C中'=='是什么意思? [英] What is the meaning of '==' in C?

问题描述

==是什么意思，它和=有什么区别?

What is the meaning of == and how does it differ from =?

我怎么知道使用哪个?

推荐答案

== 是对相等性的测试.= 是一个赋值.

== is a test for equality. = is an assignment.

任何优秀的 C 语言书籍都应该涵盖这一点(我想在这本书的早期).

Any good C book should cover this (fairly early on in the book I would imagine).

例如:

int i = 3;                       // sets i to 3.
if (i == 3) printf("i is 3\n");  // prints it.

注意那些令人发指的:

if (i = 4) { }

这是有效的 C 并且经常被人发现.这实际上将分配 4 给变量i 并将其用作if 语句中的真值.这导致很多人使用更丑但更安全的方式:

which is valid C and frequently catches people out. This actually assigns 4 to the variable i and uses that as the truth value in the if statement. This leads a lot of people to use the uglier but safer:

if (4 == i) {}

如果你不小心使用了 = 而不是 ==，这是一个编译时错误，而不是在你的程序运行时会咬你的东西:-)

which, if you accidentally use = instead of ==, is a compile-time error rather than something that will bite you on the backside while your program is running :-)

逻辑或运算符是两个竖线字符，一个接一个，不是一个字符.在这里，它与一个逻辑与和一个名为 b4 的变量对齐:

The logical-or operator is two vertical bar characters, one after the other, not a single character. Here it is lined up with a logical-and, and a variable called b4:

||
&&
b4

没有魔法.

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