如何获取方法链中字符串的第一个或最后几个字符? [英] How to I get the first or last few characters of a string in a method chain?

问题描述

如果我使用函数式方法链进行字符串操作，则不能使用 用于获取第一个或最后几个字符的常用机制:我无法访问对当前字符串的引用，因此我无法计算索引.

If I use functional-style method chains for string manipulation, I can not use the usual machinery for getting the first or last few characters: I do not have access to a reference to the current string, so I can not compute indices.

示例:

[some, nasty, objects]
    .map( { $0.asHex } )
    .joined()
    .<first 100>
    .uppercased()
    + "..."

用于截断的调试输出.

那么我如何实现，或者我必须打破链条?

So how to I implement <first 100>, or do I have to break the chain?

推荐答案

我不知道有任何 API 可以做到这一点.幸运的是，编写我们自己的代码是一项简单的练习:

I don't know of any API that does this. Fortunately, writing our own is an easy exercise:

extension String {
    func taking(first: Int) -> String {
        if first <= 0 {
            return ""
        } else if let to = self.index(self.startIndex, 
                                      offsetBy: first, 
                                      limitedBy: self.endIndex) {
            return self.substring(to: to)
        } else {
            return self
        }
    }
}

从最后开始是类似的.

在此处查找完整代码(包括变体)和测试.

Find full code (including variants) and tests here.

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