How to fill an array with a for loop?

Question

I’m having a lot of trouble learning how to use for loops to fill a new variable. As an example say if I have var year = [2010, 2000, 1992]; and var age = [];.

How would I use a for loop to fill in the age variable?

If this is a bad example, don’t use this. I just would like some help with understanding how to fill in empty arrays.

var names = ["Ace", "yoshi", "Lassie"];
var age = [25, 23, 5];
var u24 = [];
for (var i = 0; i < names.length; i++) {
 if ([age] < 24) {
 u24 += age[i]
 console.log("hello " + names + " " + "you are" + age);
 }
}

• Munna Babu: Please check this code and try yourself, You can use forEach functions and inside loop function you just subtract year with current year and store. w3schools.com/jsref/jsref_forEach.asp
• Sebastian Simon: Do you expect [age] to be a single number to be compared with 24? Are you familiar with the Array filter and push functions?
• keja: if i understand correctly then instead of u24 += age[i]you can use u24[] = age[i] or u24.push(age[i])
• Sebastian Simon: @keja No, you cannot use u24[] = age[i].
• keja: @Xufox you are right, dont know what i was thinking :D prob not thinking..

Answer 1

If all you want to do is fill in the age array with a loop, you can try this:

let years = [2010, 2000, 1992],
 age = [],
 d = new Date().getFullYear();
years.forEach(year => age.push(d - year));
console.log(age);

As regards the relationship between age and names, I think Intervalia has explained that.

Answer 2

It is better to create objects that contain relevant data. Combining the name and age into a person object would help.

var persons = [
 {
 name: "Ace",
 age: 25
 },
 {
 name: "yoshi",
 age: 23
 },
 {
 name: "Lassie",
 age: 5
 }
];
var u24=[];
for (var i =0; i < persons.length; i++) {
 var person = persons[i];
	if(person.age < 24){
		u24.push(person.age);
		console.log("hello " + person.name + " " + "you are " + person.age);
	}
}
console.log(u24);

But you can also use forEach like this:

var persons = [
 {
 name: "Ace",
 age: 25
 },
 {
 name: "yoshi",
 age: 23
 },
 {
 name: "Lassie",
 age: 5
 }
];
var u24=[];
persons.forEach(
 function(person) {
 if(person.age < 24){
 u24.push(person.age);
 console.log("hello " + person.name + " " + "you are " + person.age);
 }
 }
);
console.log(u24);

By making objects that include all relevant data your loops will never get out of sync. If you remove a person from the persons array then their name and age will go together.

UPDATE: Using a filter

var persons = [
 {
 name: "Ace",
 age: 25
 },
 {
 name: "yoshi",
 age: 23
 },
 {
 name: "Lassie",
 age: 5
 }
];
var youngPersons = persons.filter(
 function(person) {
 return (person.age < 24);
 }
);
console.log(youngPersons);

Or using an ES6 Arrow Function

 var persons = [
 { name: "Ace", age: 25 },
 { name: "yoshi", age: 23 },
 { name: "Lassie", age: 5 }
 ];
 var youngPersons = persons.filter((person) => person.age < 24);
 console.log(youngPersons);

This provides back an array of the persons that match your Age under 24 criteria.

• Sebastian Simon: Just use filter instead of forEach.
• Intervalia: Yes, filter can work if you are just trying to get a filtered list. Since @Ace didn't post what his intent was I may have assumed a different intent.
• Ace: Thank you for your help. Quick question and it may be a dumb question but what is another way of putting the new information in to u24 beside using push. As in would it be u24 =person.age?
• Intervalia: You set up u24 as an array. So I assumed you wanted an array of ages.
• Ace: @intervalia I do

Answer 3

A working version. Please compare it with your code to see the differences. Arrays always got me when I was starting out, and the syntax with different across languages, despite the reuse of bracket symbols.. AutoIt language still trips me up :P

var names = ["Ace", "yoshi", "Lassie"];
var age = [25, 23, 5];
//Use array.push() to append values
var u24 = [];
//variable i counts up to names.length
//because i++ means 'add one' to i
for (var i = 0; i < names.length; i++) {
 //if ([age] < 24) {u24 += age[i];
 //age at the count 'i' (which is
 //counting)
 //is achieved by array[at_index]
 if (age[i] < 24) {
 u24.push(age[i]); //add it to the end
 console.log("Hello " + names[i] +
 ", you are " + age[i]);
 }
}

• Ace: This is very helpful as well thank you very much. I'm still in my first month of learn javascript so every bit of info helps.
• user8112809: No problemo bro, we've all been there! There isn't anything wrong with asking questions, so don't let the grump down votes get you :P
• Ace: I know it's just hard enough having self teaching, then you come here and have people make you feel slow for asking a question.The html and css was easy but this stuff is a bit more difficult