Sort a list of objects in Flutter (Dart) by property value

Question

How to sort a list of objects by the alphabetical order of one of its properties (Not the name but the actual value the property holds)?

Solution 1

You can pass a comparison function to List.sort.

someObjects.sort((a, b) => a.someProperty.compareTo(b.someProperty));

Solution 2

If you want to sort the object "objects" by the property "name" do something like this

objects.sort((a, b) {
  return a.value['name'].toString().toLowerCase().compareTo(b.value['name'].toString().toLowerCase());
});    

Solution 3

In general, you can provide a custom comparison function to List.sort.

/// Desired relation | Result
/// -------------------------------------------
///           a < b  | Returns a negative value.
///           a == b | Returns 0.
///           a > b  | Returns a positive value.
///
int mySortComparison(SomeClass a, SomeClass b) {
  final propertyA = someProperty(a);
  final propertyB = someProperty(b);
  if (propertyA < propertyB) {
    return -1;
  } else if (propertyA > propertyB) {
    return 1;
  } else {
    return 0;
  }
}

list.sort(mySortComparison);

If you're sorting some custom class you own, you alternatively could make your class implement the Comparable interface:

class MyCustomClass implements Comparable<MyCustomClass> {
  ...

  @override
  int compareTo(MyCustomClass other) {
    if (someProperty < other.someProperty) {
      return -1;
    } else if (someProperty > other.someProperty) {
      return 1;
    } else {
      return 0;
    }
  }
}

and then you can use list.sort() directly without supplying a callback.

Note that if you're sorting by a single property that already implements the Comparable interface, implementing the comparison functions is much simpler. For example:

class MyCustomClass implements Comparable<MyCustomClass> {
  ...

  @override
  int compareTo(MyCustomClass other) =>
    someProperty.compareTo(other.someProperty);
}

---

Reversing

If you want to reverse the sort order, you can make your comparison function return a value with the opposite sign. Alternatively just explicitly reverse the list after sorting:

list = (list..sort()).reversed.toList();

---

Sorting by multiple properties (a.k.a. subsorting)

If you want to sort by multiple properties, a general way is to perform a stable sort for each property in reverse order of importance. For example, if you want to sort names primarily by surname and then subsort within surnames by given name, then you would first sort by given names, and then perform a stable sort by surname. See below for how to perform a stable sort.

Alternatively, you could sort with a comparison function that accounts for multiple properties. For example:

class Name {
  Name({String surname, String givenName})
    : surname = surname ?? "",
      givenName = givenName ?? "";

  final String surname;
  final String givenName;
}

int compareNames(Name name1, Name name2) {
  var comparisonResult = name1.surname.compareTo(name2.surname);
  if (comparisonResult != 0) {
     return comparisonResult;
  }
  // Surnames are the same, so subsort by given name.
  return name1.givenName.compareTo(name2.givenName);
}

---

Okay, I want a stable sort

List.sort is not guaranteed to be a stable sort. If you need a stable sort, package:collection provides insertionSort and mergeSort implementations that are stable.

---

But comparing might be expensive

Suppose you have a custom comparison function that looks something like:

int compareMyCustomClass(MyCustomClass a, MyCustomClass b) {
  var a0 = computeValue(a);
  var b0 = computeValue(b);
  return a0.compareTo(b0);
}

Sorting might call computeValue() on each element multiple times, which is particularly wasteful if computeValue() is expensive. In such cases, a Schwartzian transform could be faster (at the expense of using more memory). This approach maps your objects to directly sortable keys, sorts the keys, and extracts the original objects. (This is how Python's sort and sorted functions work.)

Here's one possible implementation:

class _SortableKeyPair<T, K extends Comparable<Object>>
    implements Comparable<_SortableKeyPair<T, K>> {
  _SortableKeyPair(this.original, this.key);

  final T original;
  final K key;

  @override
  int compareTo(_SortableKeyPair<T, K> other) => key.compareTo(other.key);
}

/// Returns a sorted *copy* of [items] according to the computed sort key.
List<E> sortedWithKey<E, K extends Comparable<Object>>(
  Iterable<E> items,
  K Function(E) toKey,
) {
  final keyPairs = [
    for (var element in items) _SortableKeyPair(element, toKey(element)),
  ]..sort();

  return [
    for (var keyPair in keyPairs) keyPair.original,
  ];
}

void main() {
  final list = <MyCustomClass>[ ... ];
  final sorted = sortedWithKeys(list, computeValue);
}

Solution 4

Here is my contribution to this good question. If someone is facing difficulty to understand how the @Nate Bosch answer is working & you want to sort your custom model class list then you can do this way.

1. You have to implement Comparable abstract class in your model class. It has the method compareTo which you have to override. For example, I have this StudentMarks model class which has marks property in it.

class StudentMarks implements Comparable {
  int marks;

  StudentMarks({
    this.marks,
  });


  @override
  int compareTo(other) {

    if (this.marks == null || other == null) {
      return null;
    }

    if (this.marks < other.marks) {
      return 1;
    }

    if (this.marks > other.marks) {
      return -1;
    }

    if (this.marks == other.marks) {
      return 0;
    }

    return null;
  }
}

2. Now you can call compareTo method inside the sort method.

void _sortStudents({bool reversed: false}) {
    _students.sort((a, b) {
      return a.compareTo(b);
    });

    if (reversed) {
      _students = _students.reversed.toList();
    }

    setState(() {});
  }

Refer to this link you want to know more about the Comparable class

https://api.dart.dev/stable/2.1.0/dart-core/Comparable-class.html

Solution 5

Immutable extension sortedBy for List.

extension MyIterable<E> on Iterable<E> {
  Iterable<E> sortedBy(Comparable key(E e)) =>
      toList()..sort((a, b) => key(a).compareTo(key(b)));
}

And use

list.sortedBy((it) => it.name);

View more solutions

Comments

[Richard Heap]

Possible duplicate of How can I sort a list of strings in Dart?

[Nomnom]

@RichardHeap yes the solution does answer my question but the question is asked differently I guess. Thanks for your help!

[griffins]

i wrote an answer here stackoverflow.com/a/59350472/10409567

[Mahesh Jamdade]

how do you reverse sort it

[Nate Bosch]

To reverse it you could do b.someProperty.compareTo(a.someProperty). Or sort it and then use .reversed.

[Apps 247]

To reverse, just use (b,a) instead of (a,b). It's worth notinng that compareTo returns 1, 0 or -1 if you want to do more advanced sorts.

[jasonflaherty]

This is simple and easy to use for classes.

[jamesdlin]

If you're sorting a list of objects that properly implements Comparable, you shouldn't need to explicitly call List.sort with a callback; it should be automatic.

[Pavel Shorokhov]

Pay attention for dartx library. It is very popular and has a lot of useful extensions, mostly designed like in Kotlin.

[Randal Schwartz]

I was just thinking when I got to "but comparisons might be expensive" something like "I wonder how the author will approach this?" And there it was again. :)

[Oprimus]

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review