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

    1. CL-USER> (format t "~5$" pi)
    2. 3.14159

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

    1. CL-USER> (format t "~,5f" pi)

    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.

    1. CL-USER> (format t "~:@d" 1000000)
    2. +1,000,000

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