How to find sum of values of objects in array in dart? - arrays

Question

I have an array that is like following:
[{name: Cibo, quantity: 3},
{name: Cibo, quantity: 2},
{name: Invia Regalo, quantity: 3},
{name: Invia Regalo, quantity: 1}]
How can I find the total quantity for these objects by combining the same names?
So output would look like:
[
{name: Invia Regalo, quantity: 4},
{name: Cibo, quantity: 5},
]
I have tried something like this, but does not work in Dart.
var categoryTotal = [];
category.forEach((category) {
print(category);
var categ =
categoryTotal.firstWhere((cat) => cat['name'] == category['name']);
print("Category is $categ");
if (categ != null) {
categ['quantity'] += category['quantity'];
} else {
categoryTotal.add(category);
}
});
print(categoryTotal);

Answer 1

I will in general not recommend the use of maps maps to handle objects that contains different types of values. Instead, use classes. It makes it a lot easier to write type safe code.
Also, I will not recommend using the .forEach() method when you have multiple lines of code since it can make it difficult to read. Instead, use a for-each loop.
I have made the following solution to handle your list of maps. The sumMap is for making the code more efficient by using a Map to handle the sums of quantity. The result are then created based on sumMap:
void main() {
final array = [
{'name': 'Cibo', 'quantity': 3},
{'name': 'Cibo', 'quantity': 2},
{'name': 'Invia Regalo', 'quantity': 3},
{'name': 'Invia Regalo', 'quantity': 1},
];
final sumMap = <String, int>{};
for (final map in array) {
sumMap.update(
map['name']! as String,
(sum) => sum + (map['quantity']! as int),
ifAbsent: () => map['quantity']! as int,
);
}
final result = [
for (final entry in sumMap.entries)
{'name': entry.key, 'quantity': entry.value}
];
print(result);
// [{name: Cibo, quantity: 5}, {name: Invia Regalo, quantity: 4}]
}

Answer 2

You need to add orElse: () => null to avoid exeption. So, final code would look like this:
var categoryTotal = [];
category.forEach((category) {
var categ = categoryTotal.firstWhere(
(cat) => cat['name'] == category['name'],
orElse: () => null,
);
if (categ != null) {
categ['quantity'] += category['quantity'];
} else {
categoryTotal.add(category);
}
});
print(categoryTotal);