Let's say I have an array like this:
var colors = ['blue', 'red', 'red', 'red', 'blue', 'red', 'blue', 'blue']
How would I get an array that tells me what position each of them is at? for example:
var reds = [1,2,3,5], blues = [0,4,6,7]
So far I have tried to use the indexOf(); function but that will only return 1 of the values if there are multiple matches.
You could collect all indices in an object with the color as property.
This approach features
a classic for statement for iterating the index,
a logical nullish assignment ??= to check the property and assign an array if not given,
and Array#push, to get the index into the array with the color as name.
const
colors = ['blue', 'red', 'red', 'red', 'blue', 'red', 'blue', 'blue'],
indices = {};
for (let i = 0; i < colors.length; i++) {
(indices[colors[i]] ??= []).push(i);
}
console.log(indices);You can use forEach, for loop to get the element and index and push it into respective array.
var colors = ["blue", "red", "red", "red", "blue", "red", "blue", "blue"];
const reds = [];
const blues = [];
colors.forEach((color, index) => {
if (color === "red") reds.push(index);
else if (color === "blue") blues.push(index);
});
console.log(reds);
console.log(blues);I think this will be easier to understand than the other answers:
var colors = ['blue', 'red', 'red', 'red', 'blue', 'red', 'blue', 'blue'];
var reds = [];
var blues = [];
for(var i = 0; i < colors.length; ++i)
{
if(colors[i] == 'red')
{
reds.push(i);
}
else if(colors[i] == 'blue')
{
blues.push(i);
}
}
You can also use reduce to construct an object where the properties are your colors and the values an array with the indexes
const colors = ['blue', 'red', 'red', 'red', 'blue', 'red', 'blue', 'blue'];
const indices = colors.reduce((acc, color, index) => {
return !acc[color] ?
{ ...acc, [color]: [index]} :
{ ...acc, [color]: [...acc[color], index] }
}, {})
console.log(indices);The answer of Nina Scholz looks good. And I want to give another way to help you.
You can use Array#reduce to resolve it with the highest performance O(n) time complexity like below:
const colors = ['blue', 'red', 'red', 'red', 'blue', 'red', 'blue', 'blue']
const result = colors.reduce((acc, color, index) => {
acc[color] ??= [];
acc[color].push(index);
return acc;
}, {});
console.log(result);
let arr1 = ['blue', 'red', 'blue', 'red', 'red', 'blue', 'blue'];
const redElements = arr1.filter(x => x == 'red');
const blueElements = arr1.filter(x => x == 'blue');
Filter returns an array;
