Programming in C

498 Appendix D Common Programming Mistakes

If squareRootis defined later in the program, or in another file, and is not explic-

itly declared otherwise, the compiler assumes that the function returns an int.

Furthermore, the compiler converts floatarguments to double,and _Bool,char,

and shortarguments to int. No other conversion of arguments is done.

Remember, it’s always safest to include a prototype declaration for allfunctions

that you call (either explicitly yourself or implicitly by including the correct head-

er file in your program), even if they’re defined earlier.

4. Confusing the precedences of the various operators.
   Examples
   while ( c = getchar () != EOF )
   ...
   if ( x & 0xF == y )
   ...

In the first example, the value returned by getcharis compared against the value

EOFfirst.This is because the inequality test has higher precedence than the assign-

ment operator.The value that is therefore assigned to cis the TRUE/FALSE result

of the test: 1 if the value returned by getcharis not equal to EOF, and 0 other-

wise. In the second example, the integer constant 0xFis compared against yfirst

because the equality test has higher precedence than any of the bitwise operators.

The result of this test (0 or 1) is then ANDed with the value of x.

5. Confusing a character constant and a character string.
   In the statement
   text = 'a';

a single character is assigned to text. In the statement

text = "a";

a pointer to the character string "a"is assigned to text.Whereas, in the first case,

textis normally declared to be a charvariable, in the second case, it should be

declared to be of type "pointer to char".

6. Using the wrong bounds for an array.
   Example
   int a[100], i, sum = 0;
   ...
   for ( i = 1; i <= 100; ++i )
   sum += a[i];

Valid subscripts of an array range from 0 through the number of elements minus

one.Therefore, the preceding loop is incorrect because the last valid subscript of a

is 99 and not 100.The writer of this statement also probably intended to start with

the first element of the array; therefore,ishould have been initially set to 0.

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