Hello I have an array as follows:
[[1,1],[2,8],[3,12],[4,13],[5,23],[6,30],[7,41],[8,44],[9,50]]
So lets say it is the format of [[x,y]]
As you can see the above array, the first element inside inner arrays goes from 1,2...6 which is x, and the second element of inner arrays are 1,8,12,13,23 and 30 which is y.
This is a cumulative array of y and I want to convert it into non-cumulative but with a twist.
I want to convert y to non-cumulative array by getting the difference from the value of y at last 3rd value of x
Therefore I want the final result to be as follows:
[[1,1],[2,8],[3,0],[4,1],[5,11],[6,0],[7,11],[8,14],[9,0]]
I have tried a fiddle here:
https://jsfiddle.net/gxytx6ka/1/
So far I have been able to get the difference between two array elements by some help of Dekel (stackoverflow guy) as you can see in the above fiddle:
$(document).ready(function() {
var old_arr = [ [1, 1], [2, 8], [3, 12], [4, 13], [5, 23], [6, 30], [7, 41], [8, 44], [9, 50] ];
var new_arr = old_arr.reduce(function(a, b, c, d) {
if (a.length) {
a.push([d[c][0], d[c][1] - d[c - 1][1]])
} else {
a = [b]
}
return a;
}, []);
console.log(new_arr);
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
which outputs:
[[1,1],[2,7],[3,4],[4,1],[5,10],[6,7],[7,11],[8,3],[9,6]]
but I want:
[[1,1],[2,8],[3,0],[4,1],[5,11],[6,0],[7,11],[8,14],[9,0]]
which I am not able to get.
I tried using a%3==0 in if condition, but not working...
Basically I want the final output in an efficient way.
Why reduce? A simple loop does the job perfectly fine:
let a = [[1,1],[2,8],[3,12],[4,13],[5,23],[6,30],[7,41],[8,44],[9,50]];
let out = [],
n = 0,
lastY = 0;
for (let [x, y] of a) {
if (++n % 3 == 0)
lastY = y;
out.push([x, y - lastY]);
}
console.log(JSON.stringify(out))
You could use Array#map, which returns simply an array with the result.
var array = [[1,1], [2,8], [3,12], [4,13], [5,23], [6,30], [7,41], [8,44], [9,50]],
result = array.map(function (a, i) {
(i + 1) % 3 || (this.last = a[1]);
return [a[0], a[1] - this.last];
}, { last: 0 });
console.log(result);
.as-console-wrapper { max-height: 100% !important; top: 0; }
ES6
var array = [[1,1], [2,8], [3,12], [4,13], [5,23], [6,30], [7,41], [8,44], [9,50]],
result = array.map((l => ([x, y], i) => (++i % 3 || (l = y), [x, y - l]))(0));
console.log(result);
.as-console-wrapper { max-height: 100% !important; top: 0; }
It is possible to do this with reduce — You just need to figure out when to zero-out every third value and when to find the difference between the current value and it's previous (third) value.
Naming parameters with more intelligible names also helps out.
var old_arr = [ [1, 1], [2, 8], [3, 12], [4, 13], [5, 23], [6, 30], [7, 41], [8, 44], [9, 50] ];
var new_arr = old_arr.reduce(function(result, value, index, arr) {
if (index % 3 == 2) {
return result.concat([[value[0], 0]]); // Set every third value to 0.
} else {
var prevIndex = index - (index % 3) - 1;
var prevVal = prevIndex > -1 ? arr[prevIndex][1] : 0;
return result.concat([[value[0], Math.abs(prevVal - value[1])]]);
}
}, []);
console.log(JSON.stringify(new_arr)); // [[1,1],[2,8],[3,0],[4,1],[5,10],[6,0],[7,11],[8,3],[9,0]]
With reduce you can do it like this (also using ES6 arrow function and spread operator):
var arr = [[1,1], [2,8], [3,12], [4,13], [5,23], [6,30], [7,41], [8,44], [9,50]];
var result = arr.reduce( ([arr, v], [x, y], i) =>
[[...arr,[x, i%3==2 ? 0 : y-v]], i%3==2 ? y : v], [[],0])[0];
console.log(result);
Explanation
reduce is called with a double initial value (a pair):
[[],0]
The first element is the array that will accumulate into the final result, and the second value is the current value to which subsequent y values will be offset.
