Expressions and assignment

1. Expressions and assignment • Points for discussion • Arithmetic expressions • Overloading • Logical expressions • Assignment

2. Arithmetic expressions • An arithmetic expression is a function that maps one or more numbers into a number. Operators are usually written in the infix form (unless it is an expression in Scheme J). • Operator precedence or strength: the order of evaluation of expressions with more than one operator. (Parentheses can be used to specify order explicitly.)

3. Arithmetic expressions (2) • Operators are usually grouped as follows. • exponentiation: ** • unary operators: abs, prefix + and -, etc. • (negation is also a unary operator) • multiplicative: *, /, div, mod etc. • (conjunction is also multiplicative) • additive: binary +, binary - etc. • (disjunction is also additive)

4. Precedence rules Precedence rules differ a lot between languages. • Pascal • multiplicative > additive • C • self-increment and unary >multiplicative > additive • Ada • ** > multiplicative >unary + and - > additive • Fortran • ** > multiplicative > additive

5. Associativity (1) • Let • be any binary operator. It associates: • left to right if x • y • z = (x • y) • z • Pascal, Ada, C (all normal operators) • right to left if x • y • z = x • (y • z) • C (self-increment operators ++ and --) • There also are nonassociative operators, such as exponentiation in Ada: • x**y**z is syntactically incorrect, though • (x**y)**z and x**(y**z) are OK.

6. Associativity (2) • Some language have no precedence and one associativity rule. • In APL: always right to left. • x + y * z correctly means x + (y * z) • x * y + z means x * (y + z)(!) • In Smalltalk: always left to right. • x * y + zcorrectly means (x * y) + z • x + y * z means (x + y) * z(!)

7. Conditional expressions • First introduced in Algol 60: • if x > 0 then 1 elseif x = 0 then 0 else -1 • The same is available in the C family: • (x > 0 ? 1 : (x == 0 ? 0 : -1)) • Conditional expressions are essential in Scheme—they are a principal control structure!

8. Overloading • Overloading occurs when a name or symbol has several different uses or meanings. Here are a few examples in Pascal: + integer addition, floating-point addition, string concatenation, set union * integer multiplication, floating-point multiplication, set intersection abs integer  integer, real  real

9. Overloading (2) • Overloading can be always resolved by context if all operands have known types. • In 2 + 3 the plus works with integers,in "a" + "cde" there are strings. • In Ada, overloading is an important element of the language design. Ada is extendible: a new meaning can be given to an operator in addition to the present meaning. • Overloading is also possible in C++.

10. , , b c d Overloading (3) • Overloading can be quite confusing. In C: • &bitwise conjunction and address, • *multiplication and dereferencing. • In PL/I, = denotes both equality and assignment. • In Prolog, the comma is heavily overloaded. • conjunction a :- b, c, d. • argument separator a(b, c, d) • list element separator [b, c, d] • a functor: • (b, c, d) means

11. Coercion • If objects of two numeric types appear as operands, we "upgrade" the lower type. • Numeric type hierarchy in Fortran: • integer < real < double < complex • In Java byte, short and char are almost always coerced to int before an aritmetic operation is applied—and converted back from int afterwards.

12. Logical expressions • The comparison operators have many forms • equal = == .EQ.not equal <> != .NE.less < < .LT.less or equal <= =< .LE.greater > > .GT.greater or equal >= => .GE. Pascal   Fortran IV

13. Logical operators • Pascal not,and,or • Java !,&&,&,||,| • Ada not,and,andthen, or,orelse,xor • Short-circuit operations perform lazy evaluation: they stop after the first true in "or", after the first false in "and". The operators &&,andthen,||,orelse all work like this.

14. Assignment • Assignment works in the same way in all imperative and object-oriented languages. Only the operators may look differently: • target := expression Algol, Pascal, Ada • target = expression Fortran, C, Java • target  expression Smalltalk • There is no assignment in Prolog, and nothing like it in pure Scheme.

15. Multiple assignment • Multiple assignment is more interesting. • PL/I: A, B := EXPR; • This is quite obvious: calculate the value of EXPR and send to A and to B, in any order. • Algol 60: A := B := EXPR; • (Step 1) Find the value of EXPR. • (Step 2) Assign this value to B, then to A. • (or Step 2) Assign this value to A, then to B.

16. Multiple assignment (2) • The order may be important. Consider: • I := 5; A[I] := I := 10; • The order in which target addresses are calculated may change the result. • One method: • (1) Find all target addresses. • (2) Find the value of EXPR. • (3) Assign this value to A and B. • With this method, A[5] := 10.

17. Multiple assignment (3) • Another method: • (1) Find the value of EXPR. • (2) Find target addresses left-to-right, • assign the value to every address. • With this method, again A[5] := 10. • A third method: • (1) Find the value of EXPR. • (2) Find target addresses right-to-left, • assign the value to every address. • With this method, A[10] := 10.

18. More on assignment • This statement in C is not a multiple assignment: • A = B = EXPR; • Here, B = EXPR has a value (equal to the value of EXPR). This value is next assigned to A: the assignment operator in C associates right-to-left.

19. More on assignment (2) • Another well-known syntactic variation in C, C++, Java mixes assignment with arithmetics. • A += B; means A = A + B; • A *= B; means A = A * B; • and so on. • Finally, in C++ we can have conditional targets. This may be really hard to follow. • (x != 0 ? y : z) = 17; • or (even less readable!) • x ? y : z = 17;