Dart why my code not work with negative value - dart

Question

I try a small code but I have a strange behavior that I can't explain.
I want according to a value to return the "keyvalue" of a map which is based on the key.
My code works with positive value.
If the value is not in the array then it returns null.
It also works with negative values ​​only if the value is included in my array.
If I put a negative value lower than my array then it returns not null but zero which is false!
Keys in my map must be String.
My code that you can test on dartPad :
import 'dart:collection';
void main() {
int myVar = -360;
Map<String, dynamic> values = {
"-200" : 42,
"-100" : 21,
"0" : 0,
"100" : -22,
"150" : -30,
"200" : -43,
"300" : -64
};
Map<String, dynamic> filter(int myVar, Map<String, dynamic> values) {
SplayTreeMap<String, dynamic> newval = SplayTreeMap.of(values);
String convertString = myVar.toString();
if (values.containsKey(convertString)) {
return {convertString: values[convertString]};
}
String lowerKey;
String upperKey;
if(myVar > 0){
lowerKey = newval.lastKeyBefore(convertString);
upperKey = newval.firstKeyAfter(convertString);
}
else{
lowerKey = newval.firstKeyAfter(convertString);
upperKey = newval.lastKeyBefore(convertString);
}
print(lowerKey);
print(upperKey);
return {
if (lowerKey != null) lowerKey: values[lowerKey],
if (upperKey != null) upperKey: values[upperKey],
};
}
var result = filter(myVar, values);
print('============================');
print(result);
}

Answer 1

First I want to give a minor complain about the use of dynamic in the code. It is totally fine to use dynamic in cases where the type cannot be determined on runtime like JSON parsing. But in this case, all the types can be determined and the use of dynamic is not necessary. So I have fixed the code to remove the usage of dynamic and also removed unnecessary typing:
import 'dart:collection';
void main() {
const myVar = -360;
final values = {
"-200": 42,
"-100": 21,
"0": 0,
"100": -22,
"150": -30,
"200": -43,
"300": -64
};
Map<String, int> filter(int myVar, Map<String, int> values) {
final newVal = SplayTreeMap.of(values);
final convertString = myVar.toString();
if (values.containsKey(convertString)) {
return {convertString: values[convertString]};
}
String lowerKey;
String upperKey;
if (myVar > 0) {
lowerKey = newVal.lastKeyBefore(convertString);
upperKey = newVal.firstKeyAfter(convertString);
} else {
lowerKey = newVal.firstKeyAfter(convertString);
upperKey = newVal.lastKeyBefore(convertString);
}
print(lowerKey);
print(upperKey);
return {
if (lowerKey != null) lowerKey: values[lowerKey],
if (upperKey != null) upperKey: values[upperKey],
};
}
final result = filter(myVar, values);
print('============================');
print(result);
}
Your problem is that you are using SplayTreeMap to sort your keys in values but you have used Strings to represent your numbers. This is rather confusing since numbers is valid keys. But this also means that your sorting in your SplayTreeMap is alphabetical and not by number. This is properly the reason why your code does not work as expected.
You can either change the type of your keys to int or provide a compare method to your SplayTreeMap which changes how the sorting are done.
I have made the following example where I have changed the type of keys into int which makes your code work:
import 'dart:collection';
void main() {
const myVar = -360;
final values = {
-200: 42,
-100: 21,
0: 0,
100: -22,
150: -30,
200: -43,
300: -64
};
Map<int, int> filter(int myVar, Map<int, int> values) {
final newVal = SplayTreeMap.of(values);
if (values.containsKey(myVar)) {
return {myVar: values[myVar]};
}
int lowerKey;
int upperKey;
if (myVar > 0) {
lowerKey = newVal.lastKeyBefore(myVar);
upperKey = newVal.firstKeyAfter(myVar);
} else {
lowerKey = newVal.firstKeyAfter(myVar);
upperKey = newVal.lastKeyBefore(myVar);
}
print(lowerKey);
print(upperKey);
return {
if (lowerKey != null) lowerKey: values[lowerKey],
if (upperKey != null) upperKey: values[upperKey],
};
}
final result = filter(myVar, values);
print('============================');
print(result);
}
Output
-200
null
============================
{-200: 42}