如何使用Rebol在块内创建子块

Question

我想要

[["1" "a"] ["2" "b"]] 

来自

["1" "" "a" "2" "" "b"]

什么是正确和最优雅的方式？我尝试了这个，但它没有得到我想要的结果：

block: ["1" "" "a" "2" "" "b"]

new-block: []
sub-block: []

foreach [first second third] block [
  sub-block: []
  append sub-block first
  append sub-block third
  append new-block sub-block
] 

Answer 1

newblock: copy [] forskip block 3 [repend/only newblock [block/1 block/3 ]]
== [["1" "a"] ["2" "b"]]

Answer 2

foreach [a b c] block [repend/only [] [a c]]

Answer 3

另一种方法是使用COLLECT/KEEP：

collect [
    foreach [first second third] block [
        keep/only collect [
            keep first
            keep third
        ]
    ]
]

Answer 4

另一种不太优雅的方法是：

>> parse ["1" "" "a" "2" "" "b"] [
         2 [set a string! skip set b string!
            (append x: [] reduce [a b])
         ]
   ]
>> x
== ["1" "a" "2" "b"]

Answer 5

对于更多种类，这是使用MAP-EACH

的解决方案

map-each [first second third] block [reduce [first third]]

NB。以上在Rebol（2＆amp; 3）中有效。 MAP-EACH目前在Red中不可用。