes() does not work this way. Per the Mozilla docs:
arr.includes(valueToFind[, fromIndex])
Where valueToFind is a single value and fromIndex is the index to start looking from. (In your case it looks like it ignored that because it was a string; otherwise it would have returned false.)
If what you want is to find if array A contains every element of array B, there's no standard library function that does exactly that. Here's what you can do:
function containsAll(container, contained) {
return contained.every(item => container.includes(item))
}
containsAll(A, B);
If I understand you correct you want to check if array B all of it's element includes
in array A or not it would be like this using every
B.every((el) => A.includes(el))
let A = [1, 2, 3], B = [2, 3, 4]
let result = B.every((el) => A.includes(el))
console.log(result)
and if you want at least one element from the second array to be in the first one you could do like this using some
B.some((el) => A.includes(el))
let A = [1, 2, 3], B = [2, 3, 4];
let result = B.some((el) => A.includes(el))
console.log(result)
Related
By default the indexing of every JavaScript array starts from 0. I want to create an array whose indexing starts from 1 instead.
I know, must be very trivial... Thanks for your help.
It isn't trivial. It's impossible. The best you could do is create an object using numeric properties starting at 1 but that's not the same thing.
Why exactly do you want it to start at 1? Either:
Start at 0 and adjust your indices as necessary; or
Start at 0 and just ignore index 0 (ie only use indices 1 and up).
A simple solution is to fill the zeroth item:
var map = [null, 'January', 'February', 'March'];
'First month : ' + map[1];
Semantically it would be better to use an object:
var map = {1:'January', 2:'February', 3:'March'};
'First month : ' + map[1];
Note these keys are not ints actually, object keys are always strings.
Also, we can't use dot notation for accessing. (MDN - Property Accessors)
I'd choose the first solution, which I think is less confusing.
Since this question also pops up for a Google search like "javascript start array at 1" I will give a different answer:
Arrays can be sliced. So you can get a sliced version of the Array like this:
var someArray = [0, 1, 2, 3];
someArray.slice(1);
[1, 2, 3]
someArray.slice(2, 4);
[2, 3]
Source: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/slice
You could use delete to remove the first element like so:
let arr = ['a','b','c'];
delete arr[0];
console.log(arr[0]);
console.log(arr[1]);
Or just not define it at all:
let arr = [,'b','c'];
console.log(arr[0]);
console.log(arr[1]);
If you want to make sure that you always get the first truthy element regardless of the index and have access to ES6 you can use:
arr.find(Boolean)
The question asks "How to create an array in JavaScript whose indexing starts at 1". The accepted answer states "It isn't trivial. It's impossible."
This is true, and should be understood for good reason. However, you can create an array and omit setting the first element, in which case it will still exist (hence the accepted answer being correct) but it'll be marked as empty for you.
let usernames = ['bob', 'sally', 'frank']
let myArray = [];
let arrayIndex = 1;
usernames.map(username => {
myArray[arrayIndex] = username;
arrayIndex++;
})
console.log(myArray);
Array(4) [ <1 empty slot>, "bob", "sally", "frank" ]
1: "bob"
2: "sally"
3: "frank"
length: 4
Notice that the length is "4".
console.log(myArray[0]);
undefined
Using this, there's a quirk in our favour whereby using Object.keys() on an array doesn't return empty (undefined) elements. So with the above array:
console.log(Object.keys(myArray).length);
3
Note: This is arguably a little hacky so use it with caution.
As zero of something rarely exists in our world, doing this might be useful where you are only going to access pre-defined indexes. An example would be if you have pages of a book. There isn't a page 0 as that makes no sense. And if you are always access a value directly, e.g.
const currentPage = pages[1];
Then this is fine in my opinion, as long as the code shows intent. Some will argue that ignoring a valid array index is futile, and I don't fully disagree. However, it's also futile and very annoying when I want to get page 35 of a book and the array index is 34. Meh!
When you loop your (dodgy) array with map it ignores the 0 index you didn't want:
myArray.map((value, index) => {
console.log(index);
console.log(value);
})
1
bob
2
sally
3
frank
For general use however, you should use index 0, so when you loop some data and spit things out you're not going to get caught out by the first one being empty.
