Archiwum
- Index
- Livingstone J.B. Morderstwo w British Museum
- Jacqueline_Baird_ _Grecki_multimilioner
- Zelazny, Roger El Amor es un Numero Imaginario
- javascript dla kaśźdego full scan
- Yasunari Kawabata Kraina śÂ›niegu
- HIdden Hills Shifters 2 Bare Instincts Vella Day
- Ariel Toaff Blood Passover. Internet Aaargh, 2007. (ang.)
- Hawksley Elizabeth Próba
- Susan Elizabeth Phillips Hot Shot
- Comte, Auguste The Positive Philosophy Vol II
- zanotowane.pl
- doc.pisz.pl
- pdf.pisz.pl
- marcelq.xlx.pl
[ Pobierz całość w formacie PDF ]
It's hard to generalize about which form is better. Constructions like !valid read nicely (``if not valid''), but
more complicated ones can be hard to understand.
Exercise 2-2. Write a loop equivalent to the for loop above without using && or ||.
2.7 Type Conversions
When an operator has operands of different types, they are converted to a common type according to a small
number of rules. In general, the only automatic conversions are those that convert a ``narrower'' operand into a
``wider'' one without losing information, such as converting an integer into floating point in an expression like
f + i. Expressions that don't make sense, like using a float as a subscript, are disallowed. Expressions
that might lose information, like assigning a longer integer type to a shorter, or a floating-point type to an
integer, may draw a warning, but they are not illegal.
A char is just a small integer, so chars may be freely used in arithmetic expressions. This permits
considerable flexibility in certain kinds of character transformations. One is exemplified by this naive
implementation of the function atoi, which converts a string of digits into its numeric equivalent.
/* atoi: convert s to integer */
int atoi(char s[])
{
int i, n;
n = 0;
for (i = 0; s[i] >= '0' &s[i]
n = 10 * n + (s[i] - '0');
2.7 Type Conversions 42
The C programming Language
return n;
}
As we discussed in Chapter 1, the expression
s[i] - '0'
gives the numeric value of the character stored in s[i], because the values of '0', '1', etc., form a
contiguous increasing sequence.
Another example of char to int conversion is the function lower, which maps a single character to lower
case for the ASCII character set. If the character is not an upper case letter, lower returns it unchanged.
/* lower: convert c to lower case; ASCII only */
int lower(int c)
{
if (c >= 'A' &c
return c + 'a' - 'A';
else
return c;
}
This works for ASCII because corresponding upper case and lower case letters are a fixed distance apart as
numeric values and each alphabet is contiguous -- there is nothing but letters between A and Z. This latter
observation is not true of the EBCDIC character set, however, so this code would convert more than just
letters in EBCDIC.
The standard header , described in Appendix B, defines a family of functions that provide tests
and conversions that are independent of character set. For example, the function tolower is a portable
replacement for the function lower shown above. Similarly, the test
c >= '0' &c
can be replaced by
isdigit(c)
We will use the functions from now on.
There is one subtle point about the conversion of characters to integers. The language does not specify
whether variables of type char are signed or unsigned quantities. When a char is converted to an int, can
it ever produce a negative integer? The answer varies from machine to machine, reflecting differences in
architecture. On some machines a char whose leftmost bit is 1 will be converted to a negative integer (``sign
extension''). On others, a char is promoted to an int by adding zeros at the left end, and thus is always
positive.
The definition of C guarantees that any character in the machine's standard printing character set will never be
negative, so these characters will always be positive quantities in expressions. But arbitrary bit patterns stored
in character variables may appear to be negative on some machines, yet positive on others. For portability,
specify signed or unsigned if non-character data is to be stored in char variables.
Relational expressions like i > j and logical expressions connected by && and || are defined to have value
1 if true, and 0 if false. Thus the assignment
2.7 Type Conversions 43
The C programming Language
d = c >= '0' &c
sets d to 1 if c is a digit, and 0 if not. However, functions like isdigit may return any non-zero value for
true. In the test part of if, while, for, etc., ``true'' just means ``non-zero'', so this makes no difference.
Implicit arithmetic conversions work much as expected. In general, if an operator like + or * that takes two
operands (a binary operator) has operands of different types, the ``lower'' type is promoted to the ``higher''
type before the operation proceeds. The result is of the integer type. Section 6 of Appendix A states the
conversion rules precisely. If there are nounsigned operands, however, the following informal set of rules
will suffice:
" If either operand is long double, convert the other to long double.
" Otherwise, if either operand is double, convert the other to double.
" Otherwise, if either operand is float, convert the other to float.
" Otherwise, convert char and short to int.
" Then, if either operand is long, convert the other to long.
Notice that floats in an expression are not automatically converted to double; this is a change from the
original definition. In general, mathematical functions like those in will use double precision. The
main reason for using float is to save storage in large arrays, or, less often, to save time on machines where
double-precision arithmetic is particularly expensive.
Conversion rules are more complicated when unsigned operands are involved. The problem is that
comparisons between signed and unsigned values are machine-dependent, because they depend on the sizes
of the various integer types. For example, suppose that int is 16 bits and long is 32 bits. Then -1L
because 1U, which is an unsigned int, is promoted to a signed long. But -1L > 1UL because -1L
is promoted to unsigned long and thus appears to be a large positive number.
Conversions take place across assignments; the value of the right side is converted to the type of the left,
which is the type of the result.
A character is converted to an integer, either by sign extension or not, as described above.
Longer integers are converted to shorter ones or to chars by dropping the excess high-order bits. Thus in
int i;
char c;
i = c;
c = i;
the value of c is unchanged. This is true whether or not sign extension is involved. Reversing the order of
assignments might lose information, however.
If x is float and i is int, then x = i and i = x both cause conversions; float to int causes
truncation of any fractional part. When a double is converted to float, whether the value is rounded or
truncated is implementation dependent.
Since an argument of a function call is an expression, type conversion also takes place when arguments are
passed to functions. In the absence of a function prototype, char and short become int, and float
becomes double. This is why we have declared function arguments to be int and double even when the
function is called with char and float.
2.7 Type Conversions 44
The C programming Language
Finally, explicit type conversions can be forced (``coerced'') in any expression, with a unary operator called a
cast. In the construction
(type name) expression
the expression is converted to the named type by the conversion rules above. The precise meaning of a cast is
as if the expression were assigned to a variable of the specified type, which is then used in place of the whole
construction. For example, the library routine sqrt expects a double argument, and will produce nonsense
if inadvertently handled something else. (sqrt is declared in .) So if n is an integer, we can use
sqrt((double) n)
to convert the value of n to double before passing it to sqrt. Note that the cast produces the value of n in
the proper type; n itself is not altered. The cast operator has the same high precedence as other unary
operators, as summarized in the table at the end of this chapter.
If arguments are declared by a function prototype, as the normally should be, the declaration causes automatic
coercion of any arguments when the function is called. Thus, given a function prototype for sqrt:
double sqrt(double)
the call
root2 = sqrt(2)
[ Pobierz całość w formacie PDF ]