Accepts a format template (a string), and processes it, replacing any format directives in order with one or more characters, the characters themselves dependent on the semantics of the format directive encountered. Each directive may consume one obj. It is an error if fewer or more obj values are provided than format directives that require them.
When a port is specified it must be either an output port or a boolean.
- If port is an output port, the formatted output is output into that port.
- If port is #t, output is to the current-output-port.
- If port is #f or omitted, the output is returned as a string.
It is an error if a format directive consumes an obj argument and that argument does not conform to a required type as noted in the table below.
A format directive is a two character sequence in the string where the first character is a tilde '~'. Directive characters are case-independent, i.e. upper and lower case characters are interpreted the same. Each directive code's meaning is described in the following table:
DIRECTIVE MNEMONIC ACTION CONSUMES? ~a Any (display obj) for humans yes ~s Slashified (write obj) for parsers yes ~w WriteCircular (write-with-shared-structure obj) like ~s, but handles recursive structures yes ~d Decimal the obj is a number which is output in decimal radix yes ~x heXadecimal the obj is a number which is output in hexdecimal radix yes ~o Octal the obj is a number which is output in octal radix yes ~b Binary the obj is a number which is output in binary radix yes ~c Character the single character obj is output by write-char yes ~y Yuppify the list obj is pretty-printed to the output yes ~? Indirection the obj is another format-string and the following obj is a list of arguments; format is called recursively yes ~K Indirection the same as ~? for backward compatibility with some existing implementations yes ~[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. yes ~~ Tilde output a tilde no ~t Tab output a tab character no ~% Newline output a newline character no ~& Freshline output a newline character if it is known that the previous output was not a newline no ~_ Space a single space character is output no ~h Help outputs one line of call synopsis, one line of comment, and one line of synopsis for each format directive, starting with the no directive (e.g. "~t")
The ~F fixed format directive requires some elucidation.
~wF is useful for strings or numbers. Where the string (or number->string of the number) has fewer characters than the integer width w, the string is padded on the left with space characters.
~w,dF is typically used only on numbers. For strings, the d specifier is ignored. For numbers, the integer d specifies the number of decimal digits after the decimal place. Both w and d must be zero or positive.
If d is specified, the number is processed as if added to 0.0, i.e. it is converted to an inexact value.
(format "~8,2F" 1/3) ; => " 0.33"
If no d is specified, the number is not coerced to inexact.
(format "~6F" 32) ; => " 32"
Digits are padded to the right with zeros:
(format "~8,2F" 32) ; => " 32.00"
If the number it too large to fit in the width specified, a string longer than the width is returned:
(format "~1,2F" 4321) ; => "4321.00"
If the number is complex, d is applied to both real and imaginal parts:
(format "~1,2F" (sqrt -3.9)) ; => "0.00+1.97i"
For very large or very small numbers, the point where exponential notation is used is implementation defined.
(format "~8F" 32e5) ; => " 3.2e6" or "3200000.0"