在 Kotlin 中递增 hash map 计数

Question

I have the function below. However, when I pass a string to it, I get the following error:

error: operator call corresponds to a dot-qualified call 'charCountMap.get(c).plus(1)' which is not allowed on a nullable receiver 'charCountMap.get(c)'. charCountMap.put(c, charCountMap.get(c) + 1)

private fun characterCount(inputString:String) {
  val charCountMap = HashMap<Char, Int>()
  val strArray = inputString.toCharArray()
  for (c in strArray)
  {
    if (charCountMap.containsKey(c))
    {
      charCountMap.put(c, charCountMap.get(c) + 1)
    }
    else
    {
      charCountMap.put(c, 1)
    }
  }
  
  
}

Answer 1

The Kotlin Standard Library has groupingBy and eachCount for this purpose, you don't need to do any of this manually:

private fun characterCount(inputString:String) {
    val charCountMap : Map<Char, Int> = inputString.groupingBy { it }.eachCount()
}

Note that I put the type on charCountMap for clarity, but it can be left off and inferred.

Answer 2

There is nice compute method in HashMap for this:

private fun characterCount(inputString:String) = hashMapOf<Char, Int>().also { charCountMap ->
    inputString.forEach { charCountMap.compute(it) { _, v -> if (v == null) 1 else v + 1 } }
}

Answer 3

Both the other answers are correct. Todd's answer is right, you don't need to write a function for this. Just use the standard library. And if you are going to write a function that updates maps, Михаил Нафталь's suggestion to use compute() to handle updating existing values is also good.

---

However, if you're just doing this an an exercise, here are three suggestions to fix/improve your algorithm:

1. Instead of get() , use getValue() , which does not return null. It will raise an exception if the element does not exist, but you already checked for that.
2. Use the [] operator instead of put() (no need to, it's just nicer syntax).
3. You don't need to call toCharArray() because Strings are already iterable.

if (charCountMap.containsKey(c))
{
    charCountMap[c] = charCountMap.getValue(c) + 1
}
else
{
    charCountMap[c] = 1
}

---

Rewriting the whole thing using standard formatting:

fun characterCount(inputString: String): Map<Char, Int> {
    val charCountMap = mutableMapOf<Char, Int>()
    for (c in inputString) {
        if (charCountMap.containsKey(c)) {
            charCountMap[c] = charCountMap.getValue(c) + 1
        } else {
            charCountMap[c] = 1
        }
    }
    return charCountMap
}