the "Error: reassignment to val"be thrown in function

Question

when I run the following codes, the "error: reassignment to val" be thrown, anyone knows why?

def main(args: Array[String]) :Unit = {
    var a = 10
    def Fun(a:Int):Unit = {
      while( a < 20 ){
       println( "Value of a: " + a );
       a = a + 1 // Error: reassignment to val 
      }
    }
    Fun(a)
 }

If I want to implement the function of variable a plus one in the function Fun, how to modify it？

Answer 1

You're hiding the a outer variable with the method parameter a which is a val "automatically".

You wrote the same as:

    var a = 10
    def Fun(b:Int):Unit = {
      while( b < 20 ){
       println( "Value of b: " + b );
       b = b + 1 // Error: reassignment to val 
      }
    }
    Fun(a)

I guess you want to remove the method parameter as you want to increment the outer variable which is accessible as you're defining an inner method:

    var a = 10
    def Fun():Unit = {
      while( a < 20 ){
       println( "Value of a: " + a );
       a = a + 1
      }
    }
    Fun()

Keep in mind this is not idiomatic Scala code and it's not clear what you're trying to achieve so I cannot provide a better solution for you it you definitely don't want to write such code in real projects.