Racket

racket-sprintf🔗ℹ

Hans Dijkema / hans@dijkewijk.nl

The racket-sprintf package provides sprintf and sprintf*, two small C-style string formatting procedures. The format language deliberately supports only a useful subset of C printf. The result is always a string; output is not written to a port.

1 Procedures🔗ℹ

procedure
(sprintf fmt arg ...) → string?
fmt : string?
arg : any/c

Formats the arguments according to fmt and returns the resulting string. Arguments are consumed from left to right. An exception is raised when a conversion is missing an argument, when arguments remain after all conversions have been processed, or when an argument has the wrong type for its conversion.

procedure
(sprintf* fmt args) → string?
fmt : string?
args : list?

Like sprintf, but takes the values to format in the list args. This is useful when the arguments have already been collected in a list.

2 Format syntax🔗ℹ

A conversion has this form:

%[flag][width][.precision][length]conversion

The supported conversion characters are s, d, f, x, and %. %s formats a string. %d formats a decimal number. %f formats a number using the requested precision. %x formats a number in base 16. %% represents a literal percent sign.

The optional - flag left-aligns a value within its field. Without -, a value with a field width is right-aligned. For numeric conversions, the optional 0 flag pads on the left with zeroes.

A width specifies the minimum field width. For example:

(sprintf "%10s" "Abc") ; => " Abc"

(sprintf "%-10s" "Abc") ; => "Abc "

(sprintf "%05d" 42) ; => "00042"

(sprintf "%-5d" 42) ; => "42 "

For %s, precision specifies the maximum number of characters taken from the string. Truncation happens before field-width padding. Consequently:

(sprintf "%.5s" "abcdefgh") ; => "abcde"

(sprintf "%10.5s" "abcdefgh") ; => " abcde"

(sprintf "%-10.5s" "abcdefgh") ; => "abcde "

For numeric conversions other than %d, precision is passed to ~r as the number of digits after the decimal point. Decimal integer conversion %d always uses integer-style precision zero.

3 Dynamic width and precision🔗ℹ

A width or precision can be written as *. Its value is then consumed from the argument list before the value being formatted. For example:

(sprintf "%*.*f" 8 3 1.23456)

Here 8 supplies the field width, 3 supplies the precision, and 1.23456 is the value. A dynamic width or precision must be numeric.

4 Length modifier🔗ℹ

One or more l characters are accepted before the conversion character for compatibility with existing format strings, but currently have no effect on the result.

5 Type rules🔗ℹ

The %s conversion requires a string. The numeric conversions require a number. The implementation intentionally reports a type mismatch instead of silently coercing the value.

6 Examples🔗ℹ

(sprintf "%-12s %5d" "items" 42)
(sprintf "%08x" 255)
(sprintf "%-10.4s" "abcdefgh")
(sprintf* "%s = %d" (list "answer" 42))

The implementation uses ~a and ~r from racket/format for the final field formatting.

1	Procedures
2	Format syntax
3	Dynamic width and precision
4	Length modifier
5	Type rules
6	Examples