racket条件语句

( if ‹expr› ‹expr› ‹expr› )

if语句，若2>3输出”bigger”,否则输出”smaller”

> (if (> 2 3)
      "bigger"
      "smaller")

( cond {[ ‹expr› ‹expr› ]} )(相当于C中的switch,else相当于c中的default)

> (define (reply-more s)
    (cond
      [(equal? "hello" (substring s 0 5))
       (string-append "this is " s)]
      [(equal? "goodbye" (substring s 0 7))
       (string-append "this is " s)]
      [else "hehe"]))

> (reply-more "hello")
"this is hello"
> (reply-more "dfdfllfd")
"hehe"

( and ‹expr›* )(当所有表达式为真时，返回真)

> (define (is-digit v)
    (if (and (number? v) (> v 0) (< v 10))
        "v is a number and value is between 1-9"
        "v is not a number"))
> (is-digit 5)
"v is a number and value is between 1-9"

( or ‹expr›* )(当任一表达式为真时，返回真)

> (define (str-num v)
    (if (or (string? v) (number? v))
        "v is string or number"
        "v is not string or number"))

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Ref： 1.官方文档