Foreach with Index

Get loop counter/index using for…of syntax in JavaScript

for…in iterates over property names, not values, and does so in an unspecified order (yes, even after ES6). You shouldn’t use it to iterate over arrays. For them, there’s ES5’s forEach method that passes both the value and the index to the function you give it:

var myArray = [123, 15, 187, 32];

myArray.forEach(function (value, i) {
    console.log('%d: %s', i, value);
});

// Outputs:
// 0: 123
// 1: 15
// 2: 187
// 3: 32

Or ES6’s Array.prototype.entries, which now has support across current browser versions:

for (const [i, value] of myArray.entries()) {
    console.log('%d: %s', i, value);
}

For iterables in general (where you would use a for…of loop rather than a for…in), there’s nothing built-in, however:

function* enumerate(iterable) {
    let i = 0;

    for (const x of iterable) {
        yield [i, x];
        i++;
    }
}

for (const [i, obj] of enumerate(myArray)) {
    console.log(i, obj);
}

demo

If you actually did mean for…in – enumerating properties – you would need an additional counter. Object.keys(obj).forEach could work, but it only includes own properties; for…in includes enumerable properties anywhere on the prototype chain.

How do you get the index of the current iteration of a foreach loop?

The foreach is for iterating over collections that implement IEnumerable. It does this by calling GetEnumerator on the collection, which will return an Enumerator.

This Enumerator has a method and a property:

MoveNext()
Current

Current returns the object that Enumerator is currently on, MoveNext updates Current to the next object.

The concept of an index is foreign to the concept of enumeration, and cannot be done.

Because of that, most collections are able to be traversed using an indexer and the for loop construct.

I greatly prefer using a for loop in this situation compared to tracking the index with a local variable.

Get index in ForEach in SwiftUI

This works for me:

Using Range and Count

struct ContentView: View {
    @State private var array = [1, 1, 2]

    func doSomething(index: Int) {
        self.array = [1, 2, 3]
    }
    
    var body: some View {
        ForEach(0..<array.count) { i in
          Text("\(self.array[i])")
            .onTapGesture { self.doSomething(index: i) }
        }
    }
}

Using Array's Indices

The indices property is a range of numbers.

struct ContentView: View {
    @State private var array = [1, 1, 2]

    func doSomething(index: Int) {
        self.array = [1, 2, 3]
    }
    
    var body: some View {
        ForEach(array.indices) { i in
          Text("\(self.array[i])")
            .onTapGesture { self.doSomething(index: i) }
        }
    }
}

How can i get index use forEach loop?

You can simply use a for loop and use classList.contains() to check if the class is present. Also, you need to declare a local var, to store the index returned from the loop.

const product = document.querySelector(".product");
const productList = product.querySelectorAll("li");

function getIdx() {
  let idx;
  for (var i = 0; i < productList.length; i++) {
    let list = productList[i];

    if (list.classList.contains("on")) {
      idx = i;
    }
  }
  return idx;
};

function printIdx() {
  let idx = getIdx();

  console.log(idx);
}

productList.forEach(list => {
  list.addEventListener("click", printIdx);
});

<ul class="product">
  <li>aaaaa</li>
  <li>bbbbb</li>
  <li class="on">ccccc</li>
  <li>ddddd</li>
</ul>

Automatically get loop index in foreach loop in Perl

Like codehead said, you'd have to iterate over the array indices instead of its elements. I prefer this variant over the C-style for loop:

for my $i (0 .. $#x) {
    print "$i: $x[$i]\n";
}

How to find the foreach index?

foreach($array as $key=>$value) {
    // do stuff
}

$key is the index of each $array element

How to target specific index with forEach array method?

You can remove element from setInterval because inner function setInterval has an access to the variables of outer function such as element. Such behavior is called closure.

In addition, it is possible to use arrow functions to make shorter and cleaner your code. Moreover, arrow functions:

An arrow function does not have its own this. The this value of the
enclosing lexical scope is used; arrow functions follow the normal
variable lookup rules.

sentinel.forEach((element, index) => {
   setInterval(() =>{

    if (index === 0){
        console.log(element);
    }

}, 3000);

ForEach - Index out of range?

Let me give you a simple example to show you what happened:

struct ContentView: View {
    
    @State private var lowerBound: Int = 0
    
    var body: some View {
        
        ForEach(lowerBound..<11) { index in
            Text(String(describing: index))
        }
        
        Button("update") { lowerBound = 5 }.padding()
 
    }
    
}

if you look at the upper code you would see that I am initializing a ForEach JUST with a Range like this: lowerBound..<11 which it means this 0..<11, when you do this you are telling SwiftUI, hey this is my range and it will not change! It is a constant Range! and SwiftUI says ok! if you are not going update upper or lower bound you can use ForEach without showing or given id! But if you see my code again! I am updating lowerBound of ForEach and with this action I am breaking my agreement about constant Range! So SwiftUI comes and tell us if you are going update my ForEach range in count or any thing then you have to use an id then you can update the given range! And the reason is because if we have 2 same item with same value, SwiftUI would have issue to know which one you say! with using an id we are solving the identification issue for SwiftUI! About id you can use it like this: id:\.self or like this id:\.customID if your struct conform to Hash-able protocol, or in last case you can stop using id if you confrom your struct to identifiable protocol! then ForEach would magically sink itself with that.

Now see the edited code, it will build and run because we solved the issue of identification:

struct ContentView: View {
    
    @State private var lowerBound: Int = 0
    
    var body: some View {
        
        ForEach(lowerBound..<11, id:\.self) { index in
            Text(String(describing: index))
        }
        
        Button("update") { lowerBound = 5 }.padding()
 
    }
    
}

Using Indices to get Index in a ForEach loop SwiftUI

In the variant of indices, you should use self as id

ForEach(self.postViewModel.posts.indices, id: \.self) { post in

Possible variant to have both

ForEach(Array(self.postViewModel.posts.enumerated()), 
    id: \.1.postId) { (index, post) in

Java8 forEach with index

I found a util method in eclipse collections with this post Is there a concise way to iterate over a stream with indices in Java 8?

https://www.eclipse.org/collections/javadoc/7.0.0/org/eclipse/collections/impl/utility/Iterate.html#forEachWithIndex-java.lang.Iterable-org.eclipse.collections.api.block.procedure.primitive.ObjectIntProcedure-

Iterate.forEachWithIndex(people, (Person person, int index) -> LOGGER.info("Index: " + index + " person: " + person.getName()));

The implementation https://github.com/eclipse/eclipse-collections/blob/master/eclipse-collections/src/main/java/org/eclipse/collections/impl/utility/internal/IteratorIterate.java is very similar to my util method:

public static <T> void forEach(@NonNull Iterable<T> iterable, @NonNull ObjIntConsumer<T> consumer) {
    int i = 0;
    for (T t : iterable) {
        consumer.accept(t, i++);
    }
}