125 lines
4.8 KiB
HTML
125 lines
4.8 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Originally converted to HTML using LaTeX2HTML 95 (Thu Jan 19 1995) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE> Nicer Output Using Format</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Nicer Output Using Format">
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<meta name="keywords" value="lp">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<BR> <HR>
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<A HREF="node61.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node58.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node59.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <BR>
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<A HREF="lp.html"><B>Contents</B></A>
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<B> Next:</B>
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<A HREF="node61.html"> Reading</A>
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<B>Up:</B>
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<A HREF="node58.html"> Input and Output</A>
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<B> Previous:</B>
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<A HREF="node59.html"> Basic Printing</A>
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<BR> <HR> <P>
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<H1> Nicer Output Using Format</H1>
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<P>
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Print and its kin are useful basic printing functions, but for sophisticated and easy to read output, format is more useful.
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<P>
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The basic structure of a format call is
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<P>
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<BLOCKQUOTE>
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<PRE> (format <destination> <control-string> <optional-arguments> )
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</PRE>
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</BLOCKQUOTE>
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The full use of a destination will be introduced further below; for most basic uses, the destination should be specified as t or nil. The control string is a string containing characters to be printed, as well as control sequences. Every control sequence begins with a tilde: <code>~</code>. The control sequences may require extra arguments to be evaluated, which must be provided as optional arguments to format.
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<P>
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With t as the specified destination, and no control sequences in the control-string, format outputs the string in a manner similar to princ, and returns nil.
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<P>
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<BLOCKQUOTE>
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<PRE>> (format t "this")
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this
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NIL
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</PRE>
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</BLOCKQUOTE>
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With nil as destination and no control sequences, format simply returns the string:
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<P>
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<BLOCKQUOTE>
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<PRE>> (format nil "this")
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"this"
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</PRE>
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</BLOCKQUOTE>
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Inserting <code>~</code>% in the control string causes a newline to be output:
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<P>
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<BLOCKQUOTE>
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<PRE>> (format t "~%This shows ~%printing with ~%newlines.~%")
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This shows
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printing with
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newlines.
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NIL
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</PRE>
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</BLOCKQUOTE>
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<code>~</code>s indicates that an argument is to be evaluated and the result inserted at that point. Each <code>~</code>s in the control string must match up to an optional argument appearing after the control string.
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<P>
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Here is an example of a function that uses this capability:
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<P>
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<BLOCKQUOTE>
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<PRE>(defun f-to-c (ftemp)
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(let ((ctemp (* (- ftemp 32) 5/9)))
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(format t
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"~%~s degrees Fahrenheit is ~%~s degrees Celsius~%"
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ftemp ;; first ~s
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(float ctemp)) ;; second ~s
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ctemp)) ;; return ratio value
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</PRE>
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</BLOCKQUOTE>
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(Float converts a number from integer or ratio to a floating point number, i.e. one with a decimal point.)
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<P>
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<BLOCKQUOTE>
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<PRE>> (f-to-c 82)
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82 degrees Fahrenheit is
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27.777777777777777 degrees Celsius
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250/9
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</PRE>
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</BLOCKQUOTE>
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Format simply evaluates the optional arguments in order they appear to determine the values for each <code>~</code>s. There must be enough optional arguments following the control string to provide values for all the occurrences of <code>~</code>s in the control string, otherwise an error will occur. If there are too many optional arguments, format evaluates them all (so side effects may occur) but no error is signalled.
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<P>
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Format will also preserve newlines entered directly in the control string. For example, f-to-c would behave just the same if defined as follows:
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<P>
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<BLOCKQUOTE>
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<PRE>(defun f-to-c (ftemp)
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(let ((ctemp (* (- ftemp 32) 5/9)))
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(format t "
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~s degrees Fahrenheit is
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~s degrees Celsius
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"
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ftemp
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(float ctemp)) ;; print floated value
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ctemp)) ;; return ratio value
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</PRE>
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</BLOCKQUOTE>
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In addition to these two control sequences it is useful to know about <code>~</code>T to produce tab spacing and <code>~</code><code>~</code> to print a tilde. Some other control sequences are documented in the appendix entry for format.
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<P>
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<BR> <HR>
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<A HREF="node61.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node58.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node59.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <BR>
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<A HREF="lp.html"><B>Contents</B></A>
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<B> Next:</B>
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<A HREF="node61.html"> Reading</A>
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<B>Up:</B>
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<A HREF="node58.html"> Input and Output</A>
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<B> Previous:</B>
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|
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<A HREF="node59.html"> Basic Printing</A>
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<BR> <HR> <P>
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<BR> <HR>
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<P>
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<ADDRESS>
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<I>© Colin Allen & Maneesh Dhagat <BR>
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March 2007 </I>
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</ADDRESS>
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</BODY>
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