String-append does not work; overwrites previous lines

Question

I am trying to accomplish the simple task of adding a number to a string several times and hope that this will lead to a string.

But it does not work, every call (string-append ....) seems to do nothing.

Here is my code:

 (define myString "") (string-append (number->string 4) myString) (string-append (number->string 5) myString) (string-append (number->string 6) myString) (string-append (number->string 7) myString) (display myString) 

the following is displayed:

"4"

"5"

"6"

"7"

and an empty string for (display myString)

What am I doing wrong?

also if I do it in a different way, so that it doesn't work either:

 (define myString "") (string-append myString (number->string 4)) (string-append myString (number->string 5)) (string-append myString (number->string 6)) (string-append myString (number->string 7)) (display myString) 

Thanks for any help

Answer 1

string-append does not modify an existing string; it returns a new line that is the result of the operation.

If this seems unusual, consider the addition operator + . When I write

3+4

I do not change 3 to 7 , just returning the result of the operation.

So, in your case, if you write

 (define my-string (string-append (number->string 4) (number->string 5))) (display my-string) 

You should see

 45 

To advise you, this can help find out where you are going.

Answer 2

The expression (string-append (number->string 4) myString) evaluates to "4" , so your code prints 4 . Similarly, (string-append (number->string 5) myString) evaluates to "5" and so on. None of them actually change the value of the myString variable.

To really change the value of a variable, you need to assign this variable using the Scheme language form to assign, set! . ( ! in the name, this is a schema convention saying that this is a destructive operation, that is, one that destroys the previous value in myString and replaces it with a new one). For example, you could write:

 (define myString "") (set! myString (string-append myString (number->string 4))) ;; assigns "4" to myString (set! myString (string-append myString (number->string 5))) ;; assigns "45" to myString (set! myString (string-append myString (number->string 6))) ;; assigns "456" to myString (set! myString (string-append myString (number->string 7))) ;; assigns "4567" to myString (display myString) ;; prints 4567 

But instead, you can simply create the desired expression, instead of performing all these destructive operations:

 (define myString (string-append (number->string 4) (string-append (number->string 5) (string-append (number->string 6) (string-append (number->string 7) ""))))) (display myString) ;; prints 4567 

And at this point, instead of writing code that repeats, I would do fold-right :

 (define myString (fold-right (lambda (num str) (string-append (number->string num) str)) "" (list 4 5 6 7))) (display myString) ;; prints 4567 

If you need to assign "4567" global variable called myString , you can simply replace define with set! in any of the two code snippets below. However, although from time to time set! may come in handy, an idiomatic scheme tends to use set! and other destructive operations sparingly. I like to consider the exclamation mark as "Proceed with caution!"