These two values are captured by the argument [arr, v] which is a destructuring assignment (ES6). The reduce callback will always return a new pair like that, with extended arr and (potentially) updated v.
The callback function also takes the current [x, y] pair from the original array. Again this is a destructuring assignment, so you have direct access to x and y variables. The third argument i is the current, zero-based index in the original array.
As said, the callback returns a pair. The first element in that pair is constructed as follows:
[...arr, [x, i%3==2 ? 0 : y-v]]
The ...arr notation spreads the previously collected result elements as a list, and one pair is added to that list: [x, i%3==2 ? 0 : y-v]. The x is just reproduced from the original [x, y], but the y follows the requested logic with the ternary operator: if we are at an index that has a remainder of 2 when divided by 3, then the y value should be 0. Otherwise, it should be offset against the previously set value v.
The second element in the pair, must be the new value for v:
i%3==2 ? y : v
Again, according to the requested logic, v remains unchanged when the remainder is not 2, but otherwise it is set to the current value of y.
So reduce will thus accumulate/update these two values in each iteration, and finally return a pair, of which only the first has our interest, which is why there is [0] at the end.
Notes
As you seemed to be looking for a reduce implementation, that is what I went with, but whenever your output array has just as many elements as your input array (like is the case here) you can also consider to use map as a good alternative (See answer that NinaScholz posted).
If you are open for a less functional programming way, you can also choose to use a for loop and maybe gain a bit of performance.
I'm looking for any alternatives to the below for creating a JavaScript array containing 1 through to N where N is only known at runtime.
var foo = [];
for (var i = 1; i <= N; i++) {
foo.push(i);
}
To me it feels like there should be a way of doing this without the loop.
In ES6 using Array from() and keys() methods.
Array.from(Array(10).keys())
//=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Shorter version using spread operator.
[...Array(10).keys()]
//=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Start from 1 by passing map function to Array from(), with an object with a length property:
Array.from({length: 10}, (_, i) => i + 1)
//=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
You can do so:
var N = 10;
Array.apply(null, {length: N}).map(Number.call, Number)
result: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
or with random values:
Array.apply(null, {length: N}).map(Function.call, Math.random)
result: [0.7082694901619107, 0.9572225909214467, 0.8586748542729765,
0.8653848143294454, 0.008339877473190427, 0.9911756622605026, 0.8133423360995948, 0.8377588465809822, 0.5577575915958732, 0.16363654541783035]
Explanation
First, note that Number.call(undefined, N) is equivalent to Number(N), which just returns N. We'll use that fact later.
Array.apply(null, [undefined, undefined, undefined]) is equivalent to Array(undefined, undefined, undefined), which produces a three-element array and assigns undefined to each element.
How can you generalize that to N elements? Consider how Array() works, which goes something like this:
function Array() {
if ( arguments.length == 1 &&
'number' === typeof arguments[0] &&
arguments[0] >= 0 && arguments &&
arguments[0] < 1 << 32 ) {
return [ … ]; // array of length arguments[0], generated by native code
}
var a = [];
for (var i = 0; i < arguments.length; i++) {
a.push(arguments[i]);
}
return a;
}
Since ECMAScript 5, Function.prototype.apply(thisArg, argsArray) also accepts a duck-typed array-like object as its second parameter. If we invoke Array.apply(null, { length: N }), then it will execute
function Array() {
var a = [];
for (var i = 0; i < /* arguments.length = */ N; i++) {
a.push(/* arguments[i] = */ undefined);
}
return a;
}
Now we have an N-element array, with each element set to undefined. When we call .map(callback, thisArg) on it, each element will be set to the result of callback.call(thisArg, element, index, array). Therefore, [undefined, undefined, …, undefined].map(Number.call, Number) would map each element to (Number.call).call(Number, undefined, index, array), which is the same as Number.call(undefined, index, array), which, as we observed earlier, evaluates to index. That completes the array whose elements are the same as their index.
