What is the meaning of '==' in C?

What is the meaning of ==开发者_开发百科 and how does it differ from =?

How do I know which one to use?

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

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

For example:

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

Just watch out for the heinous:

if (i = 4) { }

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) {}

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 :-)

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

No magic there.

a == b is a test if a and b are equal.

a = b is called an assignment, which means to set the variable a to having the same value as b.

(You type | with Shift-\ in the US keyboard layout.)

== tests equality = assigns a value

neither are related to ||

I might add that in Finnish and Swedish keyboards. Pipe symbol; |; of OR is AltGr (the right alt) and < key. IF you are using Mac on the other hand it is Alt-7 key.

Gave me a lot of sweat when I first started typing on these keyboards.

Now that you know the difference between '==' and '=", let me put you some words of caution. Although '==' is used as a standard test of equality between comparable variables and '=' used as an internally type-casted assignment, the following programming error is quiet common.

In the below example and similar codes, '=' is know as "Always true" conditional operator.

#include<stdio.h>
int main()
{
    int i = 10, j = 20;
    if ( i = j )
        printf("Equal\n");
    else
        printf("NOT Equal\n");
    return 0;
}

So, the word of caution is "Never use '=' in if statements, unless you have something evil in your mind."