Flutter iterate through List with access to index

Question

I tried some other solutions to this like using asMap() and forEach but kept getting different errors. Like it was saying the return type of my ChartBar isn't a 'MapEntry', as defined by anonymous closure, or The expression here has a type of 'void', and therefore cannot be used.

Row(
    mainAxisSize: MainAxisSize.min,
    mainAxisAlignment: MainAxisAlignment.spaceAround,
    crossAxisAlignment: CrossAxisAlignment.end,
    children: myList.map((data) {
      return ChartBar(
          ///etc
    }).toList(),
  )

I want the index as well.

@mirkancal That's the question I was referring to, the solutions gave me the errors I described above. — Hasen
– Hasen, Commented Aug 23, 2019 at 15:45
Can you try myList.asMap().map((MapEntry data) => data.value).toList() ? — mirkancal
– mirkancal, Commented Aug 23, 2019 at 15:47
@mirkancal As described in the question, I get the error ChartBar isn't a 'MapEntry' — Hasen
– Hasen, Commented Aug 23, 2019 at 15:49

jamesdlin · Accepted Answer · 2019-08-23 16:44:44Z

9

mirkancal's suggestion didn't work because Map.map returns another Map (and therefore the enumeration callback you pass it is expected to return a MapEntry).

You instead need to use Map.entries so that you can use Iterable.map instead to construct a List:

Row(
    mainAxisSize: MainAxisSize.min,
    mainAxisAlignment: MainAxisAlignment.spaceAround,
    crossAxisAlignment: CrossAxisAlignment.end,
    children: myList.asMap().entries.map((MapEntry entry) {
      return ChartBar(entry.key, entry.value);
    }),
  )

You alternatively can use Dart's new collection-for construct:

Row(
    mainAxisSize: MainAxisSize.min,
    mainAxisAlignment: MainAxisAlignment.spaceAround,
    crossAxisAlignment: CrossAxisAlignment.end,
    children: [for (MapEntry entry in myList.asMap().entries)
      ChartBar(entry.key, entry.value)
    ],
  )

In the above, entry.key will be the index, and entry.value will be the original value in myList.

edited Aug 23, 2019 at 16:44

answered Aug 23, 2019 at 16:31

jamesdlin

91.2k14 gold badges185 silver badges224 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

Hasen Over a year ago

With this I get the error The type 'Map<int, String>' used in the 'for' loop must implement Iterable. or The type 'Map<int, double>' used in the 'for' loop must implement Iterable. with a different List.

jamesdlin Over a year ago

@Hasen Oops, I forgot to add .entries. I've updated my answer.

Hasen Over a year ago

It's kind of odd code to me, why is it all contained in square brackets?

jamesdlin Over a year ago

@Hasen I've updated my answer with a better explanation.

Hasen Over a year ago

I guess it's the equivalent of the toList() with a map, you're constructing a List inside the square brackets.

|

Rodolfo Franco · Accepted Answer · 2019-08-23 15:51:06Z

-1

For the index you could use indexOf(value) function from the list to get index value. For example:

List list = List.generate(3, (int index) => index * index); // [0, 1, 4]

list.indexOf(4) // returns 2

As for your other issue, I'd need to see more code to see how you populate myList. I would suggest this approach (if it fits) to render your Row children. Use a function to return that list, for example:

buildList() {
 List<Widget> list = List<Widget>();

 chartBars.forEach((chart) => { //chartBars list with info
  // add your ChartBar widget to the list with appropiate info
  list.add(ChartBar(someProperty: chart.property));
 });
 return list;
}

Then on your Row widget just call that function when you build it's children:

Row(
    mainAxisSize: MainAxisSize.min,
    mainAxisAlignment: MainAxisAlignment.spaceAround,
    crossAxisAlignment: CrossAxisAlignment.end,
    children: buildList()      
)

answered Aug 23, 2019 at 15:51

Rodolfo Franco

4413 silver badges11 bronze badges

6 Comments

mirkancal Over a year ago

Splitting widgets to methods is an antipattern though. iirokrankka.com/2018/12/11/…

Hasen Over a year ago

Is there no simpler way to do it? There's no simple tweak to my existing code just to get the index to display?

Rodolfo Franco Over a year ago

@mirkancal thanks for the heads up, I'm relatively new to Flutter and may not be aware of best practices, @Hasen Have you tried using myList.indexOf(data) to show the index?

jamesdlin Over a year ago

Using indexOf() would be rather inefficient, making what should be an O(n) operation instead O(n^2).

Hasen Over a year ago

@mirkancal Yes indexOf() works but like it doesn't seem like the most eloquent way to do it.

|

Collectives™ on Stack Overflow

Flutter iterate through List with access to index

2 Answers 2

8 Comments

6 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

8 Comments

6 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related