我需要拆分含有不同数量 - 分隔符的字符串。我想取倒数第四项，并忽略拆分后片段数量不足的结果
let segments = string.split("2-33-444-5555-66666-7777-8888", on: "-")
let chosen = segments[-4] // <- This syntax is not valid.这份指南中写道 (https://tour.gleam.run/everything/#basics-lists)。
计算列表长度，或取得列表开头以外位置的元素，开销都很大，所以人们很少这样做。在 Gleam 中，通过索引访问序列的算法并不常见，而在这些场景中，列表并不是合适的数据结构。
但它没有建议另一种数据结构。怎样才能从后往前取得拆分后的片段？
I need to separate strings containing different numbers of - delimiters. I want the fourth item counting backwards, ignoring any results that have too few pieces
let segments = string.split("2-33-444-5555-66666-7777-8888", on: "-")
let chosen = segments[-4] // <- This syntax is not valid.According to this guide (https://tour.gleam.run/everything/#basics-lists):
Finding a list's size or retrieving entries somewhere beyond its beginning costs a lot, so people seldom do it. Algorithms which access sequences by index are uncommon in Gleam, and for those cases lists would be an unsuitable structure to choose.
but there's no suggestion for another data structure. How do I retrieve the separated pieces counting backwards?
先把列表反转。
let segments = list.reverse(segments)然后取第 4 个元素
let chosen = case list.drop(segments, 3) {
[fourth, ..] -> fourth
[] -> ""
}我还没在本机运行过，不过这样应该可以吧？
更符合习惯的写法如下。
pub fn fourth_from_tail(entries: String) -> Result(String, Nil) {
case list.reverse(string.split(entries, on: "-")) {
// Matches if there are at least 4 segments, binding the 4th from the end to `picked`
[_, _, _, picked, ..] -> Ok(picked)
// Discards strings with fewer than 4 segments
_ -> Error(Nil)
}
}Start by reversing it:
let segments = list.reverse(segments)then take element number 4
let chosen = case list.drop(segments, 3) {
[fourth, ..] -> fourth
[] -> ""
}I haven't tried running this locally, but wouldn't this work?
Here's an approach that's more idiomatic:
pub fn fourth_from_tail(entries: String) -> Result(String, Nil) {
case list.reverse(string.split(entries, on: "-")) {
// Matches if there are at least 4 segments, binding the 4th from the end to `picked`
[_, _, _, picked, ..] -> Ok(picked)
// Discards strings with fewer than 4 segments
_ -> Error(Nil)
}
}我找到了一种把 [-4] 这种简写语法封装到函数中的方法。
fn item_from_back(entries: List(a), position: Int) -> Result(a, Nil) {
let size = list.length(entries)
case size >= position {
True -> list.drop(entries, size - position) |> list.first
False -> Error(Nil)
}
}
let segments = string.split("2-33-444-5555-66666-7777-8888", on: "-")
case item_from_back(segments, 4) {
Ok(piece) -> io.println("piece: " <> piece)
_ -> io.println("No piece found!")
}I've managed to wrap the [-4] shorthand syntax inside a function:
fn item_from_back(entries: List(a), position: Int) -> Result(a, Nil) {
let size = list.length(entries)
case size >= position {
True -> list.drop(entries, size - position) |> list.first
False -> Error(Nil)
}
}
let segments = string.split("2-33-444-5555-66666-7777-8888", on: "-")
case item_from_back(segments, 4) {
Ok(piece) -> io.println("piece: " <> piece)
_ -> io.println("No piece found!")
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