scheme full padding example using format

Question

all

I want to change a element to formatted string, then I use format function. (the language I use is scheme )

As the document in http://www.gnu.org/software/mit-scheme/documentation/mit-scheme-ref/Format.html said, I can use ~mincolA if I want inserts spaces on the right.

So I use

(format "~4A " x) 

but I get an error like that:

format: ill-formed pattern string
  explanation: tag `~4' not allowed
  pattern string: "~4A "

I want to get the result like below:

if x is 0, then the result is space space space 0;

if x is 12, then the result is space space 12.

I know I can use

(string-append (make-string (- 4 (string-length x)) #\ ) x)

to get the result I want, but I really want use "format" function.

Thanks.

Cashandcarry

Is there no answer you'd consider acceptable? – Morphine

Answer 1

Notice that the referenced documentation is for MIT/GNU Scheme, the format function works different in Racket. Out-of-the-box, you can use the ~a function for the same effect:

(~a x #:min-width 4 #:align 'right #:left-pad-string " ") ; x can be a number or a string

For example:

(~a 0 #:min-width 4 #:align 'right #:left-pad-string " ")
=> "   0"

(~a "12" #:min-width 4 #:align 'right #:left-pad-string " ")
=> "  12"

If you don't mind importing an additional external library, @uselpa's answer is spot-on.

Demote

I would call it neither an additional nor an external library, since it is included in the Racket installation. It's just not required by default with #lang racket. – Morphine

Well, it's not there out-of-the box, that's what I mean. I'm not a big fan of importing another library just for some functionality that's already in the core language, albeit with a different syntax. It's a kind of code duplication, not to mention an additional dependency and memory overhead. – Maddock

Answer 2

You can use the format procedure from SRFI 48:

> (require srfi/48)
> (format "~4F" 0)
"   0"
> (format "~4F" 12)
"  12"

If you want to keep the original format procedure along with this one, you can give the one from SRFI 48 a prefix:

> (require (prefix-in srfi48: srfi/48))
> (srfi48:format "~4F" 0)

so the original format is still available.

Morphine

Answer 3

Scheme doesn't have a format procedure, but one is available in SRFI-48. It is not compatible with either MIT Scheme nor #!racket (the language).

#!r6rs
(import (rnrs base)
        (srfi :48))

(format "~4F " "x") ; ==> "   x" 

You can use SRFI-48 with #!racket in similar manner:

#!racket
(require srfi/48)

(format "~4F " "x") ; ==> "   x" 

F works only for numbers and strings according to the documentation:

~[w[,d]]F Fixed ~w,dF outputs a number with width w and d digits after the decimal; ~wF outputs a string or number with width w.

Also by evaluating (format "~h") you get instructions for use so for the basic reminder on syntax you don't need to visit the SRFI page.

Nordic

Wish one could tag answers. r6rs would do it for this :) – Mikael