Lecture #8: Operators and Expressions

Monday, October 18, 1999

Bob Pitha

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

what’s an operator?

• Something that acts on a variable or an expression

• Does something

– computes or retrieves a value

– changes an object

types of operators

• Arithmetic operators

• Increment/decrement operators

• Assignment operators

• Relational operators

• Equality operators

• Logical operators

• There are others...

expressions

• A complex topic, really

• For our purposes today:

– an operator and its operands

– a variable

• Expressions can be nested

expression concepts

• lvalues

• Conversions

• Operator precedence

lvalue

• This is a tough concept - but necessary!

• Some expressions refer to real memory (objects)

– can have a value assigned

– these are sometimes called lvalues

• because they can be on the left side of an equals sign

• Some expressions are just values

– calculated, used, then thrown away

examples

• Given the declaration

int i;

– i is an lvalue

• so it is legal to say:

i = 1;

– i + 1 is not an lvalue

• because you can’t assign a value to i + 1

• i + 1 is just a computed value; it has no permanent memory associated with it

– 1 is not an lvalue

• you can’t assign a value to a constant

why do we care?

• Some operators require lvalue operands

– Like =

• legal: i = 1

• illegal: i + 1 = 2

• This concept will come up again and again

conversions

• What happens here?

– remember I said the computer does integer and floating-point math differently

– so how does it mix the two?

conversions

• The answer is it finds an acceptable type, and converts whatever is necessary.

– in this example

a is converted into a double and a+b is evaluated using floating-point addition

some caveats

• Conversions follow certain rules

– depends on the operator involved

• The old variable isn’t changed

– a new object is created temporarily; you never really see it

– thus, the converted value is not an lvalue

• this isn’t an issue in the simple cases we’ve seen so far

• but it can be in some things we have yet to see

usual arithmetic conversions

• Standard conversions applied to operands to enforce type agreement.

• Most binary operators require operands of the same type

• Usually widen, not narrow, an operand

conversion rules

• Informally:

– If either operand is a long double, convert the other to long double

– Otherwise, if either operand is a 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

conversion example

ASCII character set

operator precedence and associativity

• Each expression operator in C has a precedence and associativity rule

• In a compound expression in the absence of parentheses, sometimes interpretation is ambiguous:

a + b * c

• Rules of precedence and associativity clarify such situations

precedence

• Each operator has a precedence level

• Specifies how operands are grouped with operators in ambiguous situations

• Example:

a + b * c

* has higher precedence than +, so this expression is evaluated as

a + (b * c)

associativity

• Some operators have the same precedence

• In that case, associativity rules come into play

• Associativity specifies in what order operators with equal precedence are evaluated

associativity example

a / b * c

• * and / have the same precedence

• * and / are left-associative

– which means left-to-right evaluation:

(a / b) * c

– common sense in this simple example, but less obvious sometimes

arithmetic operators

+ - * / %

• Most of these are straightforward and intuitive

• These all compute values, have no side-effects

• /

Works differently for integers, real numbers: for integers, result is truncated

• the expression 9/10 will have the value 0

%: the modulus operator

• Computes the remainder of the division

• Both operands must be integers

precedence and associativity