Swift replace string in array using for-in loop

Question

As a learning exercise, I am trying to use a for-in loop to replace a String in an Array (with a dictionary value) if the String is an existing key in a Dictionary. I already know how to do this using .stringByReplacingOccurrencesOfString, but I'd like to learn what I'm doing wrong here, and how to accomplish this with a for-in loop.

let sillyMonkeyString = "A monkey stole my iPhone"
let dictionary = ["monkey": "🐒", "iPhone":"📱"]
var toArray = sillyMonkeyString.componentsSeparatedByString(" ")
// desired result is "A 🐒 stole my 📱"

Here is what doesn't work:

for var str in toArray {
    if let val = dictionary[str] {
        str = val                  
    }
}
let backToString = toArray.joinWithSeparator(" ")

What does work:

var newArray = [String]()
for var str in toArray {
    if let val = dictionary[str] {
        str = val
    }
    newArray.append(str)
}
let backToString = newArray.joinWithSeparator(" ")

This works because I'm creating a new array and appending to the new array. However, my original Array is mutable, so why is the first solution not correctly assigning str to val in the original Array inside the for-in loop? The only other related question I found here had a great one-line answer that did not explain whether or not I can accomplish this using a for-in loop.

UPDATE: I do not recommend implementing a for-in loop for this particular use case. I asked this question to learn how to do this. If a user would like to replace parts of strings with a dictionary, I highly recommend considering using one of the more efficient and Swifty solutions below (which may not be the accepted solution)

Answer 1

'pure' Swift (no import Foundation) and 'functional' way leads to

let sillyMonkeyString = "A monkey stole my iPhone"
let dictionary = ["monkey": "🐒", "iPhone":"📱"]

let arr = sillyMonkeyString.characters.split(" ").map(String.init)
let result = arr.map { dictionary[$0] ?? $0 }.joinWithSeparator(" ")

print(result)
// A 🐒 stole my 📱

if for in loop is required, than

var resArr:[String] = []
for txt in arr {
    resArr.append(dictionary[txt] ?? txt)
}
let result2 = resArr.joinWithSeparator(" ") // A 🐒 stole my 📱"

if you would like to have mutating solution ...

let sillyMonkeyString = "A monkey stole my iPhone"
let dictionary = ["monkey": "🐒", "iPhone":"📱"]

var arr = sillyMonkeyString.characters.split(" ").map(String.init)

for (idx,value) in arr.enumerate() {
    arr[idx] = dictionary[value] ?? value
}
let result3 = arr.joinWithSeparator(" ") // "A 🐒 stole my 📱"

if you don't like enumerate(), you can use indices instead

let sillyMonkeyString = "A monkey stole my iPhone"
let dictionary = ["monkey": "🐒", "iPhone":"📱"]

var arr = sillyMonkeyString.characters.split(" ").map(String.init)

for i in arr.indices {
    arr[i] = dictionary[arr[i]] ?? arr[i]
}
let result4 = arr.joinWithSeparator(" ") // "A 🐒 stole my 📱"

Answer 2

When declaring something as "var" in a for loop or "if var"(deprecated) statement, the value is copied to that variable, not the reference. So when you change "val", you're changing that local variable and not changing the object that exists in toArray

If you want to change it within the array, this should work (not compiled fyi):

for str in toArray {
    if let val = dictionary[str] {
        toArray[toArray.indexOf(str)] = val
    }
}

Alternately, you can track the index instead with

for i in 0 ..< toArray.count {
    if let val = dictionary[toArray[i]] {
        toArray[i] = val
    }
}

To give an comparable example to the link you sent, you could do something like this:

toArray = toArray.map { dictionary[$0] ?? $0 }

If you're unfamiliar with the map function, I would suggest looking it up as it's one of the best features in swift! :)

Answer 3

str = val will only change the value in your local variable, which is valid only in the scope of the for loop. The value in the array will no be changed. What you want really is:

for i in 0..< toArray.count {
    if let val = dictionary[toArray[i]] {
        toArray[i] = val
    }
}

Answer 4

You can use forEach to iterate through your dictionary key/value pairs and replace them as follow:

var sillyMonkeyString = "A monkey stole my iPhone"
let dictionary = ["monkey": "🐒", "iPhone":"📱"]

dictionary.forEach{ sillyMonkeyString = sillyMonkeyString.stringByReplacingOccurrencesOfString($0.0, withString: $0.1) }
print(sillyMonkeyString)  // "A 🐒 stole my 📱"

Answer 5

How about this approach:

var sillyMonkeyString = "A monkey stole my iPhone"
let dictionary = ["monkey": "🐒", "iPhone":"📱"]
var toArray = sillyMonkeyString.componentsSeparatedByString(" ")

for (index, string) in toArray.enumerate()
{
  if let val = dictionary[string]
  {
    toArray[index] = val
  }
}
let finalString = toArray.joinWithSeparator(" ")

That code uses a variant of for...in that returns the objects in an array and the index as tuples. Some other answers used indexOf. indexOf is both computationally expensive, and potentially risky. Arrays can contain multiple copies of the same object, so the string in question could occur at multiple places. In this particular example, it works because we loop through the array of words front to back, replacing matching words as we go. So, indexOf will always find the first remaining occurrence of the string. Nevertheless, it is still quite slow on large arrays.

Answer 6

This is the solution I came up with. I'll consider using others since many have recommended that using .indexOf is very expensive. Rather than assigning to an ivar, I'm assigning val directly to the item at the current index in the original array. I'm all for learning and implementing what is least costly. However, you got to admit this is clean and short. (though not the most efficient)

for var str in toArray {
    if let val = dictionary[str] {
        toArray[toArray.indexOf(str)!] = val
    }
}

Answer 7

On the topic of "what was wrong" in the code that didn't work :

 for var str in toArray {
     if let val = dictionary[str] {
         str = val                  
     }
 }
 let backToString = toArray.joinWithSeparator(" ")

The str variable in your for loop is not a reference to the elements of your toArray but a copy of it. So modifying str inside the loop has no effect on the array it came from.

Answer 8

As others have said, instead of

for var str in toArray {

if must be

for str in toArray {

But I want to show another approach, using enumerate() on dictionary

var sillyMonkeyString = "A monkey stole my iPhone"
let dictionary = ["monkey": "🐒", "iPhone":"📱"]

for (_ ,v) in dictionary.enumerate() {
    sillyMonkeyString = sillyMonkeyString.stringByReplacingOccurrencesOfString(v.0, withString: v.1)
}

print(sillyMonkeyString)

prints A 🐒 stole my 📱

enumerate() returns a tuple containing of the index and the tuple with the key and value.

and to make tanzolone happy, though this is a rather stupid approach:

let sillyMonkeyString = "A monkey stole my iPhone"
let dictionary = ["monkey": "🐒", "iPhone":"📱"]
var toArray = sillyMonkeyString.componentsSeparatedByString(" ")

for (i,v) in dictionary.enumerate() {
    for (j,s) in toArray.enumerate() {
        if s == v.0 {
            toArray[j] = v.1
        }
    }
}

print(toArray.joinWithSeparator(" "))

---

enumeration with for in

var sillyMonkeyString = "A monkey stole my iPhone"
let dictionary = ["monkey": "🐒", "iPhone":"📱"]

for (k,v) in dictionary{
    sillyMonkeyString = sillyMonkeyString.stringByReplacingOccurrencesOfString(k, withString: v)
}

print(sillyMonkeyString)