Functions and procedures are the foundations of programming. They provide the structure to organize the program into logical units that can manage the various activities needed for a program.
Functions
There are two basic types of functions:
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Built-inthese are built into the programming environment and do things such as opening and closing files, printing, writing, and converting variables (e.g., text to numbers, singles to integers, etc.). |
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Application/user-specificdepending on what the program needs, you can build functions and procedures using built-in functions and procedures and variables. |
Functions are often treated like variables. For example:
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InStr(sourcestring, findstring) is a function in Visual Basic for locating the starting point of string (findstring) within another string (sourcestring). |
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The InStr function returns a number - the position of findstring. |
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Consider: i = InStr("Muffin is a cat.","cat") |
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When this function is called, "i" is set to 13 |
In this example,
InStr is treated as an integer data type.
Functions are typically called by other functions or procedures and given one or more variables to act upon.
Types of Built-in
Functions
The programming environment to accomplish activities that are often common across many applications provides built-in functions. These include:
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String handling such as finding a string within a string. |
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Numeric conversion (to and from a string, between integer and long, etc.) |
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Financial calculations such as calculating mortgage payments. |
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Trigonometric calculations. |
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Date and time calculations. |
Examples of some of these functions include:
i = abs(-10) i would be equal to 10 after the function is invoked
inline = input(#1, 100) inline (a string variable) would have the first 100 characters from the file referenced by a handle of #1 (the file handle is determined when the file is opened).
Examples of built-in functions include the following (Note: These are functions found in Visual Basic, there are similar functions found in many languages, make sure you check the proper syntax):
Math Functions:
Abs return absolute
value
Cos returns cosine
Exp raise to exponent
Int converts number to
integer
Conversion Functions:
Asc convert value to
ASCII format
CCur convert to
currency
CDate
convert to date
Chr convert ASCII value
to character
CInt convert to integer
Format Functions
Hex convert to hex
value
Oct convert to octal
Str convert to string
Val convert to number
Application-Specific
Functions
Application-specific
functions are written by programmers to accomplish the very specific tasks
required in any given computer program.
Programmer-developed
functions typically have the following structure:
Function <functionname> (<inputs>) <type of function>
<actions>
<functionname =
value>
End Function
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"functionname" must be unique. |
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"inputs" are optional. |
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"type of function" usually relates to the type of data returned when the function is called. |
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"actions" are one or more steps that are performed by the function |
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The function is set to a value prior to ending execution returning that value to the calling function |
Functions are
created for many reasons:
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To break up a program into
more manageable segments. |
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To handle a repetitive task
that might be called by a number of other functions or procedures. |
Imagine a currency exchange "function" at a bank. You give the teller $10 and ask for British Pounds. The teller takes the money and returns the equivalent in Pounds.
Example:
Function
convertdollars (amount as currency, newcurrency as string) as currency
If newcurrency = "British" then
Convertdollars = amount * Britishconversionrate
Else
process other types of currency
End if
End
Function
This function would be called from somewhere else in the program as follows:
BritPounds = convertdollars($100, "British")
Procedures
Procedures are used to perform a series of tasks. They usually include other procedures and functions within the program.
Procedures typically do not return a value, they are simply executed and return control to the calling procedure or subroutine.
Procedures in Visual
Basic are called "Subroutines," often "Sub" for short.
In JavaScript, "Functions" are used as procedures (they simply
return no or null values to whatever called them).
The structure for a procedure is:
Sub <subname> (<inputs>)
<actions>
End Sub
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"subname" must be unique |
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"inputs" are optional; if used, they are a data type supported by the programming environment. |
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"actions" are one or more steps performed by the procedure. |
In environments such
as Visual Basic and Lingo (Macromedia Director) procedures can be attached to
objects and are executed when an event is triggered. For example, you might have a button called
"button1," when it is clicked, a procedure called
"button1_click" is called.
Eval Process
Program processes can be triggered by a command line invocation or standalone code reference.
Also used to invoke program functions passed as strings enabling end users to call program procedures or functions.