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Flutter – Why is indexOf returning zero if used with json.decode

In flutter, I have this code that returns zero for all item2:

for (var item2 in json.decode(item.details?["result"]))
   Text(
     ((json.decode(item.details?["result"])).indexOf(item2) + 1).toString()
   ),

My question is why does the above code output 0, while the below code shows the correct index:

for (var item in values)
   Text(
     (values.indexOf(item) + 1).toString()
   ),

UPDATE: My actual code is below, so I cannot really declare a new variable inside the for loop…or can I?

Widget build(BuildContext context) {
   return Scaffold(
      body: Container(
         ...
         for (var item in values)
            Text(
               (values.indexOf(item) + 1).toString()
            ),
            for (var item2 in json.decode(item.details?["result"]))
               Text(
                  ((json.decode(item.details?["result"])).indexOf(item2) + 1).toString()
               ),
      ),
   );
}
Tags:

2

Answers


  1. 0

    The issue arises because json.decode generates new objects each time it’s called, and indexOf in Dart compares objects by reference for complex types like lists, maps, or other objects

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  2. 0

    You can declare a new variable to hold the decoded result outside the loop. This avoids repeatedly decoding the same JSON string and ensures consistent behavior. Here’s how:

    Widget build(BuildContext context) {
  return Scaffold(
    body: Container(
      child: Column(
        children: [
          for (var item in values)
            Text(
              (values.indexOf(item) + 1).toString(),
            ),
          for (var item in values)
            ...(() {
              // Decode once and reuse the result
              final decodedResult = json.decode(item.details?["result"]) as List;
              return decodedResult.map((item2) {
                return Text(
                  (decodedResult.indexOf(item2) + 1).toString(),
                );
              });
            })(),
        ],
      ),
    ),
  );
}
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