ruby - 为什么 Ruby 注入(inject)方法不能对没有初始值的字符串长度求和？

Question

为什么下面的代码会报错？

['hello','stack','overflow'].inject{|memo,s|memo+s.length}

TypeError: can't convert Fixnum into String
        from (irb):2:in `+'
        from (irb):2:in `block in irb_binding'
        from (irb):2:in `each'
        from (irb):2:in `inject'
        from (irb):2

如果传递了初始值，它就可以正常工作:

['hello','stack','overflow'].inject(0){|memo,s|memo+s.length}
=> 18

Answer 1

apidock 中有答案:

If you do not explicitly specify an initial value for memo, then uses the first element of collection is used as the initial value of memo.

也就是说，如果没有初始值，您将尝试执行 'hello' + 'stack'.length