05-high-order-functions.md

Lab: High Order Function

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1. Select the correct statement(s) for high order functions

   1. Function that receives a function as an argument.
   2. Function that contains a function.
   3. Function that returns another function.
   4. Function that calls another function.

   Reveal

   A. C

   These two answers correctly define a high order function.

2. What would be the output of the following program?

   package main
   
   import "fmt"
   
   func addHundred(x int) int {
       return x + 100
   }
   func partialSum(x ...int) func() {
       sum := 0
       for _, value := range x {
           sum += value
       }
       return func() {
           fmt.Println(addHundred(sum))
       }
   }
   func main() {
       partial := partialSum(1, 2, 3, 4, 5)
       partial()
   }

   • 110
   • 15
   • Error
   • 115

   Reveal

   115

   • The function partialSum adds together the values of the integers it receives and stores them in sum.
   • It then returns an anonymous function that prints the value of sum plus 100 (via call to addHundred).
   • Recall that the body of this anonymous function is an inner scope with respect to partialSum meaning it can use the variable sum.
   • The variable partial is assigned the result of the call to partialSum. Its value is the anonymous function returned by partialSum.
   • Finally the anonymous function is executed via the variable partial by applying the call operator () - i.e. partial() and the result is printed.
3. What would be the output of the following program?

   package main
   
   func addHundred(x int) int {
       return x + 100
   }
   func partialSum(x ...int) func() int {
       sum := 0
       for _, value := range x {
           sum += value
       }
       return func() int {
           return addHundred(sum)
       }
   }
   func main() {
       partial := partialSum(1, 2, 3)
       partial()
   }

   • 106
   • no output
   • Error
   • 115

   Reveal

   No output

   Quite simply there is no fmt.Println so nothing will be printed.

   This is a minor variation on the previous question. The caculated result would indeed be 106, but since the fmt.Println has been removed from the returned anonymous function, there is no output.

4. What would be the output of the following program?

   package main
   
   import "fmt"
   
   func addHundred(x int) int {
       return x + 100
   }
   func partialSum(add100 func(x int) int, x ...int) int {
       sum := 0
       for _, value := range x {
           sum += value
       }
       return add100(sum)
   
   }
   func main() {
       partial := partialSum(addHundred, 1, 2, 3)
       fmt.Println(partial)
   }

   • 6
   • 115
   • 106
   • error

   Reveal

   106

   Yet another variation on the theme, however this time...

   • The addHundred function is being passed as the first argument to partialSum, then the list of integers to sum.
   • partialSum sums the integers, then calls the addHundred function and returns its result.
   • main() assigns that value to partial (whose type will now be int), and prints it.
5. What would be the output of the following program?

   package main
   
   import "fmt"
   
   func addHundred(x int) {
       fmt.Println(x + 100)
   }
   func partialSum(add100 func(x int), x ...int) int {
       sum := 0
       for _, value := range x {
           sum += value
       }
       add100(sum)
       return 0
   }
   func main() {
       partial := partialSum(addHundred, 1, 2, 3)
       fmt.Println(partial)
   }

   • Error
   • 106
     0
   • 115
     1
   • 115
     0

   Reveal

   106
   0

   Similar to the previous question, however this time...

   • addHundred does the printing of the result of the sum.
   • partialSum simply returns zero
   • main() assigns that value to partial (whose type will now be int), and prints it (zero).