Question 257673 in Flutter Question posted in
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How to use shuffled List in for loop in flutter?

I want to clone an existing list, shuffle it and use it in a loop in a children. But when I use it I got an error.

If I use it like this:

const availablePlaycards = [
  Playcard(front: 'back.png', back: 'sheep.png'),
  Playcard(front: 'back.png', back: 'cow.png'),
  Playcard(front: 'back.png', back: 'pig.png'),
];
var doubledCards = (List.from(availablePlaycards)..addAll(clonedCards)).shuffle();

I got here

children: [
          for (final playcard in doubledCards)

the error:

A nullable expression can't be used as an iterator in a for-in loop.
and
This expression has a type of 'void' so its value can't be used.

I have added an exclamation mark:

for (final playcard in doubledCards!)

But the error persists.

I have also tries like this without shuffle().

final List doubledCards = (List.from(availablePlaycards)..addAll(clonedCards));

Then I do not get the error, but the cards are not shuffled.

If I add shuffle here:

for (final playcard in doubledCards.shuffle())

I got the error:

A nullable expression can't be used as an iterator in a for-in loop.

How can I use it with shuffle?

Tags:

2

Answers


  1. 0

    You are trying to assign the return value of the shuffle method, which is a void. You should use the cascade notation instead.

    var doubledCards = List.from(availablePlaycards)..addAll(clonedCards)..shuffle();

    Now doubledCards has a type of List so its value can be used as an iterator in a for-in loop.

    This code is equivalent to:

    var doubledCards = List.from(availablePlaycards);
doubledCards.addAll(clonedCards);
doubledCards.shuffle();
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  2. 0

    The issue is that shuffle() method doesn’t return a new list but shuffles the existing list in-place and returns void. Therefore, you are trying to iterate over void, which is causing the error.

    To shuffle a copy of the list without modifying the original list, you should shuffle the copy after creating it. You can do something like this:

    final List<Playcard> doubledCards = List.from(availablePlaycards)..addAll(clonedCards);
doubledCards.shuffle();

// Now you can use doubledCards in your loop
for (final playcard in doubledCards) {
  // Your code here
}

    By doing this, you create a new list, add elements to it, and then shuffle the new list. This ensures that your original availablePlaycards list remains unchanged.

    Remember that you need to use List<Playcard> instead of var or final if you want to correctly specify the type of the list elements. This can prevent potential issues and make your code more readable.

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