左值要求作为分配的左操作数（C程序） [英] lvalue required as left operand of assignment (C program)

问题描述

我得到的误差要求作为转让的左操作数左值。

其分配到该行：

if(ch=='.' || ch=='"' || ch=='(' || ch=')' || ch=='!' || ch==',' || ch=='?' || ch==';') { etc }

我只是想检查，如果我看的字符等于这些字符中的任何一个。在修复任何想法？

I just want to check if the character I'm looking at is equal to any one of those characters. Any idea on the fix?

感谢您，

推荐答案

CH ='）应 CH =='）。您获得的特定错误的原因是因为在 == 具有较高的precedence比 || ， = 具有较低的precedence，所以这个：

ch=')' should be ch==')'. The reason you are getting that particular error is because while == has higher precedence than ||, = has lower precedence, so this:

ch == '(' || ch = ')'

被解析为

((ch == '(') || ch) = ')'

这是试图以分配'）到的结果（CH =='（'）|| CH）这是一个右值，而不是转让。这是一个常见的​​错误，并且使用这种技术来避免它：

Which is trying to assign ')' to the result of (ch == '(') || ch) which is an rvalue and not assignable. This is a common mistake, and this technique is used to avoid it:

if ('.' == ch || '"' == ch || ...)

注意，字符文字和变量皆已周围的 == 的地方。这样一来，如果你不小心键入

Notice that the character literals and the variable have switched places around the ==. That way, if you accidentally type

if ('.' = ch)

您会得到一个编译器错误。

You'll get a compiler error.

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