Why go through the trouble of Array.apply(null, {length: N}) instead of just Array(N)? After all, both expressions would result an an N-element array of undefined elements. The difference is that in the former expression, each element is explicitly set to undefined, whereas in the latter, each element was never set. According to the documentation of .map():
callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.
Therefore, Array(N) is insufficient; Array(N).map(Number.call, Number) would result in an uninitialized array of length N.
Compatibility
Since this technique relies on behaviour of Function.prototype.apply() specified in ECMAScript 5, it will not work in pre-ECMAScript 5 browsers such as Chrome 14 and Internet Explorer 9.
Multiple ways using ES6
Using spread operator (...) and keys method
[ ...Array(N).keys() ].map( i => i+1);
Fill/Map
Array(N).fill().map((_, i) => i+1);
Array.from
Array.from(Array(N), (_, i) => i+1)
Array.from and { length: N } hack
Array.from({ length: N }, (_, i) => i+1)
Note about generalised form
All the forms above can produce arrays initialised to pretty much any desired values by changing i+1 to expression required (e.g. i*2, -i, 1+i*2, i%2 and etc). If expression can be expressed by some function f then the first form becomes simply
[ ...Array(N).keys() ].map(f)
Examples:
Array.from({length: 5}, (v, k) => k+1);
// [1,2,3,4,5]
Since the array is initialized with undefined on each position, the value of v will be undefined
Example showcasing all the forms
let demo= (N) => {
console.log(
[ ...Array(N).keys() ].map(( i) => i+1),
Array(N).fill().map((_, i) => i+1) ,
Array.from(Array(N), (_, i) => i+1),
Array.from({ length: N }, (_, i) => i+1)
)
}
demo(5)
More generic example with custom initialiser function f i.e.
[ ...Array(N).keys() ].map((i) => f(i))
or even simpler
[ ...Array(N).keys() ].map(f)
let demo= (N,f) => {
console.log(
[ ...Array(N).keys() ].map(f),
Array(N).fill().map((_, i) => f(i)) ,
Array.from(Array(N), (_, i) => f(i)),
Array.from({ length: N }, (_, i) => f(i))
)
}
demo(5, i=>2*i+1)
If I get what you are after, you want an array of numbers 1..n that you can later loop through.
If this is all you need, can you do this instead?
var foo = new Array(45); // create an empty array with length 45
then when you want to use it... (un-optimized, just for example)
for(var i = 0; i < foo.length; i++){
document.write('Item: ' + (i + 1) + ' of ' + foo.length + '<br/>');
}
e.g. if you don't need to store anything in the array, you just need a container of the right length that you can iterate over... this might be easier.
See it in action here: http://jsfiddle.net/3kcvm/
Arrays innately manage their lengths. As they are traversed, their indexes can be held in memory and referenced at that point. If a random index needs to be known, the indexOf method can be used.
This said, for your needs you may just want to declare an array of a certain size:
var foo = new Array(N); // where N is a positive integer
/* this will create an array of size, N, primarily for memory allocation,
but does not create any defined values
foo.length // size of Array
foo[ Math.floor(foo.length/2) ] = 'value' // places value in the middle of the array
*/
ES6
Spread
Making use of the spread operator (...) and keys method, enables you to create a temporary array of size N to produce the indexes, and then a new array that can be assigned to your variable:
var foo = [ ...Array(N).keys() ];
Fill/Map
You can first create the size of the array you need, fill it with undefined and then create a new array using map, which sets each element to the index.
var foo = Array(N).fill().map((v,i)=>i);
Array.from
This should be initializing to length of size N and populating the array in one pass.
