Thread: A question about '=' and '=='

In C, the symbol '=' means 'assign'. but in the selection structure, the '==' means check whether the condition is true, eg.

Code:

if (i == 2)

but if you write the statement in this way:

Code:

if (i = 2)

the check statement still works sometimes, my question is that why the '=' symbol in the check statement works, did the equality returns some value? and how the '==' works? Thanks!

its still better to use == for safety =p

if (i=2)
this works because it it checking for integer value...
but if u put a string value eg,
if (i="as")
there will be an error as it is checking for string comparison..
so using == to compare will be much safer..

thats what i think

Well, if you go:

Code:

if (i == 2)
  <code>
else
   <code>

it will check if i is 2 and then determine where to keep executing. But you are right. If you go

Code:

if (i = 2)
  <code>
else
   <code>

... i is tested, but not the way you would expect. In that line of code, 2 is assigned to i and then i is tested. An assignment returns the value that is assigned. So you end up with this:

Code:

if (i) // where i = 2
  <code>
else
   <code>

Now, in C/C++, you can do that and it tests i for a non-zero value. That means zero = false, anything else = true. So if i = zero, which it isn't, then it skips the if code and goes on to the else

== works by returning 1 or true (either way its not zero) if the two operands are equal.

= works by assigning the right value into the left variable and returning the value of the left variable.

A neat little thing you can do with = is this:

Code:

x = y = z = 3;

It sets all 3 variables to 3. Why? = has right-left associability. First it will assign 3 to z. Then the value of that (3) is set to y, then x. Then the final value of that statement is 3 but your semicolon discards that value and execution keeps on going.

if (i="as")
there will be an error as it is checking for string comparison..
so using == to compare will be much safer..

I think this statement should be:

Code:

if (strcmp(i, "as") == 0)

Actually I do not know whether you can do this comparison, I just follow the things I learned from my teacher.

You are correct, NightWalker.

Then the final value of that statement is 3 but your semicolon discards that value and execution keeps on going.

I thinks I do not totally understand this sentence, can you explain it to me with an example? Thank you very much!

Well, what happens if you don't put the semicolon? You would have something like:

x = y = z = 3

You can stick that anywhere you want. Say you stick it in another if statement

if (x = y = z = 3) <code>

Now, the value 3, which is returned by the assignments is being used. If you place a semicolon, it marks the end of a statement and whatever value is left over, is discarded.

x = y = z = 3;
i++;

^ the value of the first statement, 3, is discarded and execution moves on to the next statement (i++;)