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Programming: Simple Control Structures

Programming: Simple Control Structures. MMP 220 Multimedia Programming.

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Programming: Simple Control Structures

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  1. Programming: Simple Control Structures MMP 220 Multimedia Programming The material in this handout is taken from “Learning to Program with Alice” a project supported by the National Science Foundation under Grant NSF-0126833, NSF-0302542, and NSF-0339734. Contact Wanda Dann: wpdann@ithaca.edu, Stephen Cooper: scooper@sju.edu, Randy Pausch: pausch@cmu.edu, and Don Slater: dslater@cmu.edu http://www.aliceprogramming.net/ This adapted material was prepared for students in MMP220 as as part of a curriculum redesign project funded by National Science Foundation grant #0511209 Co PI’s Christopher Stein and Jody Culkin BMCC CUNY http://teachingmultimedia.net

  2. Control Statements • We have been using Do in order and Do together to control the way instructions are executed in your Alice program. • Control statements can also be used for • conditional execution • repetition

  3. Conditional Execution • Conditional execution is where some condition is checked and a decision is made about whether a block of the program will be executed. • Conditional execution is extremely useful in • games • simulations • real-time controls, e.g. robot systems

  4. Example • As a simple example of conditional execution, let's revisit the FirstEncounter world. • As the robot moves closer, the alien hides behind the rocks. • We want to check a condition (is robot to short to over the rock?) and then perform an action based on the whether the condition is true.

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

  6. Storyboard • A storyboard design for this conditional statement is: • The condition in an If statement is a Boolean function 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

  7. Demo • Ch03Lec2FirstEncounterIfElse • Concepts illustrated in this example program: • The condition of an If/Else statement is constructed by using a function that returns a Boolean value (true or false). • Do nothing in the Else part of the statement means that if the condition is false, Alice will skip this part and go to the next instruction.

  8. A different scenario • In some cases, the built-in functions are not sufficient for a condition that we want to check. • For example, the built-in function is shorter than compares the heights of two objects. • Suppose, however, that we wanted to compare the robot's height to 2 meters. • How can we compare the height of the robot to a specific measurement (2 meters)?

  9. Relational Operators • In situations where you need to write your own comparison, you can use a relational operator. • Relational operators are provided in the World's built-in functions.

  10. Demo • Ch03Lec2FirstEncounterRelationalOps • Concepts illustrated in this example program: • Relational operations are defined using the world'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 by either a function or a number.

  11. Need for Repetition The walking action is not realistic: the robot moves forward to the rock (a distance of several meters) but the legs are simulating a walking action only once.

  12. One meter, one step • A more realistic action is to move the robot forward one 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 and again to animate the robot moving close to the rock.

  13. Loop The Loop statement is a simple control structure that provides for repeating an instruction (or block of instructions) a counted number of times.

  14. Demo Ch03Lec2FirstEncounterLoop

  15. Demo • Ch03Lec1FirstEncounterExpressionV2 • Subtracting 2 meters from the distance is an arbitrary amount. • To be more precise, we could subtract the width of the rock. • The resulting expression subtracts the value of one function from the value of another function.

  16. Assignment • Read Chapter 3

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