The set function takes two parameters: an index of the array and a value to be assigned at that index:

intArray.set(2,3)
intArray[3] = 4
intArray.set(4,5)

Take a look at the following example:

 fun declareAndInitArray(){
 
     var intArray = IntArray(5)
     intArray[0] = 10
     intArray[1] = 20
     intArray.set(2,30)
     intArray.set(3,40)
      for (element in intArray){
         println(element)
     }
 }

The set function takes the 2 index with a value of 30 and the 3 index with a value of 40 to initialize the array.