Sams Teach Yourself C in 21 Days

Working with Memory 567

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the compiler would first use the operand,Xand the operand 2. Once in completes it

would use the resulting value and the operand Y.

Within expressions, individual operands are promoted as necessary to match the associ-

ated operands in the expression. Operands are promoted, in pairs, for each binary opera-

tor in the expression. Of course, promotion isn’t needed if both operands are the same

type. If they aren’t, promotion follows these rules:

• If either operand is a long double, the other operand is promoted to type long
  double.


• If either operand is a double, the other operand is promoted to type double.


• If either operand is a float, the other operand is promoted to type float.


• If either operand is a long, the other operand is converted to type long.
  For example, if xis an intandyis a float, evaluating the expression x/ycausesxto be
  promoted to type floatbefore the expression is evaluated. This doesn’t mean that the
  type of variable xis changed. It means that a type floatcopy of xis created and used in
  the expression evaluation. The value of the expression is, as you just learned, type float.
  Likewise, if xis a type doubleandyis a type float,ywill be promoted to double.

Conversion by Assignment

Promotions also occur with the assignment operator. The expression on the right side of

an assignment statement is always promoted to the type of the data object on the left side

of the assignment operator. Note that this might cause a “demotion” rather than a promo-

tion. If fis a type floatandiis a type int,iis promoted to type floatin this assign-

ment statement:

f = i;

In contrast, the assignment statement

i = f;

causesfto be demoted to type int. Its fractional part is lost on assignment to i.

Remember that fitself isn’t changed at all; promotion affects only a copy of the value.

Thus, after the following statements are executed

float f = 1.23;

int i;

i = f;

the variable ihas the value 1 , andfstill has the value 1.23. As this example illustrates,

the fractional part is lost when a floating-point number is converted to an integer type.

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