Key Value Iteration In For Loop With Generator

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When you use the yieldcontextual keyword in a statement, you indicate that the method, operator, or get accessor in which it appears is an iterator. Using yield to define an iterator removes the need for an explicit extra class (the class that holds the state for an enumeration, see IEnumerator<T> for an example) when you implement the IEnumerable and IEnumerator pattern for a custom collection type.

The following example shows the two forms of the yield/all-in-one-key-generator.html. statement.

Remarks

You use a yield return statement to return each element one at a time.

The sequence returned from an iterator method can be consumed by using a foreach statement or LINQ query. Each iteration of the foreach loop calls the iterator method. When a yield return statement is reached in the iterator method, expression is returned, and the current location in code is retained. Execution is restarted from that location the next time that the iterator function is called.

You can use a yield break statement to end the iteration.

For more information about iterators, see Iterators.

Iterator methods and get accessors

The declaration of an iterator must meet the following requirements:

  • The return type must be IEnumerable, IEnumerable<T>, IEnumerator, or IEnumerator<T>.

  • The declaration can't have any inref or out parameters.

The yield type of an iterator that returns IEnumerable or IEnumerator is object. If the iterator returns IEnumerable<T> or IEnumerator<T>, there must be an implicit conversion from the type of the expression in the yield return statement to the generic type parameter .

You can't include a yield return or yield break statement in:

  • Lambda expressions and anonymous methods.

  • Methods that contain unsafe blocks. For more information, see unsafe.

Exception handling

A yield return statement can't be located in a try-catch block. A yield return statement can be located in the try block of a try-finally statement.

A yield break statement can be located in a try block or a catch block but not a finally block.

If the foreach body (outside of the iterator method) throws an exception, a finally block in the iterator method is executed.

Technical implementation

The following code returns an IEnumerable<string> from an iterator method and then iterates through its elements.

The call to MyIteratorMethod doesn't execute the body of the method. Instead the call returns an IEnumerable<string> into the elements variable.

On an iteration of the foreach loop, the MoveNext method is called for elements. This call executes the body of MyIteratorMethod until the next yield return statement is reached. The expression returned by the yield return statement determines not only the value of the element variable for consumption by the loop body but also the Current property of elements, which is an IEnumerable<string>.

On each subsequent iteration of the foreach loop, the execution of the iterator body continues from where it left off, again stopping when it reaches a yield return statement. The foreach loop completes when the end of the iterator method or a yield break statement is reached.

Example

The following example has a yield return statement that's inside a for loop. Each iteration of the foreach statement body in the Main method creates a call to the Power iterator function. Each call to the iterator function proceeds to the next execution of the yield return statement, which occurs during the next iteration of the for loop.

The return type of the iterator method is IEnumerable, which is an iterator interface type. When the iterator method is called, it returns an enumerable object that contains the powers of a number.

Example

The following example demonstrates a get accessor that is an iterator. In the example, each yield return statement returns an instance of a user-defined class.

C# language specification

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

See also

The JavaScript for..of statement iterates over the values of iterable objects such as Arrays, Strings, Maps, Sets, NodeLists, and more. It was introduced in ES6 to provide a clean and concise iteration mechanism.

Syntax

The for..of statement has the following syntax:

For each iteration, the value of the property is assigned to the value variable. It can be declared with const, let, or var. The iterable is the object which has enumerable properties (strings, arrays, maps, sets, etc.) and can be iterated upon. The code block is executed once for each property.

Examples

Let us look at the following examples that demonstrate how to use for..of statement to loop over different iterable objects.

Iterating over an Array

Key Value Iteration In For Loop With Generator 2

A JavaScript array is a simple data structure that stores multiple values in a single variable. Here is an example that shows how you can iterate over an array using the for..of loop:

Iterating over a String

Strings are also an iterable data type, so you can use for..of on strings too:

Iterating over a Map

A JavaScript Map is a special data structure introduced in ES6 that allows you to create collections of key-value pairs. Both objects and primitive values can be used as a key or value.

When iterating over the map, the for..of statement returns a key-value pair for each iteration in the same order as they were inserted as shown below:

Iterating over a Set

A JavaScript Set is a special type of object introduced in ES6 that allows you to create a collection of unique values. You can store both objects and primitives as values in a set object.

The following example shows how you can use for..of to iterate over a set object:

Iterating over an Arguments Object

An arguments object is just an array-like object accessible inside functions that contains the values of the arguments passed to that function.

Using for..of loop, you can iterate over the arguments object to list all the arguments passed to a JavaScript function:

Iterating over a DOM Collection

The for..of statement can also be used to iterate over a DOM collection like a NodeList. The following example adds a img-fluid class to images that are direct children of an article:

Iterating Generators

A generator is a special kind of function in JavaScript that can be exited and later re-entered.

You can easily iterate over generators using the for..of statement as shown in the following example:

Closing Iterators

You can easily terminate a for..of loop and close the iterator by using break, return, or throw statement:

Key Value Iteration In For Loop With Generator 2

Iterating over an Object Literal

Unfortunately, for..of only works with iterables. An object literal is not iterable. However, you can use the Object.keys() method to get all property names and then iterate over them:

Instead of using for..of statement, you should consider using for..in loop for object literals.

Browser Compatibility

JavaScript for..of statement is currently supported by modern browsers only. If you want to support old browsers like Internet Explorer, you need a polyfill or use the forEach() loop instead.

Key Value Iteration In For Loop With Generator Excel

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