about "==" and "=" in if statement

Question

I have found a problem that "==" and "=" is very different in if statement.
For example, if there's a variable "a"
I type in some code :

if ( a == n ) ;
{ action1
}
if (a = m ) ;
{ action2
}

the first block is able to work normally,but the other will always be executed.
what are the differences between them?
hope that I can get your help.Thanks a lot :)
----from a Chinese starter

Answer 1

= is an assignment opperator.
== is a conditional operator, it tests for equality.

'if' statement in C++ does not accept only conditional expressions inside the
parentheses. It will consider expressions that return zero and false as 'false'.
Everything else is 'true'.

Answer 2

Firstly never ever use = in if statements the assigning operator is only used to assign values to objects/variables, always use ==. I think whats happening is that C++ checks if the assigning was successful if it was then it returns true executing the if block of code.

Answer 3

== tests for equality
= assigns the value on the right side to the variable on the left, the result is the value assigned
when the result of condition is non-zero, it is true
otherwise it is false

Answer 4

TurtleShell There many examples in which you would want to use = inside an if statement. For example:

if ((c = getchar()) == EOF)
{
//end
}
else
{
//go on with c as the next char
}

Misapon
It's also worth noting that if (a = 0) always executes.

Answer 5

Vlad Serbu
I have to desagree with you in the last point.

if(a = 0){
// this will not execute
}

https://code.sololearn.com/cYqvvmBZUAHZ/?ref=app

Answer 6

I don't know how to give thanks to each of you in this app,so I thank all of you here for your great help to me :)