Array.from({ length: N }, (v, i) => i)
In lieu of the comments and confusion, if you really wanted to capture the values from 1..N in the above examples, there are a couple options:
if the index is available, you can simply increment it by one (e.g., ++i).
in cases where index is not used -- and possibly a more efficient way -- is to create your array but make N represent N+1, then shift off the front.
So if you desire 100 numbers:
let arr; (arr=[ ...Array(101).keys() ]).shift()
In ES6 you can do:
Array(N).fill().map((e,i)=>i+1);
http://jsbin.com/molabiluwa/edit?js,console
Edit:
Changed Array(45) to Array(N) since you've updated the question.
console.log(
Array(45).fill(0).map((e,i)=>i+1)
);
Use the very popular Underscore _.range method
// _.range([start], stop, [step])
_.range(10); // => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
_.range(1, 11); // => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
_.range(0, 30, 5); // => [0, 5, 10, 15, 20, 25]
_.range(0, -10, -1); // => [0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
_.range(0); // => []
function range(start, end) {
var foo = [];
for (var i = start; i <= end; i++) {
foo.push(i);
}
return foo;
}
Then called by
var foo = range(1, 5);
There is no built-in way to do this in Javascript, but it's a perfectly valid utility function to create if you need to do it more than once.
Edit: In my opinion, the following is a better range function. Maybe just because I'm biased by LINQ, but I think it's more useful in more cases. Your mileage may vary.
function range(start, count) {
if(arguments.length == 1) {
count = start;
start = 0;
}
var foo = [];
for (var i = 0; i < count; i++) {
foo.push(start + i);
}
return foo;
}
the fastest way to fill an Array in v8 is:
[...Array(5)].map((_,i) => i);
result will be: [0, 1, 2, 3, 4]
Performance
Today 2020.12.11 I performed tests on macOS HighSierra 10.13.6 on Chrome v87, Safari v13.1.2 and Firefox v83 for chosen solutions.
Results
For all browsers
solution O (based on while) is the fastest (except Firefox for big N - but it's fast there)
solution T is fastest on Firefox for big N
solutions M,P is fast for small N
solution V (lodash) is fast for big N
solution W,X are slow for small N
solution F is slow
Details
I perform 2 tests cases:
for small N = 10 - you can run it HERE
for big N = 1000000 - you can run it HERE
Below snippet presents all tested solutions A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
function A(N) {
return Array.from({length: N}, (_, i) => i + 1)
}
function B(N) {
return Array(N).fill().map((_, i) => i+1);
}
function C(N) {
return Array(N).join().split(',').map((_, i) => i+1 );
}
function D(N) {
return Array.from(Array(N), (_, i) => i+1)
}
function E(N) {
return Array.from({ length: N }, (_, i) => i+1)
}
function F(N) {
return Array.from({length:N}, Number.call, i => i + 1)
}
function G(N) {
return (Array(N)+'').split(',').map((_,i)=> i+1)
}
function H(N) {
return [ ...Array(N).keys() ].map( i => i+1);
}
function I(N) {
return [...Array(N).keys()].map(x => x + 1);
}
function J(N) {
return [...Array(N+1).keys()].slice(1)
}
function K(N) {
return [...Array(N).keys()].map(x => ++x);
}
function L(N) {
let arr; (arr=[ ...Array(N+1).keys() ]).shift();
return arr;
}
function M(N) {
var arr = [];
var i = 0;
while (N--) arr.push(++i);
return arr;
}
function N(N) {
var a=[],b=N;while(b--)a[b]=b+1;
return a;
}
function O(N) {
var a=Array(N),b=0;
while(b<N) a[b++]=b;
return a;
}
function P(N) {
var foo = [];
for (var i = 1; i <= N; i++) foo.push(i);
return foo;
}
function Q(N) {
for(var a=[],b=N;b--;a[b]=b+1);
return a;
}
function R(N) {
for(var i,a=[i=0];i<N;a[i++]=i);
return a;
}
function S(N) {
let foo,x;
for(foo=[x=N]; x; foo[x-1]=x--);
return foo;
}
function T(N) {
return new Uint8Array(N).map((item, i) => i + 1);
}
function U(N) {
return '_'.repeat(5).split('').map((_, i) => i + 1);
}
function V(N) {
