Fall 2007ACS-1805 Ron McFadyen1 Chapter 3 Programming - Putting Together the Pieces

Fall 2007ACS-1805 Ron McFadyen2 The Pieces Chapter 3 introduces the fundamentals of programming. Construction of a program usually requires us to include: Simple instructions Control structures Functions Expressions

Fall 2007ACS-1805 Ron McFadyen3 The Pieces A simple instruction - Commanding an object to do a certain action E.g. getting the ice skater to turn left:

Fall 2007ACS-1805 Ron McFadyen4 The Pieces Control structure: Alice offers several types E.g. choosing between two alternatives - in the World.BarrelRoll method of the flight simulator:

Fall 2007ACS-1805 Ron McFadyen5 The Pieces Control structure: Alice offers several types E.g. repeating something – in the flight simulator World.BeginFlying contains the loop where collisions are tested for continuously:

Fall 2007ACS-1805 Ron McFadyen6 The Pieces Function In the flight simulator, once the plane is within a metre of a ring the plane is considered to have gone through the ring and the ring disappears. The code uses a built in function to determine this:

Fall 2007ACS-1805 Ron McFadyen7 The Pieces Expressions Certain information needed for a simulation may require us to calculate things using arithmetic The flight simulator keeps track of the number of rings acquired:

Fall 2007ACS-1805 Ron McFadyen8 Examples Consider the rock, alien and robot in the First Encounter. We shall demonstrate Simple instructions Functions Expressions Do together If/else Loop

Fall 2007ACS-1805 Ron McFadyen9 Example Consider the rock and the robot We want the robot to move towards the rock A simple instruction can command the robot to move towards the rock:

Fall 2007ACS-1805 Ron McFadyen10 Example Consider the rock and the robot We can use the distance function to find out how far apart the robot and the alien are. We can do more, consider the distance calculated using functions and an expression : The above is complex but when we know what we want Alice lets us build it fairly easily

Fall 2007ACS-1805 Ron McFadyen11 Example Consider the rock and the robot We need the robot to be walking Using the Do Together control structure we have the moving and walking occurring at the same time :

Fall 2007ACS-1805 Ron McFadyen12 Example Consider the rock and the robot If the robot is shorter than the rock we want the robot to raise its head, otherwise it doesn’t have to : The above uses an If/Else decision structure to determine if an action needs to be executed or not.

Fall 2007ACS-1805 Ron McFadyen13 Example Consider the rock and the robot Previously we had the robot walking to the rock, but the robot took 1 step with each of 2 legs regardless of the distance. Suppose we want the walking effect to appear with each metre travelled – we can use a Loop: See next slide for a more advanced “times” calculation

Fall 2007ACS-1805 Ron McFadyen14 Example Consider the rock and the robot An expression to calculate the number of times to move a single metre:

Fall 2007ACS-1805 Ron McFadyen15 Functions Functions are designed to return certain kinds of values Number (e.g. number of metres to move) Boolean (produces a value of true or false)

Fall 2007ACS-1805 Ron McFadyen16 Expressions Expressions are designed to return certain kinds of values Arithmetic expressions return numeric values Expressions involving, >=, ==, != return boolean values The values returned by the height function for the robot and the rock are compared to determine if the robot is shorter than the rock

Fall 2007ACS-1805 Ron McFadyen17 Tips/Techniques Some properties of objects and world: Texture maps: we can change the appearance of an object by altering its texture. This is done by importing a “gif” and using it as the texture for an object. The text modifies a place to give the appearance of a cookie. Fog: we can incorporate fog in a world and adjust its density for a special effect.