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Dart & Flutter: Get the Index of a Specific Element in a List

Last updated on July 27, 2021 A Goodman Loading... Loading... Post a comment

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In Dart, the List class has 4 methods that can help you find the index of a specific element in a list:

indexOf: Returns the first index of the first element in the list that equals to a given element. Returns -1 if nothing found.
indexWhere: Returns the first index in the list that satisfies thegiven conditions. If nothing found, returns -1.
lastIndexOf: Returns the index of the last element in the list that equals to a given element. Returns -1 if nothing found.
lastIndexWhere: Returns the last index in the list that satisfies thegiven conditions. If nothing found, returns -1.

The examples below will help you understand them better.Advertisements

Example 1: Finding the Index of a specific Map in a List of Maps

Lets say we have a list of people with information including id, name, and age. The first task is to find the index of the person whose id equals a given id [because the ids are unique we can have at most 1 result]. The second task is to find the index of the last person whose age is greater than 80.

The code:

// main.dart void main[] { final List _people = [ {"id": "c1", "name": "John Doe", "age": 40}, {"id": "c2", "name": "Kindacode.com", "age": 3}, {"id": "c3", "name": "Pipi", "age": 1}, {"id": "c4", "name": "Jane Doe", "age": 99}, ]; // Find index of the person whose id = c3 final index1 = _people.indexWhere[[element] => element["id"] == "c3"]; if [index1 != -1] { print["Index $index1: ${_people[index1]}"]; } // Find the last index where age > 80 final index2 = _people.lastIndexWhere[[element] => element["age"] > 80]; if [index2 != -1] { print["Index $index2: ${_people[index2]}"]; } }

Output:

Index 2: {id: c3, name: Pipi, age: 1} Index 3: {id: c4, name: Jane Doe, age: 99}

Example 2: Finding the Index of a specific Object in a List of Objects

AdvertisementsLets say we have a list of given books. The first job is to find the index of the book whose id equals a given id. The second job is to find the index of the first book in the list that has the title contains a given string.

The code:

// main.dart class Book { String id; String title; double price; Book[this.id, this.title, this.price]; } void main[] { final List _books = [ Book['b1', 'A Blue Novel', 0.99], Book['b2', 'A Green Novel', 1.99], Book['b3', 'A Cooking Hanbook', 2.99], Book['b4', 'Kindacode.com', 0.00] ]; // Find the index of the book whose id is 'b4' final int index1 = _books.indexWhere[[[book] => book.id == 'b4']]; if [index1 != -1] { print['Index: $index1']; print['Title: ${_books[index1].title}']; } // Find the index of the fist book whose title contains 'Novel' final int index2 = _books.indexWhere[[book] => book.title.contains['Novel']]; if [index2 != -1] { print['Index: $index2']; print['Title: ${_books[index2].title}']; } }

Output:

Index: 3 Title: Kindacode.com Index: 0 Title: A Blue Novel

Wrap Up

Weve walked through a few examples of finding the index of a specific element in a list. If youd like to learn more new and exciting stuff in Dart and Flutter, take a look at the following articles:

Dart: Find List Elements that Satisfy Conditions
Dart regex to validate US/CA phone numbers
Dart: Capitalize the First Letter of Each Word in a String
Dart & Flutter: 2 Ways to Count Words in a String
4 ways to convert Double to Int in Flutter & Dart

Dart & Flutter: Get the Index of a Specific Element in a List

Example 1: Finding the Index of a specific Map in a List of Maps

Example 2: Finding the Index of a specific Object in a List of Objects

Wrap Up

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