return _.range(1, N+1);
}
function W(N) {
return [...(function*(){let i=0;while(i<N)yield ++i})()]
}
function X(N) {
function sequence(max, step = 1) {
return {
[Symbol.iterator]: function* () {
for (let i = 1; i <= max; i += step) yield i
}
}
}
return [...sequence(N)];
}
[A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X].forEach(f=> {
console.log(`${f.name} ${f(5)}`);
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.20/lodash.min.js" integrity="sha512-90vH1Z83AJY9DmlWa8WkjkV79yfS2n2Oxhsi2dZbIv0nC4E6m5AbH8Nh156kkM7JePmqD6tcZsfad1ueoaovww==" crossorigin="anonymous"> </script>
This snippet only presents functions used in performance tests - it does not perform tests itself!
And here are example results for chrome
This question has a lot of complicated answers, but a simple one-liner:
[...Array(255).keys()].map(x => x + 1)
Also, although the above is short (and neat) to write, I think the following is a bit faster
(for a max length of:
127, Int8,
255, Uint8,
32,767, Int16,
65,535, Uint16,
2,147,483,647, Int32,
4,294,967,295, Uint32.
(based on the max integer values), also here's more on Typed Arrays):
(new Uint8Array(255)).map(($,i) => i + 1);
Although this solution is also not so ideal, because it creates two arrays, and uses the extra variable declaration "$" (not sure any way to get around that using this method). I think the following solution is the absolute fastest possible way to do this:
for(var i = 0, arr = new Uint8Array(255); i < arr.length; i++) arr[i] = i + 1;
Anytime after this statement is made, you can simple use the variable "arr" in the current scope;
If you want to make a simple function out of it (with some basic verification):
function range(min, max) {
min = min && min.constructor == Number ? min : 0;
!(max && max.constructor == Number && max > min) && // boolean statements can also be used with void return types, like a one-line if statement.
((max = min) & (min = 0)); //if there is a "max" argument specified, then first check if its a number and if its graeter than min: if so, stay the same; if not, then consider it as if there is no "max" in the first place, and "max" becomes "min" (and min becomes 0 by default)
for(var i = 0, arr = new (
max < 128 ? Int8Array :
max < 256 ? Uint8Array :
max < 32768 ? Int16Array :
max < 65536 ? Uint16Array :
max < 2147483648 ? Int32Array :
max < 4294967296 ? Uint32Array :
Array
)(max - min); i < arr.length; i++) arr[i] = i + min;
return arr;
}
//and you can loop through it easily using array methods if you want
range(1,11).forEach(x => console.log(x));
//or if you're used to pythons `for...in` you can do a similar thing with `for...of` if you want the individual values:
for(i of range(2020,2025)) console.log(i);
//or if you really want to use `for..in`, you can, but then you will only be accessing the keys:
for(k in range(25,30)) console.log(k);
console.log(
range(1,128).constructor.name,
range(200).constructor.name,
range(400,900).constructor.name,
range(33333).constructor.name,
range(823, 100000).constructor.name,
range(10,4) // when the "min" argument is greater than the "max", then it just considers it as if there is no "max", and the new max becomes "min", and "min" becomes 0, as if "max" was never even written
);
so, with the above function, the above super-slow "simple one-liner" becomes the super-fast, even-shorter:
range(1,14000);
Using ES2015/ES6 spread operator
[...Array(10)].map((_, i) => i + 1)
console.log([...Array(10)].map((_, i) => i + 1))
You can use this:
new Array(/*any number which you want*/)
.join().split(',')
.map(function(item, index){ return ++index;})
for example
new Array(10)
.join().split(',')
.map(function(item, index){ return ++index;})
will create following array:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
If you happen to be using d3.js in your app as I am, D3 provides a helper function that does this for you.
