Ada Fundamentals - Structured Statements

Overview

• Loops
• For Loops
• While Loops
• General Loops
• Loop and a Half
• Named Loops


• Selection Statements
• If
• Case


• Some Java Errors

For Loop Statement

• For Loop Structure:

            for i in start .. finish loop
                -- loop body here
            end loop;
            

• Execute loop for each value in range range start .. finish
• if start = finish, execute 1 time
• if start > finish, execute 0 times


• No brackets needed: Mark end of loop with end loop;
• end loop; is REQUIRED even if the loop body contains only one statement


• Loop variable does not need to be declared
• Loop variable not visible outside loop
• Loop variable hides variable with same name (Not recommended)


• Loop variable cannot be modified


• What does this do: for i in reverse 1 .. 10 loop


• What happens here: JavaFor.java
• What does the Ada version look like?

Three Kinds of Loops

• For loop

            for i in 1 .. 10 loop
              ...
            end loop;
            

• While loop

            i := 1;
            while i <= 10 loop
              ...
                i := i + 1;
            end loop;
            

• General loop (infinite loop that can have an exit)

            i := 1;
            loop
              exit when i = 10;
              ...
                i := i + 1;
            end loop;
            

Loop and a Half

• Is exiting from the middle of a loop a good idea?


• How to code input, test, process, input, test, process, input, ...


• Contrast this:

            input i
            while i /= flagValue loop
               process i
               input i
            end loop;
            

• With this:

            loop
               input i
               exit when i = flagValue;
               process i
            end loop;
            

• Which is shorter, clearer, more error prone


• Guidelines for using exit:
• Use exit when rather than if C then exit
• If possible, use only one exit statement
• Keep multiple exit statements together

Named Loops

• Loops can be named


• Example:

 
Search:  -- Name the loop
for i in 1 .. 10 loop

   for j in 1 .. 20 loop
      ...
      exit when ...  -- Exits inner loop
      ...
      exit Search when ...  -- Exits outer loop
   end loop;

end loop Search;

If Statements

• No parens around condition


• then required


• end if; is required, even if body has only a single statement


• keyword elsif
• allows multiple conditions in single if
• avoids nesting
• Can have as many elsif statements as desired
• else statement optional


• Two kinds of languages regarding brackets
1. brackets optional for single line bodies: no elsif
2. brackets always required: elsif


• Remember: if and for statements have end if; and end for;


• Remember goal: concern for programmer

Case Statement - Multiway Selection

• Java version - what is the output?

      i = 3;
      switch (i){
        case 4: System.out.println("****"); 
        case 3: System.out.println("***"); 
        case 2: System.out.println("**"); 
        case 1: System.out.println("*"); 
        default: ;
        } 

• Ada version - what is the output?

      i := 3;
      case i is 
         when 4 => put_line("****");
         when 3 => put_line("***");
         when 2 => put_line("**");
         when 1 => put_line("*");
         when others => null;
      end case;

• Ada does not fall through to next case
• when others required if all values not accounted for
• Ada uses null; instead of solely ;

Case Statement - Multiple-Value Selection

• Multiple value selection in Ada and Java:

      -- Ada:
      case i is 
         when 4|3 => put_line("****");
         when 2|1 => put_line("**");
         when others => null;
      end case;

      // Java 
      switch (i){
        case 4: 
        case 3: System.out.println("****"); break;
        case 2:
        case 1: System.out.println("**"); break;
        default: ;
        } // End of break
  

Some Java Code

• Check for division by 0

        if (b == 0)
            if (a > 0)
                res = MAX_INT;
        else
            res = a / b;
            b++;
        

• Under what conditions is i = j executed?

        if (a == b)
            if (e == f)
                g = h;
        else
            i = j;
        

• What about this code:

            // If signal is clear, increase speed
            void increase_speed_if_safe (int speed, int signal)  
            {
                if (signal == CLEAR && speed < MAX_SPEED); 
                    increase_speed ();
            }
            

• How many times is the S.o.p executed?

        i = 0;
        while (i <= 10);
            System.out.println(i++);
            

• Some test code and a prettified version


• How to code these in Ada?

Example Program

• Let's get some practice by writing a simple program


• Let's sum a group of integers and calculate and print their average


• First we'll input the number of numbers and read that many numbers


• Can we format the average of the numbers


• What if the numbers are floating point?


• Let's make sure the number of numbers is non-negative


• Now let's read until eof