1. Side Effect Operators Changing the value of variables
Copyright Curt Hill

2. 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 Curt Hill

3. 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 Curt Hill

4. 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 Curt Hill

5. 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:
Copyright Curt Hill

6. 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 Curt Hill

7. 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 Curt Hill

8. 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 Curt Hill

9. 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 Curt Hill

10. 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 Curt Hill

11. 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 Curt Hill

12. 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 Curt Hill

13. 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 Curt Hill

14. 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 Curt Hill

15. Precedence Revisited () ++ -- (postfix)
++ -- (prefix) + - (unary) casts
* / %
+ -
= *= += -= /= %=
Copyright Curt Hill

16. 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 Curt Hill

17. 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 Curt Hill

18. 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 Curt Hill

19. 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 Curt Hill