So to get an array from 0 to 4, it's as easy as:
d3.range(5)
[0, 1, 2, 3, 4]
and to get an array from 1 to 5, as you were requesting:
d3.range(1, 5+1)
[1, 2, 3, 4, 5]
Check out this tutorial for more info.
This is probably the fastest way to generate an array of numbers
Shortest
var a=[],b=N;while(b--)a[b]=b+1;
Inline
var arr=(function(a,b){while(a--)b[a]=a;return b})(10,[]);
//arr=[0,1,2,3,4,5,6,7,8,9]
If you want to start from 1
var arr=(function(a,b){while(a--)b[a]=a+1;return b})(10,[]);
//arr=[1,2,3,4,5,6,7,8,9,10]
Want a function?
function range(a,b,c){c=[];while(a--)c[a]=a+b;return c}; //length,start,placeholder
var arr=range(10,5);
//arr=[5,6,7,8,9,10,11,12,13,14]
WHY?
while is the fastest loop
Direct setting is faster than push
[] is faster than new Array(10)
it's short... look the first code. then look at all other functions in here.
If you like can't live without for
for(var a=[],b=7;b>0;a[--b]=b+1); //a=[1,2,3,4,5,6,7]
or
for(var a=[],b=7;b--;a[b]=b+1); //a=[1,2,3,4,5,6,7]
If you are using lodash, you can use _.range:
_.range([start=0], end, [step=1])
Creates an array of numbers
(positive and/or negative) progressing from start up to, but not
including, end. A step of -1 is used if a negative start is specified
without an end or step. If end is not specified, it's set to start
with start then set to 0.
Examples:
_.range(4);
// ➜ [0, 1, 2, 3]
_.range(-4);
// ➜ [0, -1, -2, -3]
_.range(1, 5);
// ➜ [1, 2, 3, 4]
_.range(0, 20, 5);
// ➜ [0, 5, 10, 15]
_.range(0, -4, -1);
// ➜ [0, -1, -2, -3]
_.range(1, 4, 0);
// ➜ [1, 1, 1]
_.range(0);
// ➜ []
the new way to filling Array is:
const array = [...Array(5).keys()]
console.log(array)
result will be: [0, 1, 2, 3, 4]
with ES6 you can do:
// `n` is the size you want to initialize your array
// `null` is what the array will be filled with (can be any other value)
Array(n).fill(null)
Very simple and easy to generate exactly 1 - N
const [, ...result] = Array(11).keys();
console.log('Result:', result);
Final Summary report .. Drrruummm Rolll -
This is the shortest code to generate an Array of size N (here 10) without using ES6. Cocco's version above is close but not the shortest.
