字符串串联错误 [英] String Concatenation Error

问题描述

我遇到语法错误.我接受这是一个语法错误，但是对于为什么这是一个语法错误，我有点好奇.

I ran into a syntax error. I accept that it's a syntax error, but I'm somewhat curious as to why it's a syntax error.

这完全符合您的预期:

(0..9).each { |n| puts n.to_s + "^2 = " + (n**2).to_s }

这会引发错误:

(0..9).each { |n| puts n.to_s +"^2 = "+ (n**2).to_s }

错误:

NoMethodError: undefined method '+@' for "^2 = ":String

奇怪的是，我可以将第二个加号移动到任何地方，而Ruby似乎对此没有问题，但是如果第一个加号碰到双引号，则会出现语法错误.

Oddly, I can move the second plus sign wherever and Ruby seems to have no problem with it, but if that first one happens to touch the double quote, I get a syntax error.

为什么会这样呢?

推荐答案

n.to_s +"^2 = "解析为n.to_s(+"^2 = ")，这在语法上是有效的，并且意味着对字符串^2 =执行一元加号操作，然后传递结果作为to_s的参数".但是，由于字符串没有一元加号操作(由方法+@表示)，您会得到NoMethodError( not 语法错误).

n.to_s +"^2 = " is parsed as n.to_s(+"^2 = "), which is syntactically valid and means "perform the unary plus operations on the string ^2 = and then pass the result as an argument to to_s". However since strings don't have a unary plus operation (represented by the method +@), you get a NoMethodError (not a syntax error).

以这种方式而不是以n.to_s() + "^2 = "进行解析的原因是，如果以这种方式进行解析，则puts +5或puts -x也必须被解析为puts() + 5和puts() - x而不是和puts(-x)-在该示例中，很明显后者是预期的.

The reason that it's parsed this way and not as n.to_s() + "^2 = " is that if it were parsed this way then puts +5 or puts -x would also have to be parsed as puts() + 5 and puts() - x rather than puts(+5) and puts(-x) - and in that example it's rather clear that the latter is what was intended.

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