Lecture 6
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Lecture 6
Reminder: Programming Assignment 2 is due
by Wednesday by 4:30pm.
Log into Windows/ACENET
Start Microsoft Visual Studio 2010 and open
"CircumferenceProgram" project. Start MS VS
again and open "HelloWorldProgram" project.
Questions?
Monday, January 24
CS 205 Programming for the Sciences - Lecture 6
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Outline
Writing methods
Default (value) parameters
Returning results
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CS 205 Programming for the Sciences - Lecture 6
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Methods
So far, all of our code has been written in the
Main( ) method. Why write our own methods?
Organizes code for readability by giving names to
computational ideas. E.g., ComputeCircumference
Allows reuse so that code does not need to be
repeated. E.g. ComputeBallSpeed for with Earth's
and the Moon's gravity constant.
Easier to prove that the computation is correct.
Prove that an individual method is correct, then that
the composition of method uses is correct.
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CS 205 Programming for the Sciences - Lecture 6
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Methods
x
y
f(x, y, z, ...)
z
.
.
.
Recall that a method is used to encapsulate
and name a computational idea and can be
viewed as a black box that receives data and
returns an unnamed result.
C# code must describe these parts.
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CS 205 Programming for the Sciences - Lecture 6
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Writing Methods
A C# method is a subprogram that is written
inside a class (but outside of the Main( )
method). It has the following syntax:
[<modifier(s)>] <returnType> <name> (<parameterList>)
{
<body of method>
}
The first line is called the method heading.
Modifiers determine what kind of method is
being defined and are optional. Today, we
need static. We'll see other modifiers later.
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Writing Methods
returnType is the type of the result value.
name is the name of the method.
parameterList is a comma-separated list of
parameter declarations. A default parameter
declaration is just like a variable declaration:
<type> <varName>
and gives (local) names to the received data.
body of a method contains the declarations
and executable statements to compute the
result.
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Default Parameters
public static void Main()
{
int x = 3, y;
x
Console.WriteLine(x);
y = f(x);
Console.WriteLine(x);
...
}
argument
3
37
static int f (int r)
{
r Console.WriteLine(r);
r = 7;
Console.WriteLine(r);
... }
parameter
When a method is called, a default parameter is
initialized with a copy of the corresponding
actual argument. A change to the parameter is
not reflected in the argument. The example
above displays 3, 3, 7, 3.
A corresponding argument to a default
parameter may be any expression.
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Returning Results
A method may communicate a result to the
method caller by returning a result. This is
done by executing a return statement. The
syntax for this is:
return <expression>;
where the type of <expression> must match the
type giving in the method heading.
When a return statement is executed, control
immediately goes back to the point of the
method call with the result replacing the call.
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Example: Cube Method
Here is a method that computes and returns the
cube of its number parameter.
modifier
return type
name
static double Cube (int n)
{
double nCubed;
nCubed = n * n * n;
return nCubed;
}
parameter list
return statement
Type this into the HelloWorld program. It goes
above the Main method, but inside the class
as shown on the next slide.
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CS 205 Programming for the Sciences - Lecture 6
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Example: Cube Method
inside the class
Cube method
definition
above Main( )
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Example: Cube Method
Since this Cube( ) method is defined inside the
same class as the Main( ) method, it is called
without any prefix.
For example, to ask the user for a number and
then compute and display the cube of the
number, add the following to Main ( ):
Console.Write ("Enter a number: ");
userInput = Console.ReadLine ();
double number = double.Parse (userInput);
double numberCubed = Cube (number); // method call
Console.WriteLine ("The cube of {0:F} is {1:F}",
number, numberCubed);
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Example: Cube Method
The flow of control for the method call to Cube
is shown below:
method call
matches up
argument to
parameter
and goes to
start of the
method body
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method return
goes back to
point of the call
with the result
replacing the
call
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In-Class Exercise
Modify the Circumference program by writing
two new methods, ComputeCircumference
and ComputeArea, that receive a radius
number and return the circumference and
area, respectively, of a circle with that radius.
Both functions should used the pre-defined
constant Math.PI
Then modify the Main program so that it calls
the new methods to compute the circumference
and area instead of computing these values
directly in Main.
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