(function(n){for(a=[];n--;a[n]=n+1);return a})(10)
But the undisputed winner of this Code golf(competition to solve a particular problem in the fewest bytes of source code) is Niko Ruotsalainen . Using Array Constructor and ES6 spread operator . (Most of the ES6 syntax is valid typeScript, but following is not. So be judicious while using it)
[...Array(10).keys()]
https://stackoverflow.com/a/49577331/8784402
With Delta
For javascript
smallest and one-liner
[...Array(N)].map((v, i) => from + i * step);
Examples and other alternatives
Array.from(Array(10).keys()).map(i => 4 + i * 2);
//=> [4, 6, 8, 10, 12, 14, 16, 18, 20, 22]
[...Array(10).keys()].map(i => 4 + i * -2);
//=> [4, 2, 0, -2, -4, -6, -8, -10, -12, -14]
Array(10).fill(0).map((v, i) => 4 + i * 2);
//=> [4, 6, 8, 10, 12, 14, 16, 18, 20, 22]
Array(10).fill().map((v, i) => 4 + i * -2);
//=> [4, 2, 0, -2, -4, -6, -8, -10, -12, -14]
[...Array(10)].map((v, i) => 4 + i * 2);
//=> [4, 6, 8, 10, 12, 14, 16, 18, 20, 22]
Range Function
const range = (from, to, step) =>
[...Array(Math.floor((to - from) / step) + 1)].map((_, i) => from + i * step);
range(0, 9, 2);
//=> [0, 2, 4, 6, 8]
// can also assign range function as static method in Array class (but not recommended )
Array.range = (from, to, step) =>
[...Array(Math.floor((to - from) / step) + 1)].map((_, i) => from + i * step);
Array.range(2, 10, 2);
//=> [2, 4, 6, 8, 10]
Array.range(0, 10, 1);
//=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Array.range(2, 10, -1);
//=> []
Array.range(3, 0, -1);
//=> [3, 2, 1, 0]
As Iterators
class Range {
constructor(total = 0, step = 1, from = 0) {
this[Symbol.iterator] = function* () {
for (let i = 0; i < total; yield from + i++ * step) {}
};
}
}
[...new Range(5)]; // Five Elements
//=> [0, 1, 2, 3, 4]
[...new Range(5, 2)]; // Five Elements With Step 2
//=> [0, 2, 4, 6, 8]
[...new Range(5, -2, 10)]; // Five Elements With Step -2 From 10
//=>[10, 8, 6, 4, 2]
[...new Range(5, -2, -10)]; // Five Elements With Step -2 From -10
//=> [-10, -12, -14, -16, -18]
// Also works with for..of loop
for (i of new Range(5, -2, 10)) console.log(i);
// 10 8 6 4 2
As Generators Only
const Range = function* (total = 0, step = 1, from = 0) {
for (let i = 0; i < total; yield from + i++ * step) {}
};
Array.from(Range(5, -2, -10));
//=> [-10, -12, -14, -16, -18]
[...Range(5, -2, -10)]; // Five Elements With Step -2 From -10
//=> [-10, -12, -14, -16, -18]
// Also works with for..of loop
for (i of Range(5, -2, 10)) console.log(i);
// 10 8 6 4 2
// Lazy loaded way
const number0toInf = Range(Infinity);
number0toInf.next().value;
//=> 0
number0toInf.next().value;
//=> 1
// ...
From-To with steps/delta
using iterators
class Range2 {
constructor(to = 0, step = 1, from = 0) {
this[Symbol.iterator] = function* () {
let i = 0,
length = Math.floor((to - from) / step) + 1;
while (i < length) yield from + i++ * step;
};
}
}
[...new Range2(5)]; // First 5 Whole Numbers
//=> [0, 1, 2, 3, 4, 5]
[...new Range2(5, 2)]; // From 0 to 5 with step 2
//=> [0, 2, 4]
[...new Range2(5, -2, 10)]; // From 10 to 5 with step -2
//=> [10, 8, 6]
using Generators
const Range2 = function* (to = 0, step = 1, from = 0) {
let i = 0,
length = Math.floor((to - from) / step) + 1;
while (i < length) yield from + i++ * step;
};
[...Range2(5, -2, 10)]; // From 10 to 5 with step -2
//=> [10, 8, 6]
let even4to10 = Range2(10, 2, 4);
even4to10.next().value;
//=> 4
even4to10.next().value;
//=> 6
even4to10.next().value;
//=> 8
even4to10.next().value;
//=> 10
even4to10.next().value;
//=> undefined
For Typescript
class _Array<T> extends Array<T> {
static range(from: number, to: number, step: number): number[] {
return Array.from(Array(Math.floor((to - from) / step) + 1)).map(
(v, k) => from + k * step
);
}
}
_Array.range(0, 9, 1);
Solution for empty array and with just number in array
const arrayOne = new Array(10);
console.log(arrayOne);
const arrayTwo = [...Array(10).keys()];
console.log(arrayTwo);
var arrayThree = Array.from(Array(10).keys());
console.log(arrayThree);
const arrayStartWithOne = Array.from(Array(10).keys(), item => item + 1);
console.log(arrayStartWithOne)
✅ Simply, this worked for me:
[...Array(5)].map(...)
