Preventing Precedence Errors: EqualitytInequality

A very commonly observed precedence error during usage of expressions is inadequate parenthesizing.  Here we take examples of Equality/Inequality expressions:

if(var1 & var2 == 0)

The above is incorrect because == has a higher precedence than &, therefore in the above expression  var2==0  will be computed first, which is a bool. Then that bool will be bit_AND'ed with var1.  And this was not our intention.

The correct way  is:

if((var1 & var2) == 0)

In practice, the test for equality/inequality must be the last logical operation, but it can happen otherwise, if there are operators of lower precedence than  ==  or   !=  in the expression.

If we recall our three liner:

S—U—Ari

Re—Bit—Lo—C

Assignment—Comma

The  “e”  in Re stands for equality/inequality, and when they are there in an expression with operators of lower precedence then the  ==  or !=  will get executed earlier. To change the order of evaluation we need to appropriately parenthesize these operators. These operators(with precedence lower than  ==  and   != )  are   Bit—Lo—C—Assignment—Comma.   In practice we will find bit and assignment operator mixed with  ==  and   != operators.

So when we see this mixture, i.e.,==  or  !=, &,  | ,  ^, =   in a statement,  parenthesis are to be taken care.

The following are some examples where precedence errors are very common.

a)    Test if FLAG_A is clear in message incorrect

if(message & FLAG_A == 0)

correct

if((message & FLAG_A) == 0)

b)    Test if FLAG_ A is set in message  incorrect

if(message & FLAG_A != 0)

correct

if((message & FLAG_A) != 0)

c)    Test if all flags are clear incorrect

if ( message & (FLAG_A | FLAG_B | FLAG_C) == 0)

 correct

if (( message & (FLAG_A | FLAG_B | FLAG_C)) ==0)

d)    Incorrect

if ( c = getchar() != EOF)

correct

if (( c = getchar()) != EOF)

In all the above cases, parenthesizing is to be done for operands with precedence lower than  == or !=  so that they are executed earlier.

In short, when and expression has   equality/inequality operator as the final operand and we see that there are operators in the expression whose precedence levels and are still lower than equality / inequality, then we should get the parenthesis correct. The operators with lower precedence than  “Re” are  Bit—Lo—C—Assignment—Comma