CSI 101 Elements of Computing Spring 2009 Lecture #10 – Functions and Subroutines Monday, March 16th

2 Procedural Programming  Method of organizing computer program  Breaking up code into smaller segments Makes it more efficient Allows multiple programmers to produce Facilitates reuse  Tie segments together logically Code statements in segment perform a particular service or function

3 How does Procedural Programming work?  Think of it like an assembly line Each part of assembly line works on a specific task or piece of the final product Dedicated workers focus their efforts just on their part of the process Foreman oversees entire process and makes sure it runs smoothly

4 Types of procedural segments  3 types in Visual Basic  Event procedure Invoke when a particular action occurs on a screen form  Subroutine procedure Invoked via a CALL statement  Function procedure Invoked by referring to its name Example: result = Foo()

5 Parameters  Input to procedure  Data type must match general type of procedure Doesn’t have to be exact match For instance, integral values could pass a Long into an Integer if value does not exceed Integer’s maximum  If more than one parameter, corresponds in order

6 Parameter Examples  Foo has 3 inputs: Integer, Integer, String Dim A as Integer, B as Integer, C as Integer Dim S1 as String, S2 as String Dim X As Long, Y as Single, Z as Double  CALL Foo(A,B,S1) works  CALL Foo(S1,A,B) fails – WHY? S1 is 1 st parameter, would correspond to Integer B is 3 rd parameter, would correspond to String

7 Parameter Examples, cont  Foo has 3 inputs: Integer, Integer, String Dim A as Integer, B as Integer, C as Integer Dim S1 as String, S2 as String Dim X As Long, Y as Single, Z as Double  CALL Foo(A,X,S1) Works if X's value is within 2 billion  CALL Foo(A,Y,S2) works – WHY? Removes any fractional value from Y

8 Defining Subroutines  SUB statement begins it Like BEGIN from pseudocode, it must name the subroutine  END SUB ends it Unlike END in pseudocode, does not need name Cannot define another subroutine within a subroutine, so the END SUB would refer to the previous SUB statement

9 Subroutine arguments  Arguments are the subroutine’s corresponding parameters  Parameters are considered what the calling process uses, while arguments are defined by the subroutine  Arguments have variable names and data types just like parameters Uses same structure as DIM without DIM keyword

10 Defining Subroutine example  Let’s define our previous FOO: Sub Foo(InVal1 As Integer, InVal2 As Integer, InStr As String) : End Sub  Note each argument is given a unique name and has a defined data type

11 Parameter passing  2 ways:  Pass by reference Called routine does not make a copy, but works in original register Thus, any change to argument in called routine is reflected in original procedure’s parameter value This is Virtual Basic’s default  Pass by value Called routine makes a copy of data value when called Any change to called routine’s copy is not reflected in the original procedure’s variable

12 Pass by Reference Example  Foo(Int1, Int2, String1) If Int1 > Int2, String1 set to “Higher” If Int1 < Int2, String1 set to “Lower” If Int1 = Int2, String1 set to “Equal”  CALL Foo(A, B, S1) A and B retain value, but S1 changes

13 Pass by Value  Need to specify ByVal in subroutine definition  Note that this is controlled by subroutine, not by calling routine  Thus, could change Foo by: Sub Foo2(Int1 As Integer, Int2 As Integer, ByVal Str1 As String)  Note ByVal keyword goes before argument name

14 Pass by Value Example  Foo2(Int1, Int2, String1) If Int1 > Int2, String1 set to “Higher” If Int1 < Int2, String1 set to “Lower” If Int1 = Int2, String1 set to “Equal”  CALL Foo2(A, B, S1) No variables have their values changed when Foo2 ends

15 Subroutine return values  Pass by reference is how subroutines return a value to its calling procedure  Must use an argument, so calling procedure must have variable of correct data type to use as argument

16 Functions  These are special processing segments that are invoked by using their name  They have a single return value, and it is placed in a variable that is NOT a parameter Computer creates a variable with the same name as the function. By default, that is the variable used to return the function's return value  Visual Basic has many predefined functions, but programmers are free to create their own as well

17 Form of a Function  Begins with FUNCTION keyword  Concludes with END FUNCTION  Arguments defined as in subroutine  Includes definition of return value  Example: Function Foo3(Val1 As Integer, Val2 As Integer) As Integer : End Function

18 Return keyword  Simultaneously sets return value and ends the function Return A + B  Can be placed anywhere before END FUNCTION  Often used with IF statement to end function on particular condition IF index > 255 Then 'Exceed max value Return 0 'Value for error Else 'Continue processing

19 Function Example  Let’s create a function as an example  Our function finds the minimum value of three numbers Call it MIN  We'll restrict our values to integers  Return value is one of them, so it’s also an integer

20 MIN Function Function Min(A As Integer, B As Integer, C As Integer) As Integer If A<B Then 'A smaller If A<C Then Min=A 'Note we can set Min 'Min is the variable created to hold the return value Else Min=C Endif Elseif B<C Then Min=B Else Min=C EndIf End Function 'Note no RETURN statement  RETURN not necessary if variable named for function contains the return value

21 Invoking MIN Dim A,B,C,M as Integer 'At some point, we get values for A, B, and C M = Min(A,B,C) 'M now holds the smallest value