Transcript Side Effect Operators

Side Effect Operators
Changing the value of
variables
Copyright 2004 - 2006 Curt Hill
There are operators and there
are operators
• Suppose the following statement:
a = 2+(b = c+d*e);
• The values of a and b are changed
• The values of c, d and e are not
• The assignment operator is a side
effect operator, while add and
multiply are not
Copyright 2004 - 2006 Curt Hill
How many side effect
operators are there?
• Most arithmetic or comparison
operators are not of the side effect
variety
• However, there are several side
effect operators
• They are added for convenience and
conciseness
Copyright 2004 - 2006 Curt Hill
The Unary Side effect
Operators
• There are two unary operators that can
each be used in two ways
• The two operators:
++
--
• These operators have a different effect if
before or after the variable
• These are handy for statements like:
x = x + 1;
• They are very common in many programs
Copyright 2004 - 2006 Curt Hill
The Increment Operator
• The ++ increments a variable
• The exact effect depends on position
• If the ++ follows the variable the effect
is:
– Obtain the value for the rest of the
expression
– Increment the variable
• If the ++ precedes the variable the
effect is:
– Increment the variable
– Obtain the value for the rest of the
expression
Copyright 2004 - 2006 Curt Hill
Example
• Consider the following code:
int a = 2, b = 2, c, d;
c = ++a; // Prefix
d = b++; // Postfix
• What values will result?
• The variables a, b and c will end with
three
• Variable d will end in 2
Copyright 2004 - 2006 Curt Hill
Reconsidered
• Another look:
int a = 2,c;
c = ++a;
• The increment makes a equal to 3
• Then the assignment occurs and both end
at 3
• Then:
int b = 2, d;
d = b++;
• The old value of b (2) is given to d
• Then the increment raises b to 3
Copyright 2004 - 2006 Curt Hill
The Decrement
• The -- works about the same except that it
decrements
• The positioning is similar:
– If it follows the decrement occurs after
obtaining the value
– If if precedes the decrement occurs before
the value is obtained
Copyright 2004 - 2006 Curt Hill
Miscellaneous Rules
• Both ++ and -- have very high
precedence, like any unary operator
• Only a variable may be incremented or
decremented
– Like any side effect operator
– So
++i++
is not legal
• These may be used as a statement
i++;
or as part of an expression:
d = a * ++b – c--;
Copyright 2004 - 2006 Curt Hill
More Side Effect Operators
• The increment and decrement are a nice
shorthand for a common operation
• There are others
• If you look at a lot of code, you see a lot of
assignments of the form:
x = x # expr;
where # is common operator and expr is
any expression
• The most common are:
x = x + 1; x = x - 1;
but others also exist
Copyright 2004 - 2006 Curt Hill
Compound Assignments
• A compound assignment operator is
composed of an arithmetic operator
attached to an assignment operator
• Thus:
a += 4;
is the same as:
a = a + 4;
• Most of the two operand arithmetic
operators have corresponding compound
assignments:
-=
*=
/=
%=
Copyright 2004 - 2006 Curt Hill
Precedence
• The only unusual thing about the
compound assignment is the
precedence
• The precedence of all assignments
is very low
– It must be to allow the other operators
to produce a value
• This causes some problems
concerning when an operation
should occur
Copyright 2004 - 2006 Curt Hill
Example
• Consider the two assignments:
x *= a + b;
x = x * a + b;
• These are not equivalent, though we
might expect them to be
• We expect multiply to have higher
precedence than addition
• The compound assignment involving
multiplication still has the lower
precedence of assignment
Copyright 2004 - 2006 Curt Hill
In other words
• Thus:
x *= a + b;
is equivalent to:
x = x * (a + b);
but not with:
x = x * a + b;
• This is directly from the precedence
rules
Copyright 2004 - 2006 Curt Hill
Precedence Revisited
• ()
• ++ -- (postfix)
• ++ -- (prefix) + - (unary)
casts
•* / %
•+ • = *= += -= /= %=
Copyright 2004 - 2006 Curt Hill
An Example Expression
• a *= b-- + ++c/2;
• All variables are integers and start
at 3
• First, b’s value of 3 is obtained
before decrement
• Next, c is incremented to 4
• Division reduces value to 2
• Addition of 3 and 2 give 5
• The value of a (3) multiplied with 5
• Results: a set to 15, b to 2, c to 4
Copyright 2004 - 2006 Curt Hill
A Slightly Different Example
• a *= --b + ++c/2;
• All variables are integers and start
at 3
• First, b’s value of 3 is decremented
• Next, c is incremented to 4
• Division reduces value to 2
• Addition of 2 and 2 give 4
• The value of a (3) multiplied with 4
• Results: a set to 12, b to 2, c to 4
• Only difference is position of –
Copyright 2004 - 2006 Curt Hill
Where can they be?
• Usually find side effect operators
alone or in an assignment:
a += 2*b++ - c--;
• They may also be in conditions:
if(++a == c)
or any other place a variable can be:
System.out.println(a++);
• Not on left hand side of an
assignment:
a++ = 2*r;
is illegal
Copyright 2004 - 2006 Curt Hill
Last Thought
• Do not use a changed variable twice in an
expression such as:
c = a++ * 2 – b*a;
• The problem is: does the a get
incremented before or after the multiply?
• Java has a precise way of handling it, so
that it is consistent on every machine
• However, it is difficult to understand for
people
• Instead use:
c = a * 2 – b*a;
a++;
Copyright 2004 - 2006 Curt Hill