Program Structures

[horace1]

Program Structures
Structured programming has only three structures:
Sequence, Selection and Iteration.

Selection allows choice of paths in program.

The if statement

if (condition)
then operation

e.g. in pseudo code

        if month is October
        then write "new term"
        if month is June
        then write "exams!"

This may be extended to include an else clause:
a)

        if mark is greater than or equal to 40
        then write "pass"
        else write "fail"

To take this further:
b) [code]
if mark is greater than or equal to 70
then write "brilliant"
else if mark is greater than or equal to 40
then write "pass"
else write "fail" [code]

Indentation: line else up with corresponding if.

Indentation makes meaning clear. Else is lined up to match the if that it belongs to.
Assume that mark is int
a)

        if (mark>=40)
             System.out.println("pass");
        else System.out.println("fail");

b)

        if (mark>=70)
             System.out.println("brilliant");
        else if (mark>=40)
                  System.out.println("pass");
             else System.out.println("fail");

The comparison operators are:
< <= > >= == !=

A program using Scanner to read data from the keyboard and if-then-else:

import java.util.*;

public class Ifmarks
{
  public static void main(String args[])
    {
     Scanner keyboard=new Scanner(System.in); // open Scanner
     final int PASS = 40;
     final int DISTINCTION = 70;
     System.out.print("type a mark: ");
     int mark = keyboard.nextInt();           //read an integer
     if (mark>=DISTINCTION)
          System.out.println(" brilliant");
     else if (mark>=PASS)
               System.out.println(" pass");
          else System.out.println(" fail");
    }
}

Here we also see the use of constants: if a value will not change it may be declared as a constant which, by convention, are written in upper case. This has the advantage that if a constant changes (such as the distinction mark above) it need only be changed once rather than having to edit multiple occurances in a large program.

More than one statement can be placed on a branch of an if by including in {}. e.g. assuming that i is int indicate if it 0, positive or negative and if negative change its sign

   if (i<0)
        {
          System.out.println("i is negative, inverting");
          i=-i;
        }
   else
          if (i==0) System.out.println("zero");
          else
          {
            System.out.println("i is positive");
            System.out.println("no change");
          }

The else is lined up with corresponding if and the {} are lined up vertically