Place-Holder

          When I was doing Exercise 9.2.2, which requires developing a function named repeat that takes in a string and returns that string appended to itself, I wrote like that:

 

(check-expect  repeat “hello” (string-append “hello””hello”))

 

(define  repeat (string-append “hello””hello”))

 

Then, I found that this new defined function could only be applied to the string “hello”, which was different than what I expected. So, I went to office hour, and asked professor this question. Then I know that I lacked a place-holder, which has a technical name- parameter. So the correct expressions should be:

 

(check-expect(repeat “hello”)(string-append “hello””hello”)

 

(define (repeat string)(string-append string string))

 

   

Then, I tried a different string in Interactions Plane:

 

>(repeat “lol”)

“lollol”

 

          The result shows that the new defined function- repeat- now works for any string.