Scala-计算字符串中的特定字符（按功能样式）

Question

I can count a specific string with the below code

 var count = 0
for (i <- "HelloWorldHHHH") {
if (i == 'H') {
  count = count + 1
}
println(count)

or use builtins like

"HelloWorldHHHH".count(_ == 'H')

can someone help me to write the same functionality using pure functional way

I tried the below, but not sure where i am going wrong

val count1 = "Hello".foldLeft(0)((x,_) match { case k if(k == 'h') => x +1})

the last code has some errors, can someone help me to fix the error or give the correct solution

Answer 1

"HelloWorldHHHH".count(_ == 'H') is already in pure functional style, and it seems the preferred option to me since it is very short and readable.

More options:

folding :

"HelloWorldHHHH".foldLeft(0){case(count, char) => if (char == 'H') count + 1 else count }

summing after a for-comprehension . notice that you can use a guard (if):

val hs = for {
  i <- "HelloWorldHHHH"
  if i == 'H'
} yield 1
hs.sum

collecting :

"HelloWorldHHHH".collect{ case 'H' => 1 }.sum

Answer 2

In case you actually want to do this in production code, I suggest to use the stdlib count method:

"HelloWorldHHHH".count(_ == 'H')

For learning purposes you can roll your own with foldLeft :

"HelloWorldHHHH".foldLeft(0) { case (sum, char) =>
  if (char == 'H') sum + 1 else sum
}

Answer 3

You can drop the if and just insert your character in the case block

"Hello".foldLeft(0) {(acc,c) => c match {
    case 'l' => acc + 1
    case _ => acc
  }
}