What's the difference of

Projects Programming
1

Hi, everyone.
what's the difference between: if (value = 1011) and if (value == 1011)?
int value = 1011.
Thanks
adam

2

From the reference for the if structure:

Beware of accidentally using the single equal sign (e.g. if (x = 10) ). The single equal sign is the assignment operator, and sets x to 10 (puts the value 10 into the variable x). Instead use the double equal sign (e.g. if (x == 10) ), which is the comparison operator, and tests whether x is equal to 10 or not. The latter statement is only true if x equals 10, but the former statement will always be true.

This is because C++ evaluates the statement if (x=10) as follows: 10 is assigned to x (remember that the single equal sign is the (assignment operator)), so x now contains 10. Then the 'if' conditional evaluates 10, which always evaluates to TRUE, since any non-zero number evaluates to TRUE. Consequently, if (x = 10) will always evaluate to TRUE, which is not the desired result when using an 'if' statement. Additionally, the variable x will be set to 10, which is also not a desired action.

1 Like
3

Great!
Sorry for my messed.
then I feel that it's easy to compare when used: if (value == 1) , no easy to use: if (value == 1011) or other number like 2011, why?

4

No problem. We see this often and I even forget == once in a while. If you enable all compile warnings in File, Preferences, you will get a warning if you do make that mistake.

enable verbose output
enable verbose output764×613 68.7 KB

int number = 17;

void setup()
{
   Serial.begin(115200);
    if(number = 17)
    {
      Serial.println("equal");
    }
}

void loop()
{
  
}

\sketch_jul11a.ino:6:15: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
if(number = 17)
~~~~~^~

2 Likes
5

Thank you groundFungus.

6

What do you mean by that?

1 Like
7

If you flip it around it will cause an error and not let you make that mistake at all.

int number = 17;

void setup()
{
   Serial.begin(115200);
    if(17 = number)
    {
      Serial.println("equal");
    }
}

void loop()
{
  
}
sketch_jul11a:6:13: error: lvalue required as left operand of assignment

     if(17 = number)

             ^~~~~~

exit status 1

lvalue required as left operand of assignment
8

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