somewhat OT: creating a lookup hash in JS

[Felipe Gasper]

(Sorry, this is slightly OT.)

var the_array = ["foo", "bar", "baz", "qux"];

//Is there a simpler way to do the following than what�s here?
var the_lookup = {};
the_array.forEach( function(i) { the_lookup.i = true } );

========

In Perl this is easy:
my @array = qw(foo bar baz qux);
my %lookup = map { $_ => 1 } @array;


Anything of the sort coming in JS, does anyone know? Maybe in some of
the newer ES5 goodies?

-FG

[mscdex]

On Oct 30, 5:50 pm, Felipe Gasper <fel...@felipegasper.com> wrote:
> Anything of the sort coming in JS, does anyone know? Maybe in some of
> the newer ES5 goodies?

Does this help?: https://github.com/joyent/node/wiki/ECMA-5-Mozilla-Features-Implemented-in-V8

[Felipe Mobus]

You could use ES5's array.reduce() method, but it's gonna be a bit ugly

[1,2,3,4,5,6,7,8].reduce(function(prev, cur) {
prev[cur] = (cur % 2) == 0;
return prev;
}, {})

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[Felipe Mobus]

pardon me for the (cur % 2) == 0 part, completely unnecessary to your case

[Trevor Norris]

To supplement what mscdex has already posted, here are two more articles:

http://dailyjs.com/2012/10/15/preparing-for-esnext/

https://brendaneich.com/2012/10/harmony-of-dreams-come-true/

You'l want to check out Set's. Enable them with `node --harmony` at runtime. Also, v8 doesn't support Set initialization with an Array. So you'll have to use .add() for every element. But they are about 6 times faster than using an Object lookup. Sort of like the following:

var the_set = new Set();

the_array.forEach(function(i) { the_set.add(i); });

As a side note, your syntax is incorrect. The forEach statement should look like the following:

the_array.forEach(function(i) { the_lookup[i] = true; });

[Ted Young]

var MAX = 1000000;
var arr = [], obj = {}, obj2 = {},foo, start;

for(i = 0; i < MAX; i++){
  arr.push(i.toString());
}

start = Date.now();
arr.reduce(function(obj2,i){
    obj2[i] = true;
    return obj2;
},obj2);
console.log('reduce',Date.now() - start);

start = Date.now();
arr.forEach(function(i){
    obj[i] = true;
});
console.log('forEach',Date.now() - start);

var MAX = 1000000;
var arr = [], obj = {}, obj2 = {},foo, start;

for(i = 0; i < MAX; i++){
  arr.push(i.toString());
}

start = Date.now();
arr.reduce(function(obj2,i){
    obj2[i] = true;
    return obj2;
},obj2);
console.log('reduce',Date.now() - start);

start = Date.now();
arr.forEach(function(i){
    obj[i] = true;
});
console.log('forEach',Date.now() - start);

var MAX = 1000000;
var arr = [], obj = {}, obj2 = {},foo, start;

for(i = 0; i < MAX; i++){
  arr.push(i.toString());
}

start = Date.now();
arr.reduce(function(obj2,i){
    obj2[i] = true;
    return obj2;
},obj2);
console.log('reduce',Date.now() - start);

start = Date.now();
arr.forEach(function(i){
    obj[i] = true;
});
console.log('forEach',Date.now() - start);

A quick speed test, comparing forEach and reduce:

var arr = [], obj = {}, obj2 = {}, start;

var MAX = 1000000;

for(i = 0; i < MAX; i++){

  arr.push(i.toString());

}

// test reduce

start = Date.now();

arr.reduce(function(obj,i){

    obj[i] = true;

    return obj;

},obj);

console.log('reduce',Date.now() - start);

// test forEach

start = Date.now();

arr.forEach(function(i){

    obj2[i] = true;

});

console.log('forEach',Date.now() - start);

var MAX = 1000000;
var arr = [], obj = {}, obj2 = {},foo, start;

for(i = 0; i < MAX; i++){
  arr.push(i.toString());
}

start = Date.now();
arr.reduce(function(obj2,i){
    obj2[i] = true;
    return obj2;
},obj2);
console.log('reduce',Date.now() - start);

start = Date.now();
arr.forEach(function(i){
    obj[i] = true;
});
console.log('forEach',Date.now() - start);

var MAX = 1000000;
var arr = [], obj = {}, obj2 = {},foo, start;

for(i = 0; i < MAX; i++){
  arr.push(i.toString());
}

start = Date.now();
arr.reduce(function(obj2,i){
    obj2[i] = true;
    return obj2;
},obj2);
console.log('reduce',Date.now() - start);

start = Date.now();
arr.forEach(function(i){
    obj[i] = true;
});
console.log('forEach',Date.now() - start);

results are:

in node:

reduce 169

forEach 147

in node if you reverse the order:

forEach 170

reduce 145

in firefox:

reduce 956

forEach 1906

reversed in firefox:

forEach 2057

reduce 944

If this is for the browser, reduce seems twice as fast in firefox.  In node they appear equivalent. 

forEach is about as concise as it gets, but hey, one line of code ain't bad. :)

Ted

[Rick Waldron]

On Tue, Oct 30, 2012 at 7:02 PM, Trevor Norris <trev....@gmail.com> wrote:

To supplement what mscdex has already posted, here are two more articles:

http://dailyjs.com/2012/10/15/preparing-for-esnext/

https://brendaneich.com/2012/10/harmony-of-dreams-come-true/

You'l want to check out Set's. Enable them with `node --harmony` at runtime. Also, v8 doesn't support Set initialization with an Array. So you'll have to use .add() for every element. But they are about 6 times faster than using an Object lookup. Sort of like the following:

var the_set = new Set();

the_array.forEach(function(i) { the_set.add(i); });

Ugh, it's too bad v8's implementation is outdated, because the correct way to do exactly this is:

var set = new Set(the_array); // creates a unique set with lookup API for free! yay!

Rick

 

As a side note, your syntax is incorrect. The forEach statement should look like the following:

the_array.forEach(function(i) { the_lookup[i] = true; });

On Tuesday, October 30, 2012 2:50:25 PM UTC-7, Felipe Gasper wrote:

(Sorry, this is slightly OT.)

var the_array = ["foo", "bar", "baz", "qux"];

//Is there a simpler way to do the following than what�s here?

var the_lookup = {};
the_array.forEach( function(i) { the_lookup.i = true } );

========

In Perl this is easy:
my @array = qw(foo bar baz qux);
my %lookup = map { $_ => 1 } @array;


Anything of the sort coming in JS, does anyone know? Maybe in some of
the newer ES5 goodies?

-FG

--