Okay, according to #cletus you couldn't do that because it's a built-in javascript feature but you could go slightly different way if you still want that. You could write your own index-dependent functions of Array (like reduce, map, forEach) to start with 1. It's not a difficult task but still ask yourself: why do I need that?
Array.prototype.mapWithIndexOne = function(func) {
const initial = []
for (let i = 1; i < this.length + 1; i++) {
initial.push(func(this[i - 1], i))
}
return initial
}
const array = ['First', 'Second', 'Third', 'Fourth', 'Fifth']
console.log(array.mapWithIndexOne((element, index) => `${element}-${index}`))
// => ["First-1", "Second-2", "Third-3", "Fourth-4", "Fifth-5"]
Codepen: https://codepen.io/anon/pen/rvbNZR?editors=0012
Using Array.map
[,1,2,3].map((v, i) => ++i)
Just wanted to point out that an index in c ish languages is also the offset from the first element. This allows all sorts of offset math where you don't have to subtract 1 before doing the math, only to add the 1 back later.
if you want a "1" array because the indexes are mapped to other values, that's the case for an enumeration or a hash.
First add this function to your javascript codes:
var oneArray = function(theArray)
{
theArray.splice(0,0,null);
return theArray
}
Now use it like this:
var myArray= oneArray(['My', 'name', 'is', 'Ram']);
alert(myArray[1]); << this line show you: My
See live demo
Just prepend a null:
a = [1, 2, 3, 4]
a.unshift(null)
a[3] // 3
Simple, just make two changes to the classic Javascript for loop.
var Array = ['a', 'b', 'c'];
for (var i = 1; i <= Array.length; i++) {
//"i" starts at 1 and ends
//after it equals "length"
console.log(i);
}
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A very basic question but still learning.
I have a 1D array say = [a,b,c].
and another 2D array = [[1,2,3],[4,5,6],[7,8,9]].
How do I push the array into beginning of each row of 2D array so that my array result looks like this.
[[a,1,2,3],[b,4,5,6],[c,7,8,9]].
You can iterate each item from 2D array and add 1D array value into it.
For more help please check Unshift
var test = ['a', 'b', 'c'];
var tests = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
var index = 0;
tests.map(item => { //Iterate each item from 2D araay
if (index < test.length) { // Check if we have any item in 1D array to avoid crash
item.unshift(test[index++]); // Unshift each item so you can add value at 0 index.
}
});
console.log(tests);
array1 = ["a","b","c"];
array2 = [[1,2,3],[4,5,6],[7,8,9]];
for(var i = 0; i< array2.length;i++){
array2[i].unshift(array1[i]);
}
console.log(array2);
You can loop over one of the array and use unshift to push value at 0 index.
var x = ['a','b','c'];
var y = [[1,2,3],[4,5,6],[7,8,9]];
y.forEach((item, index) => item.unshift(x[index]));
console.log(y);
Here a solution with .map() and the rest operator
let singleArr = ["a","b","c"];
let multiArr = [[1,2,3],[4,5,6],[7,8,9]];
let result = multiArr.map((el, i) => [singleArr[i],...el])
console.log(result);
Put simply, I want to subtract one array from another.
The arrays are arrays of objects. I understand I can cycle through one array and on each item, comparing values in the other array, but that just seems a little messy.
Thanks for the help, hopefully this question isnt too basic, I have tried googling it with no luck :(
EDIT:
The Objects in the Arrays I wish to remove will have identical values but are NOT the same object (thanks #patrick dw). I am looking to completely remove the subset from the initial array.
This answer is copied from https://stackoverflow.com/a/53092728/7173655, extended with a comment and a solution with objects.
The code filters array A. All values included in B are removed from A.
const A = [1, 4, 3, 2]
const B = [0, 2, 1, 2]
console.log(A.filter(n => !B.includes(n)))
The same with objects:
const A = [{id:1}, {id:4}, {id:3}, {id:2}]
const B = [{id:0}, {id:2}, {id:1}, {id:2}]
console.log(A.filter(a => !B.map(b=>b.id).includes(a.id)))
http://phpjs.org/functions/index
There is no built-in method to do this in JavaScript. If you look at this site there are a lot of functions for arrays with similar syntax to PHP.
http://www.jslab.dk/library/Array
This site has some js functions on "sets"
I think you need the diff function.
