CS 330 Organization of Programming Languages

1. CS 330 Organization of Programming Languages Soundararajan Ezekiel Department of Computer Science Ohio Northern University ADA, Ohio 45810 e-mail: s-ezekiel@onu.edu http://www.onu.edu/user/FS/sezekiel

2. FORTRAN CODE WALK PROBLEMS • 1. Wrtie a program to read two n-dimensional vectors and then evaluate • the norm of each vector • unit vectors having the same direction as the vectors • the sum, difference and dot product • the cosine of the angle between the vectors calculated using cos0 = a.b /(|a||b|)

3. 2. Write a program that reads a list of numbers , count them, and the calculate their mean, variance, and standard deviation. Print how many numbers there are and their mean, variance, and SD

4. Expressions and Assignment Statements • Introduction • Arithmetic expressions • Overloaded Operators • Type Conversions • Relational and Boolean Expressions • Short-Circuit Evaluation • Assignment Statements • Mixed-Mode Assignments

5. Introduction • Expressions are the fundamental means of specifying computations in a programming language • Programmers should understand both syntax and semantics of expression • To understand evaluation -- understand the order of operator and operand evaluation • An assignment statement can simply cause a value to be copied from one memory cell to another-- in many case it include expressions with operators, which cause values to be copied to the processor and to be operated on, and the result to be copied back to memory • simple assignment statements specify an expression to be evaluated and a target location in which to place the result of the expression evaluation • we will see number of variation on this basic form

6. 2. Arithmetic Expression • Most of the characteristic of Arithmetic expression were inherited from Math • Consists of operators, operands, parentheses, and function calls • operator can be • unary--- single operands • binary -- double operands • C, C++, Java, - ternary-- three operands • Most cases-- binary operators are infix-- between their operands • In Perl-- some of them are prefix-- precede their operands

7. The purpose is to specify an arithmetic computation • Implementation of such computation must cause two action • fetching the operands-- usually from memory • executing arithmetic operations on those operands • Design issues • 1. What are the operator precedence rules • 2. What are the operator associativity rules • 3. What is the order of operand evaluation • 4. Are there restrictions on operands evaluation side effect • 5. Does the language allow user-defined operator overloading • 6. What mode mixing is allowed in expression

8. Precedence • Consider the following expression -- a+b*c • suppose a=3, b=4, c=5---- evaluate left to right=> 35, right to left=> 23 • FORTRAN:- **,* /+- • Pascal: *,/,div, mod+ - • Ada: **, abs*/ unary +/binary +- • C: postfix ++, --prefix ++.-- , unary +-,*/&binary +- • APL is odd among languages because it has single level of precedence

9. Associativity • consider the following expression a-b+c-d • here + - are the same level of precedence-- this case which operator is evaluated first is answered by the associativity rule of the language • Fortran:- left to right ( exponential right to left A**B**C ) • Ada: exponential is nonassociative-- A**B**C is illegal () should be used that is (A**B)**C or A** (B**C) • FORTRAN and Ada have the exp operator

10. Language Associativity Rule FORTRAN Left: */+- Right: ** Pascal Left: all C Left: postfix++, postfix--, */%binary+ binary- Right: prefix++ prefix--, unary + unary- C++ Left: */%binary+ binary- Right: ++, --, unary -, unary + Ada Left: all except ** Nonassociativity ** in APL: AxB+C (x means multiplication) A=3, B=4, C=5 then 27 How: associative right to left-- addition first then multiplication

11. Parentheses • programmers can alter the precedence and associativity rules by placing () in expression --- (A+B)*C • programmer would specify the desired order of evaluation with parentheses • the disadvantage of this scheme is that it makes writing expression more tedious and it also seriously compromises the readability of the code

12. Conditional expression • we will look at the the ternary operator ?: which is part of C, C++ Java • this operator is used to form conditional expression • sometimes if then else statements are used to perform a conditional expression assignment • example • if (count = = 0) • average =0; • else • average= sum/count; • In C, C++ and Java this can be specified more conveniently in an assignment statement using condition expression

13. expression_1 ? Expression_2 :expression_3 • expression_1 is Boolean expression--- if that is true do expression_2 otherwise expression_3 • average= (count = = 0)? 0 : sum/count;

14. Operand evaluation order • A less commonly discussed design characteristics of expression is the order of evaluation of operands • Side Effects:- A side effect of a function called a functional side effect, occurs when the function changes either of its parameters or global variable ( variable declared outside of the function)

15. Example: • consider the expression a+fun(a) • if fun does not have the side effect of changing a, the order of evaluation is two operands a fun(a) has no effect on value of the expression • if fun changes the value of a say divide by 2 and change its parameter to have the value 20 then it matters • a=10; • b=a+fun(a) • two answers--- 15 from left to right and 25 from right to left

16. C example • the following c code will the same problem like before • int a=5; • int fun1(){ • a=17; • return 3; • } /* of fun1*/ • void fun2(){ • a=a+fun1(); • } /* of fun2*/ • void main(){ • fun2(); • }/* of main */ The value computed for a in fun2 depends on the order of evaluation of the operands in the expression a+fun1() the value will be 8 or 20

17. Solution • 1. The language designer could disallow functional side effect • 2. Avoid the problem by stating in the language definition that operands in expressions are to be evaluated in a particular order and demand that implementors guarantee that order

18. Overloaded operators • Arithmetic operators are often used for more than one purpose • example:- + for addition In Java it is used for string catenation • This multiple use of an operator is called operator overloading

19. Example • consider the use of ampersand in (&) in C • As a binary operator--- it specifies bitwise logical AND operation • As a Unary operator-- with variable as its operand, the expression value is the address of that variable-- this case & is called address-of operator • example x=&y execution of this causes the address of y to be placed in x