Programming: Simple Control Structures

Programming:Simple Control Structures

Alice

Control Statements

We have been using Do in order andDo together to control the way instructionsare executed in your Alice program.

Control statements can also be used for

conditional execution

repetition

Conditional Execution

Conditional execution is where somecondition is checked and a decision is madeabout whether a block of the program will beexecuted.

Conditional execution is extremely useful in

games

simulations

real-time controls, e.g. robot systems

Example

As a simple example ofconditional execution,let's revisit theFirstEncounter world.

As the robot moves closer,the alien hides behind therocks.

We want to check acondition (is robot to shortto over the rock?) and thenperform an action based onthe whether the condition istrue.

If/Else

In Alice, an If/Else control statement is used tocheck a condition and make a decision.

Storyboard

A storyboard design for this conditionalstatement is:

The condition in an If statement is a Booleanfunction that yields a true or false value.

If spiderRobot is shorter than rock

Do in order

spiderRobot neck move up

spiderRobot neck move down

Else

Do nothing

Demo

Ch03Lec2FirstEncounterIfElse

Concepts illustrated in this exampleprogram:

The condition of an If/Else statement isconstructed by using a function that returns aBoolean value (true or false).

Do nothing in the Else part of the statementmeans that if the condition is false, Alice willskip this part and go to the next instruction.

Another Example

As another example ofconditional execution, let'srevisit the Egyptian scene inthe Hollywood movie set.

The camera angle canmislead our perception of thescene

We want to check a condition(is the mummy taller than thepharaoh?) and then performan action based on thewhether the condition is true.

Storyboard

In our example, we can demonstrate which object is thetallest by having that object turn around.

A storyboard design for this action is:

The condition in an If statement is a Boolean functionreturning a Boolean (true or false) value.

If mummy is taller than pharaoh

mummy turns 1 revolution

Else

pharaoh turns 1 revolution

Demo

Ch03Lec2mummyIfElse

A different scenario

In some cases, the built-in functions arenot sufficient for a condition that we wantto check.

For example, the built-in function is shorterthan compares the heights of two objects.

Suppose, however, that we wanted tocompare the robot's height to 2 meters.

How can we compare the height of the robotto a specific measurement (2 meters)?

Relational Operators

In situations where youneed to write your owncomparison, you can usea relational operator.

Relational operators areprovided in the World'sbuilt-in functions.

Demo

Ch03Lec2FirstEncounterRelationalOps

Concepts illustrated in this example program:

Relational operations are defined using theworld's built-in functions.

Placeholder values are first selected for the "a"and "b" components of the relational expression.Then the placeholder values are each replaced byeither a function or a number.

Another scenario

Let’s compare the height of the mummy to a specificmeasurement (2 meters).

Ch03Lec2mummyRelationalOps

Need for Repetition

The walking action is not realistic: the robotmoves forward to the rock ( a distance ofseveral meters) but the legs are simulating awalking action only once.

One meter, one step

A more realistic action is to move the robot forwardone meter for each step made by the legs.

But, now the robot is moving forward only 1 meter.This block of code will have to be repeated again andagain to animate the robot moving close to the rock.