What is the difference between equals equals and equals in C programming?

Answer 1

a = b changes the value of a and makes it the same as the value of b.

a == b does not change the values of a or b. It is an expression that is equal to 1 if a and b are the same or to 0 if a and b are different.

For example:

if ( a == b) { c = d;}

means if a and b are the same, then set c equal to d.

C does let you write the following:

if ( a = b) { c = d;}

This sets a equal to the value of b, and then if the new value of a is non-zero, it sets c equal to d. You can do this, but if you see a single equal sign in an "if" condition, that usually (but not always) is a mistake.