## Type Conversions in C (String to Integer, isdigit() etc)

**By:** Abinaya in C Tutorials on 2007-09-20

`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 `char`s 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; }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 <= '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 `<ctype.h>`, 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 <= '9'can be replaced by

isdigit(c)We will use the

`<ctype.h>`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

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 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. 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`.

`float`s in an expression are not automatically converted to

`double`; this is a change from the original definition. In general, mathematical functions like those in

`<math.h>`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 `char`s 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 `<math.h>`.) 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; }

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