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### CS 330 Organization of Programming Languages

Soundararajan Ezekiel

Department of Computer Science

Ohio Northern University

ADA, Ohio 45810

e-mail: [email protected]

http://www.onu.edu/user/FS/sezekiel

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|)

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

Expressions and Assignment Statements

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

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

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

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

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

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

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

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

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

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;

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)

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

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

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

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

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

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