#MonthOfCode - Day 14: loop

My entry for the 14th day of the month of code. The theme for today is: loop.

Today’s code illustrates Scala’s abilities to define new control structures.

Code after the break.

You know the while loop. It executes some code while a condition is true:

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while (condition) {
  body
}

Let’s create a while_growing control structure that executes code as long as the “condition” return growing integers:

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while_growing (integer_function) {
  body
}

To create this new control structure, we define a while_growing function that takes two blocks of code: an integer function and a generic block of code. Then, we execute the block and check if the integers returned by the function are growing.

loop.scalaview raw
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// define the new control structure (a simple function)
def while_growing (f: => Int) (body: => Unit) = {
  var value = f // f is executed
  var oldValue = 0
  do {
    body // body is executed
    oldValue = value
    value = f // f is executed
  } while (value > oldValue)
}

// test it!
var x = 0
while_growing ( -(x-3)*(x-3) ) {
  x = x + 1
  println(x, -(x-3)*(x-3) )
}

Output: 

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% scala loop.scala
(1,-4)
(2,-1)
(3,0)
(4,-1)

As soon as the function decreases, the loop stops.

You can try this code by running scala loop.scala or by copy-pasting it on Simply Scala.