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Asked: 2026-06-16T02:31:09+00:00

I’m trying to compare two booleans : (if (equal? #f (string->number 123b)) not a

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I’m trying to compare two booleans : 

(if (equal? #f (string->number "123b"))
      "not a number"
      "indeed a number")

When I run this in the command line of DrRacket I get "not a number" , however , when I
put that piece of code in my larger code , the function doesn’t return that string ("not a number") , here’s the code : 

(define (testing x y z)

    (define badInput "ERROR")  

    (if (equal? #f (string->number "123b"))
          "not a number"
          "indeed a number")  

    (display x))

And from command line : (testing "123" 1 2)
displays : 123

Why ?

Furthermore , how can I return a value , whenever I choose ?

Thanks

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1 Answer

  1. Editorial Team

    Try this:

    (define (testing x)
  (if (equal? #f (string->number "123b"))
      (display "not a number")
      (display "indeed a number"))
  (display x))

    You were discarding the result of the if expression. The condition was working fine, but nothing was done with the resulting string, it just went ignored. The whole procedure now is returning #<void>, because display is a side-effecting operation with no value of its own. Also, I removed the badInput variable and the y and z parameters because they were not being used at all.

    To return a value from a procedure, simply put the expression with the value you want to return at the end of the procedure’s body – this explains why your code wasn’t working, only the last expression returns a value, although you can do side-effecting operations (for example, calling display) with any expression before and including the last one.

    As a matter of fact, your code can be written in a more idiomatic by returning a value, noticing that the condition in the if now actually depends on the x parameter being passed:

    (define (testing x)
  (if (not (string->number x))
      (string-append "not a number " x)
      (string-append "indeed a number " x)))

(displayln (testing "123"))
=> indeed a number 123

(displayln (testing "123b"))
=> not a number 123b

    EDIT:

    Regarding the last edit to your question, I believe you’re looking for something like this:

    (define (convert originalNumber s_oldBase s_newBase)
  (if (or (not (string->number originalNumber)) ; validate error conditions first
          (not (string->number s_oldBase))
          (not (string->number s_newBase)))
      "ERROR"     ; if one of the input values is wrong, return an error message
      (begin      ; else
        <body>))) ; put the rest of the procedure's body in here

    Or this, if you prefer to use a cond:

    (define (convert originalNumber s_oldBase s_newBase)
  (cond ((or (not (string->number originalNumber)) ; validate error conditions
             (not (string->number s_oldBase))
             (not (string->number s_newBase)))
         "ERROR")  ; if one of the input values is wrong, return an error message
        (else      ; else
         <body>))) ; put the rest of the procedure's body in here
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