Notes

Outline

Basics of Procedures Procedures encapsulate useful computation in a form that can be reused. In this regard they extend the capability of the computer since the procedure can be used as if it were a primitive instruction.

Importance of Procedures Procedures encapsulate functionality so that it can be reused.  This will be the primary emphasis in today’s lecture. Another important aspect: procedures help manage complexity.  This aspect becomes obvious only when you start writing much more complex programs.

A Scenario: Reading Email You are reading email and your friend living outside the US says the temperature is 38o That’s Celsius, of course. What is it in Fahrenheit? Is it hot or cold, you wonder.  Why doesn’t your computer have a Celsius-to-Fahrenheit converter? This situation arises all of the time … there are many things a computer could do for you, but the software is not available You can step through the process yourself, i.e. convert to Centigrade But what you’d like is to solve the problem once-and-for-all and have the solution packaged-up to be always available What you want is a procedure

The Idea of Procedures Procedures encapsulate computation for general application A procedure’s operation should be hidden from view It must be possible to give data to a procedure and get results back from the procedure All of the possible eventualities must be considered The procedure concept has two parts: A procedure “declaration” -- defines how computation goes Many procedure “calls” -- requests to have the procedure performed

Anatomy Of A Procedure Procedures have the following features Name, a brief description of operation performed Parameters, variables used for passing input in, output out Body, the statements that perform the desired computation The VB6 procedure to convert Celsius to Fahrenheit Name is c2f  (Snyder book uses convertC2F … shortened to fit on slides …) Parameters: input is c; output is f Body is standard conversion equation Blue -- key words and and symbols that are required

Using the c2f Procedure

A Guessing Game Develop a program to guess a person’s weight It starts with a guess of 0 and always stays below the correct answer A weight guess is formulated as: loSide + increment Questions are asked in increments of 100, then 10, then 1

Operation ...

Braining Out The Logic When will guesses be made? Initially, when the program begins (called form_load) In response to a Yes answer In response to a No answer In addition to the first guess what happens at start Initialize loSide = 0 increment = 100 In addition to a guess, what happens on a Yes? Add-in increment, as weight is more than loSide + inc In addition to a guess, what happens on a No? Reduce the increment by dividing by 10 Check if the increment is below 1 … that’ll be the answer

Including A Procedure The fact that a guess must be made in three places is motivation to define a procedure to make the guess (despite the fact that it is a trivial computation)

The Yes/No Logic The “Yes” logic only adds-in, but the “No” logic reduces the increment and must also test for completion

Procedural Abstraction Whenever the same operations are performed in different places in a program, there is an opportunity for procedural abstraction Procedural abstraction gives a name to the operations It also encapsulates the operations so they can be executed out-of-view, receiving input via parameters and influencing the calling environment only by the result(s) returned

Mini-Exercise #1 What is the value of x after the form has been loaded?

Mini-Exercise #1 -- Answer What is the value of x after the form has been loaded?

Mini-Exercise #2 What is the value of y after the form has been loaded?

Mini-Exercise #2 -- Answer What is the value of y after the form has been loaded?