1. Copyright © 2004-2007 Curt Hill The Assignment Operator and Statement The Most Common Statement you will use

2. Copyright © 2004-2007 Curt Hill Changing variables What can change in a variable? –Only the value –The type and name are set only at compile time What statements can change the value? –Declaration might or might not initialize –An assignment –Some types of method calls –Input statements are a type of method call

3. Copyright © 2004-2007 Curt Hill Assignment Statement Simple form: variable = expression ; Simple meaning: the value computed from the expression is stored in the variable Similar to the assignment in very many languages

4. Copyright © 2004-2007 Curt Hill What is an expression? A legal combination of the following: Constants Variables Operators The legal combinations are usually intuitive

5. Copyright © 2004-2007 Curt Hill Legal Examples Suppose the following declarations: int a,b,c; double x,y,z; The following are legal: a = 5; // constant x = y; // variable y = 2*z – x/y; // expression

6. Copyright © 2004-2007 Curt Hill General rules Item on left must be a variable The two sides should be of the same type with a few exceptions –A weaker type may be converted to a stronger type –A float is weaker than a double –A double is stronger than an int The right hand side items are not changed The old value of the left hand side is lost when a new value is stored

7. Copyright © 2004-2007 Curt Hill More Examples Suppose the following declarations: int a,b,c; double x,y,z; The following are not legal: 5 = -c; // cannot change a = 5.8; // loses precision x = false; // incompatible x = y // no semicolon x = y + * z; // malformed expr The following are legal: x = a; // Stronger type

8. Copyright © 2004-2007 Curt Hill Expression Values What is the value of: 2 + 3*2 + 1 Two reasonable candidates: 11 = ((2+3) * 2) + 1 –Left to right evaluation –Some calculators do it this way 9 = 2 + (3*2) + 1 –The multiplication precedes addition –Algebra uses this way We must choose one of these

9. Copyright © 2004-2007 Curt Hill Precedence Clearly we cannot have both 11 and 9 correct or we never know what a computation produces Java like most (but not all) programming languages follows Algebra Multiplication precedes addition This gives rise to a precedence chart –Shows the level of importance for operators

10. Copyright © 2004-2007 Curt Hill Precedence Chart Highest to lowest –( ) –- + (Unary sign change) –* / % –+ - (Binary arithmetic) There are several others which we will encounter soon enough

11. Copyright © 2004-2007 Curt Hill Algebra and the Assignment There is often confusion about the assignment because it looks like an equation An equation is a true or false statement An assignment is a command to perform some action

12. Copyright © 2004-2007 Curt Hill Equations and Assignments The equation: x = y + 5 –States that the value of x is five larger than y’s value –This may be true or false depending on the value of x and y –Usually our job is to find an x and y that makes it true – this is called a solution The assignment: x = y + 5; –Commands that the value of x is computed to be five plus y’s value

13. Copyright © 2004-2007 Curt Hill Another Example The equation: x = x + 1 has no solution –It can never be true –No value may equal itself plus one The assignment: x = x + 1; increments x –It is a command and will be carried out

14. Copyright © 2004-2007 Curt Hill Another Example Suppose: int a=2, b=5, c=-7; What happens to the variable values when: c = a*b - b%c+1; b = a-c/(1+a*2); is executed? The value of c becomes 6 and the value of b becomes 1

15. Copyright © 2004-2007 Curt Hill How does this work? int a=2, b=5, c=-7; c = a*b - b%c+1; c = 2*5 – 5%-7+1 c = 10 – 5 +1 c = 6 b = a – c/(1+a*2); b = 2 – 6/(1+2*2) b = 2 – 6/(1+4) b = 2 – 6/5 b = 2 – 1 b = 1

16. Copyright © 2004-2007 Curt Hill The assignment operator Some languages have an assignment statement –Pascal –BASIC, Visual BASIC The C family (C, C++, Java) treats = as a side-effect operator What is the difference?

17. Copyright © 2004-2007 Curt Hill = as operator The following is legal: a = b = 5; The 5 is first assigned to b Next b is assigned to a This is known as a multiple assignment and some other languages support this There is more

18. Copyright © 2004-2007 Curt Hill = really is an operator The following statement is also legal: a = (b = 2*b) - (c=2); It is equivalent to the following sequence: c=2; b = 2*b; a = b – c;

19. Copyright © 2004-2007 Curt Hill Side-Effect Operator Any operator that changes one of its operands is called a side-effect operator Most operators produce a value but do not change their operands For now the assignment operator is the only one, but there are more Method calls may also have side-effects An input statement must have a side- effect while an output will usually not

20. Copyright © 2004-2007 Curt Hill Another Property of Operators What is the value: 8 – 4 – 3 Is it? (8 – 4) – 3 = 1 8 – (4 – 3) = 7 How about exponentiation: 2 3 2 Is it (2^3)^2 = 64 or 2^(3^2)=512?

21. Copyright © 2004-2007 Curt Hill Associativity The concept of associativity is the property that determines the order of execution when the same operator is used twice in a row Subtraction is left to right Exponentiation is right to left Most operators in Java are left to right The exception is the assignment operator

22. Copyright © 2004-2007 Curt Hill Assignment Operator Properties The associativity of = is right to left Thus a = b = c; is the same as a = (b = c); The assignment operator also has low precedence If it did not, then it would not allow all computations on the right to complete This is why this needs parentheses: a = (b = 2*b) - (c=2);

23. Copyright © 2004-2007 Curt Hill Precedence Chart Again Highest to lowest –( ) –- + (Unary – sign change) –* / % –+ - (Binary arithmetic) –=–= Yes, there are quite a few more!