Книга: Practical Common Lisp

Floating-Point Directives

Floating-Point Directives

Four directives format floating-point values: ~F, ~E, ~G, and ~$. The first three of these are the directives based on FORTRAN's edit descriptors. I'll skip most of the details of those directives since they mostly have to do with formatting floating-point values for use in tabular form. However, you can use the ~F, ~E, and ~$ directives to interpolate floating-point values into text. The ~G, or general, floating-point directive, on the other hand, combines aspects of the ~F and ~E directives in a way that only really makes sense for generating tabular output.

The ~F directive emits its argument, which should be a number,[197] in decimal format, possibly controlling the number of digits after the decimal point. The ~F directive is, however, allowed to use computerized scientific notation if the number is sufficiently large or small. The ~E directive, on the other hand, always emits numbers in computerized scientific notation. Both of these directives take a number of prefix parameters, but you need to worry only about the second, which controls the number of digits to print after the decimal point.

(format nil "~f" pi) ==> "3.141592653589793d0"
(format nil "~,4f" pi) ==> "3.1416"
(format nil "~e" pi) ==> "3.141592653589793d+0"
(format nil "~,4e" pi) ==> "3.1416d+0"

The ~$, or monetary, directive is similar to ~F but a bit simpler. As its name suggests, it's intended for emitting monetary units. With no parameters, it's basically equivalent to ~,2F. To modify the number of digits printed after the decimal point, you use the first parameter, while the second parameter controls the minimum number of digits to print before the decimal point.

(format nil "~$" pi) ==> "3.14"
(format nil "~2,4$" pi) ==> "0003.14"

All three directives, ~F, ~E, and ~$, can be made to always print a sign, plus or minus, with the at-sign modifier.[198]

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