It should remove all values from list a, which are present in list b keeping their order.
let a = [0, 2, 5, 6, 1];
let b = [2, 6, 2, 5, 0];
function arrayDiff() {
for (i of b) {
for (j of a) {
if (i === j) {
a.splice(a.indexOf(j), 1);
}
}
}
return a;
}
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Its kinda weird that the JavaScript Array class does not offer a last method to retrieve the last element of an array. I know the solution is simple (Ar[Ar.length-1] ), but, still, this is too frequently used.
Any serious reasons why this is not incorporated yet?
You can do something like this:
[10, 20, 30, 40].slice(-1)[0]
console.log([10, 20, 30, 40].slice(-1)[0])
The amount of helper methods that can be added to a language is infinite. I suppose they just haven't considered adding this one.
It's easy to define one yourself. That's the power of JavaScript.
if(!Array.prototype.last) {
Array.prototype.last = function() {
return this[this.length - 1];
}
}
var arr = [1, 2, 5];
arr.last(); // 5
However, this may cause problems with 3rd-party code which (incorrectly) uses for..in loops to iterate over arrays.
However, if you are not bound with browser support problems, then using the new ES5 syntax to define properties can solve that issue, by making the function non-enumerable, like so:
Object.defineProperty(Array.prototype, 'last', {
enumerable: false,
configurable: true,
get: function() {
return this[this.length - 1];
},
set: undefined
});
var arr = [1, 2, 5];
arr.last; // 5
Because Javascript changes very slowly. And that's because people upgrade browsers slowly.
Many Javascript libraries implement their own last() function. Use one!
i = [].concat(loves).pop(); //corn
icon cat loves popcorn
Another option, especially if you're already using UnderscoreJS, would be:
_.last([1, 2, 3, 4]); // Will return 4
Array.prototype.last = Array.prototype.last || function() {
var l = this.length;
return this[l-1];
}
x = [1,2];
alert( x.last() )
Came here looking for an answer to this question myself. The slice answer is probably best, but I went ahead and created a "last" function just to practice extending prototypes, so I thought I would go ahead and share it. It has the added benefit over some other ones of letting you optionally count backwards through the array, and pull out, say, the second to last or third to last item. If you don't specify a count it just defaults to 1 and pulls out the last item.
Array.prototype.last = Array.prototype.last || function(count) {
count = count || 1;
var length = this.length;
if (count <= length) {
return this[length - count];
} else {
return null;
}
};
var arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];
arr.last(); // returns 9
arr.last(4); // returns 6
arr.last(9); // returns 1
arr.last(10); // returns null
Here is another simpler way to slice last elements
var tags = [1, 2, 3, "foo", "bar", "foobar", "barfoo"];
var lastObj = tags.slice(-1);
lastObj is now ["barfoo"].
Python does this the same way and when I tried using JS it worked out. I am guessing string manipulation in scripting languages work the same way.
Similarly, if you want the last two objects in a array,
var lastTwoObj = tags.slice(-2)
will give you ["foobar", "barfoo"] and so on.
pop() method will pop the last value out. But the problem is that you will lose the last value in the array
Yeah, or just:
var arr = [1, 2, 5];
arr.reverse()[0]
if you want the value, and not a new list.
By default the indexing of every JavaScript array starts from 0. I want to create an array whose indexing starts from 1 instead.
I know, must be very trivial... Thanks for your help.
It isn't trivial. It's impossible. The best you could do is create an object using numeric properties starting at 1 but that's not the same thing.
Why exactly do you want it to start at 1? Either:
Start at 0 and adjust your indices as necessary; or
Start at 0 and just ignore index 0 (ie only use indices 1 and up).
A simple solution is to fill the zeroth item:
var map = [null, 'January', 'February', 'March'];
'First month : ' + map[1];
Semantically it would be better to use an object:
var map = {1:'January', 2:'February', 3:'March'};
'First month : ' + map[1];
Note these keys are not ints actually, object keys are always strings.
Also, we can't use dot notation for accessing. (MDN - Property Accessors)
I'd choose the first solution, which I think is less confusing.
