Книга: Practical Common Lisp

FORMAT Directives

FORMAT Directives

All directives start with a tilde (~) and end with a single character that identifies the directive. You can write the character in either upper- or lowercase. Some directives take prefix parameters, which are written immediately following the tilde, separated by commas, and used to control things such as how many digits to print after the decimal point when printing a floating-point number. For example, the ~$ directive, one of the directives used to print floating-point values, by default prints two digits following the decimal point.

CL-USER> (format t "~$" pi)
3.14
NIL

However, with a prefix parameter, you can specify that it should print its argument to, say, five decimal places like this:

CL-USER> (format t "~5$" pi)
3.14159
NIL

The values of prefix parameters are either numbers, written in decimal, or characters, written as a single quote followed by the desired character. The value of a prefix parameter can also be derived from the format arguments in two ways: A prefix parameter of v causes FORMAT to consume one format argument and use its value for the prefix parameter. And a prefix parameter of # will be evaluated as the number of remaining format arguments. For example:

CL-USER> (format t "~v$" 3 pi)
3.142
NIL
CL-USER> (format t "~#$" pi)
3.1
NIL

I'll give some more realistic examples of how you can use the # argument in the section "Conditional Formatting."

You can also omit prefix parameters altogether. However, if you want to specify one parameter but not the ones before it, you must include a comma for each unspecified parameter. For instance, the ~F directive, another directive for printing floating-point values, also takes a parameter to control the number of decimal places to print, but it's the second parameter rather than the first. If you want to use ~F to print a number to five decimal places, you can write this:

CL-USER> (format t "~,5f" pi)
3.14159
NIL

You can also modify the behavior of some directives with colon and at-sign modifiers, which are placed after any prefix parameters and before the directive's identifying character. These modifiers change the behavior of the directive in small ways. For instance, with a colon modifier, the ~D directive used to output integers in decimal emits the number with commas separating every three digits, while the at-sign modifier causes ~D to include a plus sign when the number is positive.

CL-USER> (format t "~d" 1000000)
1000000
NIL
CL-USER> (format t "~:d" 1000000)
1,000,000
NIL
CL-USER> (format t "~@d" 1000000)
+1000000
NIL

When it makes sense, you can combine the colon and at-sign modifiers to get both modifications.

CL-USER> (format t "~:@d" 1000000)
+1,000,000
NIL

In directives where the two modified behaviors can't be meaningfully combined, using both modifiers is either undefined or given a third meaning.

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