clojure学习（1）——判断语句

clojure是操作符前缀的语言。

；为单行注释

nil相当于Java里面的null，if判断里面的false

if语句除了false和nil，其他都为true，如果else后面没内容，为false时就返回nil。

user=> (defn is-small [number] (if (< number 100) "yes" "no"))#'user/is-smalluser=> (is-small 20)"yes"user=> (is-small 150)"no"

user=> (defn is-small [number] (if (< number 100) "yes" ))#'user/is-smalluser=> (is-small 110)nil

if-not：和if相反的判断

user=> (if-not (zero? 0) "no" "yes")"yes"user=> (if-not (zero? 123) "no" "yes")"no"

if-let 用法：第一个参数为绑定变量，第二个参数为表达式。并根据第二个表达式参数返回的值确定执行then、else语句。

user=> (defn if-let-test [arg] (if-let [x arg] "true" "false"))#'user/if-let-testuser=> (if-let-test 1)"true"user=> (if-let-test nil)"false"user=> (if-let-test false)"false"

when

当不为false或nil时才执行后面的语句，否则返回nil

user=> (when false (println "is true") "return true")niluser=> (when false (println "is true") "return true" "return false")niluser=> (when true (println "is true") "return true" "return false")is true"return false"user=> (def has-value (when true (println "ni hao") "returned value"))ni hao#'user/has-valueuser=> has-value"returned value"

when-not

为false或nil时候才执行后面语句，否则返回nil

user=> (when-not false (println "is false") "return true" "return false")is false"return false"user=> (when-not false (println "is false") (println "return true") "return false")is falsereturn true"return false"

when-let

类似if-let，当绑定变量部位false或nil才执行后面语句，否则返回nil

user=> (when-let [a true] (println "true") "return true")true"return true"user=> (when-let [a false] (println "true"))niluser=> (when-let [a nil] (println "true"))

cond

类似Java里面的switch-case-default语句

user=> (defn f [n] (cond (< n 0) "<0" (< n 10) "<10" :else ">=10"))#'user/fuser=> (f -56)"<0"user=> (f 56)">=10"user=> (f 156)">=10"user=> (f 6)"<10"

case

类似switch里的case

user=> (let [mystr "ni hao"](case mystr"" 0"ni hao" (count mystr)))6

user=> (let [mystr "no one"](case mystr"" 0"ni hao" (count mystr)"default"))"default"

多值匹配

user=> (defn f [x] (case x(5 10) "*5"(3 6 9) "*3""others"))#'user/fuser=> (f 5)"*5"user=> (f 10)"*5"user=> (f 3)"*3"user=> (f 11)"others"

do

执行多条语句，返回最后一条语句值

user=> (def v (do (println 123) (println 321) (println 543) 23 0))123321543#'user/vuser=> v0