Thursday 2/24 Week 2

Chapter 2

Student Notes

Objectives:

Questions about project 1

Using Loops

Looking at an Exception

Using conditional statements

1. Loops

· Loops are a way to handle

·

· First loop we will look at is the

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· FOR counterName IN StartNumber..EndNumber LOOP

.

.

.

loop body

.

.

.

END LOOP;

· Can also loops.

·

· FOR counter1 IN start1..end1 LOOP

FOR counter2 IN start2..end2 LOOP

Loop body;

END LOOP;

END LOOP;

· FOR loops are also useful because

·

·

· Example: Spiral

FOR Line IN 1..10 LOOP

Spider.ChangeColor(NewColor => Spider.RandomColor);

-- inner loop takes its bound from out count

FOR Count IN 1..Line LOOP

Spider.Step;

END LOOP;

Spider.TurnRight;

End LOOP;

1. Dealing with an Exception

· What is an exception?

·

· These exceptions can be generated by packages and programs

· If we were to have the Spider walk 12 paces from its starting position, it would run into the wall. When this happens, our program crashes an we get an error which looks like this:

Raised SPIDER.HIT_THE_WALL

Traceback Information

Program Name File Name Line

------------ ---------- ----

spider.step spider.adb 264

spider_crash spider_crash.adb 16

1. dealing with this exception in a more graceful way

· The Spider package gives us a function called AtWall which will return a Boolean value when the Spider is next to the wall

· Boolean value =

·

· FUNCTION AtWall RETURN Boolean;

-- pre: None

-- post: Returns True if the Spider is standing next to a wall.

· Exception handling is a subject that we will spend a lot of time on later, however

·

·

·

· conditional statements

· IF Spider.AtWall THEN

EXIT;

END IF;

· What’s happening here?

·

·

· Why can we just say Spider.AtWall?

2. The General Loop

· Structure:

LOOP

IF Spider.AtWall THEN

EXIT;

END IF;

Spider.Step;

END LOOP;

· A Warning: Beware the infinite loop.

·

·

· We would have to forcibly quit the program (^C)

· Notice that we could have used a more concise wording of the IF statement by using the EXIT WHEN statement. This is very useful for loops.

· LOOP

EXIT WHEN Spider.AtWall;

Spider.Step;

END LOOP;