CODEWITHSUNDEEP
schemelisp
David542
David542

Reputation: 110093

Printf in scheme

How would I do something like the following in scheme?

printf("I went to the park at %d with %s", 4, "Bob");

The closest I have right now is:

(define TIME 4)
(define NAME "Bob")
(display "I went to the park at ") (display TIME) (display " with ") (display NAME) (display ".")

Upvotes: 1

Views: 2210

Answers (2)

ad absurdum
ad absurdum

Reputation: 21318

You can't do this in standard Scheme. Many Scheme implementations have a format procedure that is modeled on the Common Lisp format. For example, Chez Scheme has a pretty complete implementation of format, and also printf which is just a wrapper around format. I am so used to using format that I never think to use printf in lisps:

> (format #t "I went to the park at ~A with ~A~%" 4 "Bob")
I went to the park at 4 with Bob
> (printf "I went to the park at ~A with ~A~%" 4 "Bob")
I went to the park at 4 with Bob

Here format sends the output to the current output port when the first argument is #t; printf automatically sends output to the current output port. Common Lisp style format directives are prefixed with a tilde (~). The ~A, or aesthetic, directive prints objects in human-readable form, and is what you want most of the time. There are other directives for formatting numbers; I added the ~% directive, which emits a newline. Your original example did not include a newline, and printf, at least in C, does not add a newline at the end of output (ordinarily this is desirable). The format procedure should allow much more control over results than the mother of all printfs, namely C's fprintf.

The specific facilities for printing formatted output will depend on the implementation, but Chez Scheme, MIT Scheme, Gauche Scheme, and Guile all implement format. Chicken Scheme implements format, and also implements printf, fprintf, and sprintf which all use the same format directives as format. Racket has a host of formatted output procedures, including format, printf, and fprintf; all of these use Common Lisp style format directives, too.

You will have to consult the documentation of a specific implementation to understand which format directives are supported and how they work; the Chez Scheme documentation contains some information, but suggests consultation of the Common Lisp HyperSpec for complete documentation.

There are also SRFI-28 (Basic Format Strings) and SRFI-48 (Intermediate Format Strings) which provide some of this functionality to implementations that support them.

Upvotes: 4

Óscar López
Óscar López

Reputation: 235984

It really depends on what Scheme interpreter you're using. For example, in Racket you can use printf for a similar effect:

(printf "I went to the park at ~a with ~a" 4 "Bob")
=> I went to the park at 4 with Bob

Check the documentation for more formatting modifiers.

Upvotes: 2

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