CP2003 Tutorial - 3

1. Abstraction is referred to as a ``powerful and useful tool''. Do you agree or disagree with this statement?


Solution:

Yes, agree with the statement. Use of abstraction allows us to
concentrate on general ideas rather than on specific 
manifestations of these ideas. In programming, abstraction
refers to what a piece of code does rather that how it is
implemented. For example, in C/C++ consider the difference 
between a .h file (prototype declaration-what the program 
does)and a .c file (how the program does it, i.e., its 
implementation).

2. Give an example of a parameterless abstraction (i.e. a procedure, or a function which has no parameters) that does not perform exactly the same computation every time it is called. Under what circumstances, in general, will a parameterless abstraction behave like this?


Solution:

  int a;  // global variable a

  void test() {
    cout << 2*a;
  }
  ...
  int main() {
    a = 2;
    test();
    a = 3;
    test();
    ...
  }
In general, a parameterless abstraction may not perform exactly 
the same computation every time it is called if it relies on the 
exterior environment for at least one of the values that it uses.

3. Consider the following C function:

  void multiply (int m, int n) {
    m = m*n;
    cout <<m<< "," <<n<< endl;
  }

Suppose the function is called with actual parameters i,j where i=2,j=3. If we are using call-by-value show what is printed when called with:

  (a) multiply (i,j) and (b) multiply (i,i).

Now suppose that the parameters could be replaced by call-by-value-result parameters. Repeat parts (a) and (b) and explain the different effect (if any). Assume that the instructions are not executed sequentially.

Solution:

Assuming the instructions are not executed sequentially:

Using call-by-value:
  (a) "6,3"
  (b) "4,2"

Using call-by-value-result:
  (a) "6,3"
  (b) "4,4"

The different effect is that when calling using call-by-value-
result, an alias is set up for the actual parameter, referred to 
via the formal parameter. So, in part b using call-by-value-
result, two aliases are set up for the variable i (one referred 
to by m and one referred to by n), so when m is changed in the 
line "m=m*n" this effectively changes the value of i (because of 
the alias), which (because of the other alias) effectively 
changes n.

4. State the visibility, lifetime, initialized value and purpose of local, global and static variables in C/C++ program.

Storage Type        Local           Static        Global
-------------------------------------------------------------
Visibility         function         function       file
Lifetime           function         program        program
Initialized Value  not initialized    0             0
Purpose            variable used   same as local  variable used
                   by a single     local, but     by several
                   function        retains value  functions

5. In what unusual place can you use a function call when a function returns a values by reference? (hint: function call is usually used at the right hand side of equal sign =). Give a simple example.

Answer:
On the left hand side of the equals sign. Following is a simple
example.

        #include<iostream.h
        int x;                  // global variable
        int& setx();            //function declaration

        
        void main()
           {                    // set x to a value, using
            setx()=92;          // using function call on the left side
            cout <<"\nx=" <<x; //display new value in x
            }

        int& setx()
            {
             return x;          // returns the value to be modified
            }

 

 

In this program the function setx() is declared with a reference type, int&, as the return type: int& setx(). The function returns the values x by : return x. Now though it looks strange, you can put a call to this function on the left side of the equals sign: setx()=92. The result is that the variable returned by the function is assigned the value on the right side of the equals sign. The output of the program is x=92.
 
 

6. (a) What is the principle reason for passing arguments by reference?

(b) Suppose a function has the following declaration: void person(char* name, int age); Modify the declaration so that default value for name is unknown and age 20.

        Answer:
          (a)To modify the original argument.
          (b)void person(char* name="unknown", int age=20);

 

7.  Why modules are important in large programming environment?

Answer: Modularity was introduced to support construction of
large programs. A module is a large program units that can be 
implemented as an independent entity. A well-designed module 
typically has a single purpose and represents a narrow interface 
to other modules. A module typically makes only a few components
visible outside. Such components are said to be exported by the 
module. Other modules remain hidden inside the module and used 
for implementing the visible components. Modules are designed to 
be resuable.

 

8. What are eager, normal-order and lazy evaluation? Consider the following ML function:

fun sqr(n: int)= n*n;
and function call sqr(p+q)with p=2 & q=5

• eager - evaluate at the point of call, evaluate the parameter once and substitute its value for each occurrence of formal parameter.
• normal-order - evaluate whenever needed, re-evaluate again and again.
• lazy - evaluate only when first needed, cache result and use cached value thereafter, this is also known as call-by-need (e.g. in the language Miranda)

1. Write a simple Java program that converts Fahrenheit temperature between 0 to 300 degree into equivalent Celsius. Here is how to write a Java program.

Answer:


class FahrToCel {
          public static void main(String arg[]) {
                int fahr, cel;
                int lower, upper, step;
                lower=0;
                upper=300;
                step=20;

                fahr=lower;
                while (fahr <=upper) {
            cel=5*(fahr-32/9);
            System.out.print(fahr);
            System.out.print("   ");
            System.out.println(cel);
            fahr=fahr+step;
                }
          }
}

How to A Java Program

There are two kinds of Java programs:

Application programs: these independently run.

Each application program must have a main() method;

The name of the java file must have the same name of the class if the class has a public modifier.

The commands to compile the java programs and run the byte-code files are:

   javac programName.java  ---generates a programName.class
   java  programName        --No need the class extension 

// HelloJava.java prints "Hello from Java".
class HelloJava {
      public static void main(String args[]) {
         System.out.println("Hello from Java!");
      }
}

Applets: these programs require supporting environment like Netscape, they do not run independently.

import java.awt.Graphics;
  public class HelloJavaApplet extends java.applet.Applet {
     public void paint(Graphics g) {
       g.drawString("Hello from a Java applet!", 150, 50);
     }
   }

<html>
<applet code="HelloJavaApplet.java" width=275 height=55>
</applet>
<html>

      appletviewer  HelloJava.html