Pascal Programming

1
Pascal Programming

• Functions, Procedures and Parameters
• Carl Smith
• National Certificate Year 2 Unit 4

2
Overview

• Procedures and functions are referred to
  collectively as routines or modules
• They are self-contained statement blocks that can
  be called from different locations in a program.
• A function is a routine that returns a value when
  it executes.
• A procedure is a routine that can change values
  in the main program (or not!).

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Parameters

• Parameters are variables or values passed to a
  function/proc from the calling program, in Pascal
  there are two types-
• Value parameters are formal parameters that pass
  data into a procedure or function, but not out of
  a procedure or function.
• Variable parameters are formal parameters that
  pass data into a procedure and out of a
  procedure. Variable parameters can be used to
  return the results of a procedure.
• E.G. x sqrt (integer_var) built in
  function MYPROC (var1, var2, var3) user
  defined

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Four differences between functions and procedures

1. Functions return only one valueProcedures can
   return or change any number of values including
   none.
2. A function call must be part of an expression, a
   procedural call is an individual statement.
3. Functions return their results via the name of
   the function, the function_identifier.
   Procedures only return its results via its
   parameters - never the procedure_identifier.
4. Functions can use only value parameters.
   Procedures can use value and variable parameters.

5
Exiting procedures

• When a procedure or function completes it will
  naturally return back to the point from which the
  routine was called.
• You can return from a procedure by using Exit
  within the body of any procedure or function (It
  is normally used within an IF statement).
• Exit halts execution of the routine where it
  occurs and immediately passes program control
  back to the point from which the routine was
  called.

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Why use functions and procedures?

• The use of functions and procedures enables you
  to write modular programs which are more
  structured and easier to debug and modify
• Enables the use of re-useable code
• Provides encapsulation which hides the details
  of the subprogram. We dont have to worry how a
  subprogram works, just when it should be
  called.This allows teams to work on large
  systems more effectively.

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When to use a function

• Questions to ask when creating a function
• What will the function do, remember that the
  function should only do one task.
• What parameters will be required ?
• What will the type of the parameters be ?
• What will be the type of the function result ?
• What local variables may be declared ?

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Procedure Declaration

• procedure procedureName(parameterList)
  localDeclarationsbegin statementsend
• where procedureName is any valid identifier,
  statements is a sequence of statements that
  execute when the procedure is called, and
  (parameterList), and localDeclarations are
  optional.

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Example Procedure

• procedure PerformComputations (PizzaCost real
• PizzaSize Integer
• VAR PricePerSquareInch real)
• DATA Received Cost and Size of Pizza
• INFO Returned Price per square inch of pizza
• LOGIC Given the diameter, find the radius then
  Compute the area using Area Pi sqr(Radius)
• VAR
• Radius, Area real local variables
• BEGIN
• Radius PizzaSize / 2
• Area Pi sqr(Radius)
• PricePerSquareInch PizzaCost / Area
• END of Procedure PerformComputations

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Function Declarations

• function functionName(parameterList) returnType
  localDeclarationsbegin statementsend
• where functionName is any valid identifier,
  returnType is any data type, statements is a
  sequence of statements that execute when the
  function is called, and (parameterList), and
  localDeclarations are optional.

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Example Function

• Function PricePerSquareInch (Cost, Size real)
  real
• DATA Received Cost and Size of Pizza
• INFO Returned Price per square inch of pizza
• LOGIC Given the diameter, find the radius then
  Compute the area using Area Pi sqr(Radius)
• VAR
• Radius, Area real local variables
• BEGIN
• Radius PizzaSize / 2
• Area Pi sqr(Radius)
• PricePerSquareInch PizzaCost / Area
• END of FUNCTION PricePerSquareInch

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User DefinedCube Function

• Function Cube (X Integer) Integer
• DATA Received number to be cubed
• INFO Returned number cubed
• LOGIC received integer is multiplied by itself
  3 times
• BEGIN
• Cube X X X
• END

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Using functions in programs

• Using assignment statementsA 5B Cube(A)
• Using arithmentic expressionsA 5B 3
  Cube(A) 2
• Using output statementsA 5Writeln
  (Cube(A)17)

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Demo of Variable andValue Parameters

• DEMO PROGRAM

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How modules help us to analyse and solve problems

• Using procedures relates to modular programming
  techniques
• Each module can be tested separately to see if it
  is producing the desired result using pseudo code
• Modules can then be transferred to Pascal (or
  other language)

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Pseudo code 1st Hit

• Program FtoC Fahrenheit to Centigrade
• Declare variables and modules
• Begin main program
• IntroScreen
• GetUserInput
• DoCalculation
• OutputResult
• End Main program

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Module Specification

• IntroScreen
• DATA Received None
• INFO Returned None
• LOGIC Clears screen then displays user
  instructions
• GetUserInput
• DATA Received None
• INFO Returned Temperature in F
• LOGIC Get user input (integer or real)
• By examining the module specification we can see
  what parameters are needed (or not!)

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Pseudo code (2nd hit)

• IntroScreen
• 1.1 Clear the screen
• 1.2 Draw user input message
• GetUserInput
• 1.1 Read input from user
• 1.2 Validate input as necessary

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Summary

• What defines a procedure
• What defines a function
• The use of procedures in modular programming
• Value and Variable parameter passing
• We looked at example programs