Remove a key from a dictionary for a given value - ios

Question

How can I remove a key from a dictionary where the value is X? I need a dictionary.removeKeyForValue(...) function.
I'd like to optimise the following code. I have a text which is associated with a certain category and a dictionary that associates all keywords to categories. Although my text is already categorised I'd like check whether it should fall into a different category.
let text = "he said hello and then ran away" // This is taken from the "activity" category
// Dictionary associating keywords to categories
let categoryRules = ["hi" : "greeting", "hello" : "greeting", "jogging" : "activity", "joy" : "feeling"]
let keywords = Array(categoryRules.keys)
// Make out of text an Array of words.
let textWordArray = text.lowercaseString.characters.split{$0 == " "}.map(String.init)
// I SHOULDN'T HAVE TO GO THROUGH THE KEYS ASSOCIATED WITH "ACTIVITY" BECAUSE TEXT IS ALREADY IN IT.
for keyword in keywords {
// If text contains the rule
if let index = textWordArray.indexOf(keyword) {
// Get the associated category
if let category = categoryRules[keyword] {
print("The text should fall into the category of \(category)")
break
}
}
}

Answer 1

To remove all keys from a dictionary with a certain value, you can use a for loop with where clause to select the keys to remove and then assign nil to remove them:
var categoryRules = ["hi" : "greeting", "hello" : "greeting", "jogging" : "activity", "joy" : "feeling"]
for (key, value) in categoryRules where value == "greeting" {
categoryRules[key] = nil
}
print(categoryRules) // ["jogging": "activity", "joy": "feeling"]
You can add removeKeysForValue by adding an extension to Dictionary that works for values that are Equatable (can be compared with ==):
extension Dictionary where Value: Equatable {
mutating func removeKeysForValue(value: Value) {
for (key, val) in self where val == value {
self[key] = nil
}
}
}
var categoryRules = ["hi" : "greeting", "hello" : "greeting", "jogging" : "activity", "joy" : "feeling"]
categoryRules.removeKeysForValue("greeting")
print(categoryRules) // ["jogging": "activity", "joy": "feeling"]

Answer 2

Here's an alternate solution that I find elegant which uses filters.
var categoryRules = ["hi" : "greeting", "hello" : "greeting", "jogging" : "activity", "joy" : "feeling"]
let keysToRemove = dict.keys.filter { dict[$0]! == "greeting" }
for key in keysToRemove {
dict.removeValueForKey(key)
}
// categoryRules = ["jogging": "activity", "joy": "feeling"]
keysToRemove will have "hi" and "hello" because they matched the given filter of having the value "greeting".
Edit:
OP mentioned that he wanted a dictionary.removeKeyForValue() function. You can create an extension with the above code to increase readability and avoid code duplication if you plan on doing this action often.
Example:
extension Dictionary {
mutating func removeKeysForValue(value: NSObject) {
let keysToRemove = self.keys.filter { self[$0]! as! NSObject == value }
for key in keysToRemove {
self.removeValueForKey(key)
}
}
}
var dict = ["hi" : "greeting", "hello" : "greeting", "jogging" : "activity", "joy" : "feeling"]
dict.removeKeysForValue("greeting") // dict = ["jogging": "activity", "joy": "feeling"]

Answer 3

Since it will take O(n) time to remove keys from the dictionary (where n is the size of the dictionary) it might be a better idea to create a list of dictionaries (instead of just one.) Each dictionary would exclude the necessary elements. So when looking up values out of the dictionary you just have to choose the correct dictionary.
This will have better performance then removing keys from the dictionary at runtime. Especially when doing lookups multiple times in loops.