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] <= '9'; ++i) n = 10 *
n + (s[i] - '0'); return n;
}
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 <=
'Z') 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
c >= '0' && c <= '9'
can
be replaced by
isdigit(c)
We will use
the
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 noncharacter 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
d = c >= '0' && c <= '9'
sets
d to 1 if c is a digit, and 0 if not. However,
functions like isdigit
may return any nonzero 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. If there are no unsigned 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 < 1U, 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.
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) coerces
the integer 2 into the double value 2.0 without any need for a cast.
The standard
library includes a portable implementation of a pseudo-random number generator
and a function for initializing the seed; the former illustrates a cast:
unsigned long int next = 1;
/* rand:
return pseudo-random integer on 0..32767 */ int rand(void)
{
next = next * 1103515245 + 12345; return (unsigned int)(next/65536) %
32768;
}
/* srand:
set seed for rand() */ void
srand(unsigned int seed)
{
next = seed;
}
Revison . Write a function htoi(s), which converts a string of
hexadecimal digits (including an optional 0x or 0X) into its equivalent integer value. The allowable digits are 0 through 9, a through f, and A through F.
0 comments:
Post a Comment