CSCD-255
2015-01-26


More Options for Expression

FULL table of operators and precedence — expanded beyond our book's Table 9.1:
Precedence.pdf

Compound Assignment Operators  Tweak available for the assignment operator:  short-hand for the cases where the variable on the left is identical with the variable at the beginning of the right side.  C was initially generated as a "mid-level" language to replace assembly language, a "low-level" langauge.  This provides the logic seen in assembly language for instructions that modify a variable or register by naming the operation and providing the right-hand side in the expression.
for instance:  Prod = Prod * k   or   Sum = Sum + k.  These can be handled by putting one arithmetic operator before the equal sign.
  Prod *= k   or   Sum += k.
Factorial.txt       Factorial.c       Factorial.exe      Not using the compound assignment operators
Factorial_2.txt   Factorial_2.c   Factorial_2.exe   Using the compound assignment operators

More Operators (Ch. 13)  Since incrementing (adding one) and decrementing (subtracting one) are very common operations, C provides increment and decrement operators. again mimicking something available in some assembly languages..  They are "unary" operators, meaning that they have only one operand.  They also have the second-highest precedence.  The value of the increment or decrement expression depends on whether the operator precedes the variable or follows it.  It can only be used on a variable.  If the operator comes before the variable name, the value of the expression is the value of the variable after it has been incremented or decremented.  If the operator comes after the variable name, the value of the expression is the value of the variable before it has been incremented or decremented.
Increment.txt       Increment.c       Increment.exe

Another unary operator is the sizeof operator, which reports the number of bytes required for its operand, which can either be a data type or a variable or an array.  We saw this briefly 9 January as we were examining the kinds of variables in C, but here is that code amplified to also show having a variable as the operand.  When we do arrays we will see a further use of the sizeof.
Size.txt       Size.c       Size.exe

Finally, the operator that can replace a simple if/else structure — and can lead to confusing code if not used carefully:  the conditional operator.  Unlike the unary operators, it has a very weak precedence.  it is three expressions separated by '?' and ':'.
<logical expression> ? <first arithmetic expression> : <second arithmetic expression>
If the logical expression evaluates as true, the value of the conditional expression is the first arithmetic expression.
If it evaluates as false, the value of the conditional expression is the second arithmetic expression.

It's precedence is right above that of the assignment operators and the comma.
Conditional.txt       Conditional.c       Conditional.exe