Ruby - 如何从字符串中选择一些字符

Question

我正在尝试找到一个用于选择的功能，例如字符串的前 100 个字符。在 PHP 中，存在 substr function

Ruby 有类似的功能吗？

Answer 1

试试foo[0...100]，任何范围都可以。范围也可以为负数。是Ruby的well explained in the documentation。

Answer 2

使用[]-运算符（docs）：

foo[0, 100]  # Get 100 characters starting at position 0
foo[0..99]   # Get all characters in index range 0 to 99 (inclusive!)
foo[0...100] # Get all characters in index range 0 to 100 (exclusive!)

Update for Ruby 2.7: Beginless ranges 现在在这里（截至 2019 年 12 月 25 日）并且可能是“返回数组的第一个 xx”的规范答案：

foo[...100]  # Get all chars from the beginning up until the 100th (exclusive)

使用.slice 方法（docs）：

foo.slice(0, 100)  # Get 100 characters starting at position 0
foo.slice(0...100) # Behaves the same as operator [] 

为了完整性：

foo[0]         # Returns the indexed character, the first in this case
foo[-100, 100] # Get 100 characters starting at position -100
               # Negative indices are counted from the end of the string/array
               # Caution: Negative indices are 1-based, the last element is -1
foo[-100..-1]  # Get the last 100 characters in order
foo[-1..-100]  # Get the last 100 characters in reverse order
foo[-100...foo.length] # No index for one beyond last character

Update for Ruby 2.6:Endless ranges 现在在这里（截至 2018 年 12 月 25 日）！

foo[0..]      # Get all chars starting at the first. Identical to foo[0..-1]
foo[-100..]   # Get the last 100 characters