How can I access a char in string in at specific number?

The equivalent of Javas String.charAt() in Kotlin is String.get(). Since this is implemented as an operator, you can use [index] instead of get(index). For example

val firstChar: Char = "foo"[0]

or if you prefer

val someString: String = "bar"
val firstChar: Char = someString.get(0)

The beauty of Kotlin is that you can do it in few ways, eg.

1. You can simply access it by index:

   while (x[i] != '+') {
       i++
   }
   

2. Converting to CharArray

   val chars: CharArray = x.toCharArray()
   
   while (chars[i] != '+') {
       i++
   }
   

3. You can also use idiomatic Kotlin (preferred):

   • forEach

     x.forEach { c ->
         if (c == '+') return@forEach
     }
     

   • forEachIndexed if you care about index

     x.forEachIndexed { index, c ->
         if (c == '+') {
             println("index=$index")
             return@forEachIndexed
         }
     }
     

In both cases, your character is accessed with c