There is another way in ES6, using Array.from which takes 2 arguments, the first is an arrayLike (in this case an object with length property), and the second is a mapping function (in this case we map the item to its index)
Array.from({length:10}, (v,i) => i)
this is shorter and can be used for other sequences like generating even numbers
Array.from({length:10}, (v,i) => i*2)
Also this has better performance than most other ways because it only loops once through the array.
Check the snippit for some comparisons
// open the dev console to see results
count = 100000
console.time("from object")
for (let i = 0; i<count; i++) {
range = Array.from({length:10}, (v,i) => i )
}
console.timeEnd("from object")
console.time("from keys")
for (let i =0; i<count; i++) {
range = Array.from(Array(10).keys())
}
console.timeEnd("from keys")
console.time("apply")
for (let i = 0; i<count; i++) {
range = Array.apply(null, { length: 10 }).map(function(element, index) { return index; })
}
console.timeEnd("apply")
Fast
This solution is probably fastest it is inspired by lodash _.range function (but my is simpler and faster)
let N=10, i=0, a=Array(N);
while(i<N) a[i++]=i;
console.log(a);
Performance advantages over current (2020.12.11) existing answers based on while/for
memory is allocated once at the beginning by a=Array(N)
increasing index i++ is used - which looks is about 30% faster than decreasing index i-- (probably because CPU cache memory faster in forward direction)
Speed tests with more than 20 other solutions was conducted in this answer
Using new Array methods and => function syntax from ES6 standard (only Firefox at the time of writing).
By filling holes with undefined:
Array(N).fill().map((_, i) => i + 1);
Array.from turns "holes" into undefined so Array.map works as expected:
Array.from(Array(5)).map((_, i) => i + 1)
In ES6:
Array.from({length: 1000}, (_, i) => i).slice(1);
or better yet (without the extra variable _ and without the extra slice call):
Array.from({length:1000}, Number.call, i => i + 1)
Or for slightly faster results, you can use Uint8Array, if your list is shorter than 256 results (or you can use the other Uint lists depending on how short the list is, like Uint16 for a max number of 65535, or Uint32 for a max of 4294967295 etc. Officially, these typed arrays were only added in ES6 though). For example:
Uint8Array.from({length:10}, Number.call, i => i + 1)
ES5:
Array.apply(0, {length: 1000}).map(function(){return arguments[1]+1});
Alternatively, in ES5, for the map function (like second parameter to the Array.from function in ES6 above), you can use Number.call
Array.apply(0,{length:1000}).map(Number.call,Number).slice(1)
Or, if you're against the .slice here also, you can do the ES5 equivalent of the above (from ES6), like:
Array.apply(0,{length:1000}).map(Number.call, Function("i","return i+1"))
Array(...Array(9)).map((_, i) => i);
console.log(Array(...Array(9)).map((_, i) => i))
for(var i,a=[i=0];i<10;a[i++]=i);
a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
It seems the only flavor not currently in this rather complete list of answers is one featuring a generator; so to remedy that:
const gen = N => [...(function*(){let i=0;while(i<N)yield i++})()]
which can be used thus:
gen(4) // [0,1,2,3]
The nice thing about this is you don't just have to increment... To take inspiration from the answer #igor-shubin gave, you could create an array of randoms very easily:
const gen = N => [...(function*(){let i=0;
while(i++<N) yield Math.random()
})()]
And rather than something lengthy operationally expensive like:
const slow = N => new Array(N).join().split(',').map((e,i)=>i*5)
// [0,5,10,15,...]
you could instead do:
const fast = N => [...(function*(){let i=0;while(i++<N)yield i*5})()]