Since this question also pops up for a Google search like "javascript start array at 1" I will give a different answer:
Arrays can be sliced. So you can get a sliced version of the Array like this:
var someArray = [0, 1, 2, 3];
someArray.slice(1);
[1, 2, 3]
someArray.slice(2, 4);
[2, 3]
Source: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/slice
You could use delete to remove the first element like so:
let arr = ['a','b','c'];
delete arr[0];
console.log(arr[0]);
console.log(arr[1]);
Or just not define it at all:
let arr = [,'b','c'];
console.log(arr[0]);
console.log(arr[1]);
If you want to make sure that you always get the first truthy element regardless of the index and have access to ES6 you can use:
arr.find(Boolean)
The question asks "How to create an array in JavaScript whose indexing starts at 1". The accepted answer states "It isn't trivial. It's impossible."
This is true, and should be understood for good reason. However, you can create an array and omit setting the first element, in which case it will still exist (hence the accepted answer being correct) but it'll be marked as empty for you.
let usernames = ['bob', 'sally', 'frank']
let myArray = [];
let arrayIndex = 1;
usernames.map(username => {
myArray[arrayIndex] = username;
arrayIndex++;
})
console.log(myArray);
Array(4) [ <1 empty slot>, "bob", "sally", "frank" ]
1: "bob"
2: "sally"
3: "frank"
length: 4
Notice that the length is "4".
console.log(myArray[0]);
undefined
Using this, there's a quirk in our favour whereby using Object.keys() on an array doesn't return empty (undefined) elements. So with the above array:
console.log(Object.keys(myArray).length);
3
Note: This is arguably a little hacky so use it with caution.
As zero of something rarely exists in our world, doing this might be useful where you are only going to access pre-defined indexes. An example would be if you have pages of a book. There isn't a page 0 as that makes no sense. And if you are always access a value directly, e.g.
const currentPage = pages[1];
Then this is fine in my opinion, as long as the code shows intent. Some will argue that ignoring a valid array index is futile, and I don't fully disagree. However, it's also futile and very annoying when I want to get page 35 of a book and the array index is 34. Meh!
When you loop your (dodgy) array with map it ignores the 0 index you didn't want:
myArray.map((value, index) => {
console.log(index);
console.log(value);
})
1
bob
2
sally
3
frank
For general use however, you should use index 0, so when you loop some data and spit things out you're not going to get caught out by the first one being empty.
Okay, according to #cletus you couldn't do that because it's a built-in javascript feature but you could go slightly different way if you still want that. You could write your own index-dependent functions of Array (like reduce, map, forEach) to start with 1. It's not a difficult task but still ask yourself: why do I need that?
Array.prototype.mapWithIndexOne = function(func) {
const initial = []
for (let i = 1; i < this.length + 1; i++) {
initial.push(func(this[i - 1], i))
}
return initial
}
const array = ['First', 'Second', 'Third', 'Fourth', 'Fifth']
console.log(array.mapWithIndexOne((element, index) => `${element}-${index}`))
// => ["First-1", "Second-2", "Third-3", "Fourth-4", "Fifth-5"]
Codepen: https://codepen.io/anon/pen/rvbNZR?editors=0012
Using Array.map
[,1,2,3].map((v, i) => ++i)
Just wanted to point out that an index in c ish languages is also the offset from the first element. This allows all sorts of offset math where you don't have to subtract 1 before doing the math, only to add the 1 back later.
if you want a "1" array because the indexes are mapped to other values, that's the case for an enumeration or a hash.
First add this function to your javascript codes:
var oneArray = function(theArray)
{
theArray.splice(0,0,null);
return theArray
}
Now use it like this:
var myArray= oneArray(['My', 'name', 'is', 'Ram']);
alert(myArray[1]); << this line show you: My
See live demo
Just prepend a null:
a = [1, 2, 3, 4]
a.unshift(null)
a[3] // 3
Simple, just make two changes to the classic Javascript for loop.
var Array = ['a', 'b', 'c'];
for (var i = 1; i <= Array.length; i++) {
//"i" starts at 1 and ends
//after it equals "length"
console.log(i);
}