Nuts and Volts - November 2008

SMILEY’S WORKSHOP ☺

like you think they should. An example of the kind of confusion you can run into is when you use the ‘=’ assignment operator and the ‘==’ ‘is equal to’ operator:

You could write this as:

if( temp > 150 )
 turnFan(ON);

else

turnFan(OFF);

So the answer is 40, right? Wrong, according to C, it is – 25. The compiler does the division and multiplication first, then the addition and subtraction:

x = y;
if(x==y) _delay_loop_2(30000);

Or, you could use the C conditional operator ?: as below:

The first statement assigns x the value of y. The second statement calls the _delay_loop_2(30000) function if x ‘is equal to’ y. What about:

temp > 150 turnFan(ON) :
turnFan(OFF);

x = 50 + 10 / 2 – 20 4
x = 50 + 10 / 2 – 80
x = 50 + 5 – 80
x = 55 – 80
x = -25

if(x=y) _delay_loop_2(30000); //BAD
STATEMENT

This will set x equal to y and then call the _delay_loop_2(30000) function. The ‘if’ is checking to see if the statement is true, meaning that it is not equal to 0. In our case, the delay will always run unless y is 0, then it will never run. Either way, it isn’t what you thought you were testing. The WinAVR compiler will think something is strange and issue this warning:

The operation has the form expresson1 expression2 : expression3, and follows the rule that if expression1 is true (non-zero value), then use expression2, otherwise use expression3. This operator seems a little gee-wiz-impress-your-friends and not as clear as the if-else expression, but you’ll see this a lot so get used to it.

Precedence and Order of Evaluation

When a statement has a sequence of operators such as:

x = 50 + 10 / 2 – 20 4;

Warning: suggest parentheses around assignment used as truth value which will scroll by so fast you won’t see it, so you’ll assume the compile was good. It is a very easy mistake to make, and you will feel really dumb after an hour of debugging, looking for something obscure, only to find a lousy missing ‘=’ character. I do this all the time.

the compiler follows an order of calculation based on operator precedence. But what the compiler does may not be what you intended. Calculate the value of x. Did you get 40? If you performed the calculations sequentially beginning at the left, you get:

Some C gurus will memorize the precedence and associativity table and actually write statements like x = 50 + 10 / 2 – 20 4. Such clever programmers are dangerous and should be avoided when possible. The Germans have a word for clever: kluge. And in programming, ‘kluge’ is a well-deserved insult.

Don’t be clever, be clear. Clever programming is difficult to read and understand. If the clever programmer gets run over by a truck (hopefully), his code will be inherited by some poor guy who will have to figure things out. DO NOT memorize the Table of Operator Precedence and Associativity in C (which I refuse to even show). DO use ’(‘ and ‘)’ to make your program clear!

Which is clearer:

x = 50 + 10 / 2 – 20 4;
 or:

x = 50 + 10 / 2 – 20 4
x = 60 / 2 – 20 4
x = 30 – 20 4
x = 10 4
x = 40

x = 50 + (10 / 2) – (20 4);

Assignment Operators

The assignment operators provide a kind of shorthand technique for arithmetic operations. The following statements are equivalent:

The second adds nothing for the compiler, but tells the reader what

Defined

Put the value of y into x

myByte = myByte + yourByte;

Same as:

myByte += yourByte;

Conditional Expressions

You will frequently need to make decisions based on external conditions. For example, if the temperature is above 150° F turn the fan on, otherwise turn the fan off.

Operator = += -= *= /= %= <<= >>= &= ^= |=

Name Example Assignment x=y Add x += y Subtract x -= y Multiply x *= y Divide x /= y Modulo x %= y Left Shift x <<= y Right Shift x <<= y Bitwise AND x &= y Bitwise XOR x ^= y Bitwise OR x |= y