Let's start by firing up an example.
if (x = y)
printf("Equal values");
Simple enough. If y is not zero, print "Equal values". And, by the way, replace the value of x with the value of y. Isn't it nice that C lets you do assignment within an if statement expression?
But maybe you thought this code really meant: If x is equal to y, print "Equal values"? No, the code for that is
if (x == y)
printf("Equal values");
If this example tripped you up, don't worry. Typing = (assignment) instead of == (equality) occasionally gets the best C programmers, too. The problem isn't in comprehending the different meanings of = and ==. The problem is that it's easy to mistype = when you mean ==, especially because = is the standard mathematical symbol for equality, and = represents equality in many other widely used programming languages (e.g., PL/I, COBOL, and Pascal). Unfortunately, C treats this easy-to-make typo as an intentional assignment operation. The resulting code will execute, and the error may be hard to diagnose.
Hard-core C programmers may try to convince you it's your inexperience, not C's syntax, that causes this type of coding error. But there's a booming market in C source-code checkers (known as "lint" filters) to help experienced C programmers protect themselves from just these kinds of sneaky problems. If C's pitfalls weren't so pervasive, lint utility vendors would be out of business.
All programmers are not created "equal equal," so if you want to be an A++ C programmer (why be just a C++ programmer?), the first rule is don't use assignment in an if statement expression, unless it is absolutely necessary. In addition, use a compiler warning level or a lint utility that will catch = in if statement expressions. Be forewarned, however, that you may never be acknowledged as a "real" C programmer unless you're willing to take some risks to speed up your code by a few nanoseconds. Another good technique -- if you can handle accusations of "wimp" programmer -- is to define the macro
#define EQ ==
and never use == at all. Instead, you can write logical expressions, such as
if (x EQ y)
printf("Equal values");
In addition to = and ==, C also has & (bitwise AND), && (logical AND), | (bitwise OR), and || (logical OR). The bitwise and logical operators work the same, when their operands are 0 or 1. In other cases, however, the results are different. For example,
2 && 4
is 1, which is considered "true" in an if statement, whereas
2 & 4
is 0, which is "false." Because, in many cases, & and | produce the same effect as && and || in if statement expressions (i.e., zero or non-zero), incorrect use of the bitwise operators can cause infrequent and hard-to-diagnose errors. If you'd rather rely on something more than luck for correct programs, you may want to define the following four macros and use them instead of &, |, &&, and ||.
#define and( a, b ) ( ( a ) & ( b ) )
#define or( a, b ) ( ( a ) | ( b ) )
#define AND &&
#define OR ||
