Procedures and Functions

1
Procedures and Functions

• Computing Module 1

2
What is modular programming?

• Most programs written for companies will have
  thousands of lines of code.
• Rather than writing one large program the problem
  is split into lots of smaller problems.
• A separate module is written to solve each of
  these problems.
• A module should be less than 100 lines of code.

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Advantages of Modular Programming

• Modules can be used in several different parts of
  a program, or even in several different programs.
• Easier to understand and debug the code.
• Program maintenance is easier.
• Several programmers can work on the same program,
  each is given different modules to work on.

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Advantages of Modular Programming continued

• More experienced programmers can be given more
  complex modules to work on.
• Modules can be tested individually, which means
  the time taken to get the whole program working
  is reduced.
• It is easier to monitor and control the project.

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Subroutine

• A subroutine is written for each module. There
  are two types of subroutine
• Procedure
• Function

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Function

• A function is a subroutine that returns a value.
• A function must form part of an expression.
• Pascal has some built-in functions, though you
  can also create your own functions.

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The sqrt function

• sqrt is a built-in function that returns the
  square root of a number
• sqrt real ? real
• Examples of sqrt being used
• answersqrt(9)
• finalsqrt(total)
• The first example would store the value 3 in the
  answer variable. The second would store the
  square root of the contents of the total variable
  in the variable called final.

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More built-in functions

• There are lots of other built-in functions.
• The table shows a few examples.

sqr Takes a number returns the square of that number
round Takes a number and rounds it to nearest whole number (real ? integer).
length Takes a string and returns an integer value representing the number of characters in the string.
9
Creating a new function

• As well as using the built-in functions, you can
  create your own functions.
• In this example a factorial function has been
  written.
• Remember Factorial of 5 is 54321 120.
  Factorial of n is n (n-1).. 1. Factorial of
  1 is 1.

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Factorial Function

• function factorial (n integer) integer
• var count, product integer
• begin
• product1
• for count 1 to n
• do product product count
• result product
• end

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Factorial Function

• The factorial function can now be used in a
  program.
• program project
• var number, answer integer
• begin
• write(Enter a number)
• readln(number)
• answerfactorial(number)
• writeln(The factorial of , number, is ,
  answer)

Note this is only part of a program
12
Procedure

• A procedure is a subroutine that does not return
  a value.
• Pascal has some built-in procedures, e.g. write,
  readln - though you can also create your own
  procedures.

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Creating a new procedure

• In this example a procedure countdown that takes
  a number and prints all the numbers between that
  number and zero has been written.
• So given the number 5, 543210 will be printed.

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Countdown procedure

• procedure countdown (n integer)
• var count integer
• begin
• count n
• repeat
• write(count)
• countcount-1
• until count 0
• end

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Countdown procedure

• The countdown procedure can now be used in a
  program.
• program project
• var start integer
• begin
• write(Enter a number)
• readln(start)
• countdown(start)
• countdown(start5)

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Variable scope

• The scope of a variable refers to where a
  variable can be used in a program.
• A variable can be either global or local.
• A global variable can be used anywhere in a
  program.
• A local variable can only be used in the
  subroutine that it was declared in.

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Local Variables

• In the function Factorial count and product are
  local variables.
• The scope of count and product is the function
  Factorial. They can only be seen inside this
  function and cannot be seen by anything outside
  the function.

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Global Variables

• In program Project number and answer are global
  variables.
• They can be seen anywhere in the program. They
  can also be seen from inside the Factorial
  function.

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Local vs Global

• Whenever possible local variables are used.
• Programs that use local variables instead of
  global variables tend to have fewer errors.
• This is because each module is more
  self-contained what happens in one module does
  not effect any other module.

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Advantages of Local Variables

• Modules that use local variables are easier to
  reuse in other programs.
• It is easier to find errors in a self-contained
  module as each module can be tested separately.

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Parameters

• If a program is using local variables then there
  needs to be a way to allow the modules to
  communicate (to share data) with each other.
• Values required by subroutines are passed to the
  routine by using parameters. A subroutine can
  have any number of parameters.
• The number of parameters passed to the
  procedure/function (actual parameters) must match
  the number of parameters stated in the
  procedure/function declaration (formal
  parameters).

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Parameter example

• The procedure Countdown takes one parameter an
  integer.
• When it is called it is passed one integer value.
  
• In the examples on the right start is passed as a
  parameter to the countdown procedure. The second
  time countdown is called, start5 is passed as a
  parameter.

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Passing Parameters

• There are two methods of passing parameters
• Passing by value
• Passing by reference (also called passing by
  variable)

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Passing by Value

• When a parameter is passed the value of the
  parameter does not change in the
  procedure/function.
• In this example the actual value of start is
  passed to the memory location n.

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Passing by Reference

• calchours(noofhours)
• procedure calchours(var h integer)

• When a parameter is passed the value of the
  parameter can change in the procedure/function.
• In this example the memory address of noofhours,
  not the value, is passed to the procedure
  calchours. This means that if the value of h is
  changed in calchours then the value of the
  noofhours will also change.

Notice that the word var is used in the formal
parameter