Flutter how to search an object in complex json - json

Question

My application is a simple calculator but I am a little lost in flutter...
I have a text field entered by the user which is a distance.
Depending on this distance, I need to look in my local values ​​of the application stored in a json for the closest value and retrieve the index.
Here is my json:
{
"charges" : [
{
"type" : 1,
"distance": 800,
"setting": 1122,
"bond" : 44,
"index" : 1
},
{
"type" : 1,
"distance": 850,
"setting": 1076,
"bond" : 50,
"index" : 2
},
{
"type" : 2,
"distance": 800,
"setting": 1336,
"bond" : 37,
"index" : 3
},
{
"type" : 2,
"distance": 900,
"setting": 1299,
"bond" : 39,
"index" : 4
}
]
}
If the user enters 836m for example, I have to compare the different ammunition (type 1 and 2) and look at the distance closest to the bottom.
Here the closest distance to the bottom is 800 for ammunition 1 and 2.
{
"type" : 1,
"distance": 800,
"setting": 1122,
"bond" : 44,
"index" : 1
},
{
"type" : 2,
"distance": 800,
"setting": 1336,
"bond" : 37,
"index" : 3
}
But I have to find only one charge at the end which corresponds to my need.
For that I have to look at the setting which must be closest to 1100 at the superior.
The setting of the charge 1 is the closest so I need to keep the index 1.
How to do that ? Is it the good method to use json file ? The structure of my json file is good fot do that ?
This is my Homepage.dart
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'dart:convert';
import 'dart:async';
class HomeController extends StatefulWidget {
HomeController({Key key, this.title}) : super(key: key);
final String title;
#override
_HomeControllerState createState() => _HomeControllerState();
}
class _HomeControllerState extends State<HomeController> {
String _distance;
final _formKey = GlobalKey<FormState>();
// Widget ************************************
Widget _buildDistance() {
return TextFormField(
decoration: InputDecoration(
border: OutlineInputBorder(),
labelText: 'Distance',
hintText: "Enter a distance",
),
keyboardType: TextInputType.number,
validator: (String value){
int distance = int.tryParse(value);
if(distance == null){
return "Distance is needed";
}
if(distance <= 0){
return "Distance must be greater than zero";
}
},
onSaved: (String value){
_distance = value;
},
);
}
// End Widget ************************************
#override
void initState() {
super.initState();
}
#override
Widget build(BuildContext context) {
return GestureDetector(
onTap: (() => FocusScope.of(context).requestFocus(FocusNode())),
child: Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: FutureBuilder(
future : DefaultAssetBundle.of(context).loadString('assets/jsons/charge.json'),
builder : (context, snapshot){
var myjson = json.decode(snapshot.data.toString());
return Container(
margin: EdgeInsets.all(24),
child: Form(
key: _formKey,
child: Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: <Widget>[
_buildDistance(),
SizedBox(height: 100),
RaisedButton(
child: Text(
'Calculer',
style: TextStyle(color:Colors.blue, fontSize : 16),
),
onPressed: (){
if(!_formKey.currentState.validate()){
return;
}
_formKey.currentState.save();
print(_distance);
Map<String, dynamic> data = {
"distance" : _distance,
};
print('result : ${data['distance']}');
print(myjson);
},
),
],
),
),
);
},
),
),
);
}
}

Answer 1

You need to parse the JSON. I would personally parse it whenever the application starts and keep it in memory (unless this JSON is really long!!). For instance, you can store the information in an array in which each element is an object of class Charge that is a simple POJO class that stores the information needed for a charge (type, distance...).
Then when you're searching for the selected Charge, you can simply traverse the array and keep track of which charge has the closest distance to the user-specified
var minDistance = double.maxFinite;
var correctCharge = null;
for(var charge in charges){
double curDistance = abs(charge.distance - userSpecifiedDIstance);
if(curDistance < minDistance){
minDistance = curDistance;
correctCharge = charge;
}
}
If you want to optimise this so you don't traverse the whole array every time, you can use search methods like binary search (if you don't have a lot of charges, you don't need to optimize.)