Parameters. Overview A Reminder Why Parameters are Needed How Parameters Work Value Parameters...
-
Upload
luke-bradley -
Category
Documents
-
view
241 -
download
2
Transcript of Parameters. Overview A Reminder Why Parameters are Needed How Parameters Work Value Parameters...
Overview
• A Reminder
• Why Parameters are Needed
• How Parameters Work
• Value Parameters
• Reference Parameters
• Out Parameters
You’ve Seen ParametersDefined:
private void buttonAdd_Click(object sender, EventArgs e)private void FormMain_Load(object sender, EventArgs e)private void buttonAddDigit_Click(object sender, EventArgs e)
Invoked:
listBoxNames.Items.Add(textBoxInput.Text);
choice = MessageBox.Show("Do you want to exit the program?", "Quit?", MessageBoxButtons.YesNo, MessageBoxIcon.Question, MessageBoxDefaultButton.Button2);
Parameters (A Reminder)• Functions cannot see each other’s variables
(scope)• Special variables used to “catch” data being
passed• This is the way functions have to
communicate with each other• Located between parentheses ( )• If no parameters are needed, leave the
parentheses empty
Why Do We Need Parameters
• Functions are good– Reuse– Modularity– Good design
• Scope is limited– A variable declared in a function is only visible
inside that function– Using global variables is a bad idea
• So we need a way of passing data between functions
Parameter Example• Defined:private static void DoSomething(int x, double y,
string name)
• Invoked:DoSomething(42, 3.14, “Bob Smith”);
• Notice above that there is a 1-1 matching, and the types must match too
Parameter Details• By default, all primitive types are
passed by value– Remember primitive types are all non-
class types (int, float, double, string, char, etc.)
– The function gets a copy of the variable– So if you change the variable in the
function, the original value in the caller will remain unchanged
By Value Examplepublic static void IncreaseByTwo(int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(i); Console.WriteLine("Now the value of i is " + i);}
The OutputThe value of i is 4The value of x is 6Now the value of i is 4
The variable “i” is not changed because the function gets a copy of the value.
Let’s see this traced out…
By Value Examplepublic static void IncreaseByTwo(int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 0i
By Value Examplepublic static void IncreaseByTwo(int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 4i
By Value Examplepublic static void IncreaseByTwo(int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 4i
OUTPUTThe value of i is 4
By Value Examplepublic static void IncreaseByTwo(int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 4i
OUTPUTThe value of i is 4
IBT 4x
The value in i is copiedinto the variable x
By Value Examplepublic static void IncreaseByTwo(int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 4i
OUTPUTThe value of i is 4
IBT 6x
Wai
ting
here
By Value Examplepublic static void IncreaseByTwo(int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 4i
OUTPUTThe value of i is 4The value of x is 6
IBT 6x
Wai
ting
here
By Value Examplepublic static void IncreaseByTwo(int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 4i
OUTPUTThe value of i is 4The value of x is 6Now the value of i is 4
IBT 6x
Ref Parameters
• We can alter how parameters are handled using the “ref” keyword
• This makes the parameter a“By Reference” parameter
• Now the parameter IS NOT a copy of the value; it refers to the original
By Reference Examplepublic static void IncreaseByTwo(ref int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(ref i); Console.WriteLine("Now the value of i is " + i);}
The OutputThe value of i is 4The value of x is 6Now the value of i is 6
The variable “i” is changed because the function refers to the original variable i.
Let’s see this traced out…
By Ref Examplepublic static void IncreaseByTwo(ref int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(ref i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 0i
By Ref Examplepublic static void IncreaseByTwo(ref int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(ref i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 4i
By Ref Examplepublic static void IncreaseByTwo(ref int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(ref i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 4i
OUTPUTThe value of i is 4
By Ref Examplepublic static void IncreaseByTwo(ref int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(ref i); Console.WriteLine("Now the value of i is " + i);}
MEMORY
Main 4i
OUTPUTThe value of i is 4
IBT Rx
x is a reference to i
By Ref Examplepublic static void IncreaseByTwo(ref int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(ref i); Console.WriteLine("Now the value of i is " + i);}
OUTPUTThe value of i is 4
Wai
ting
here
MEMORY
Main 6i
IBT Rx
By Ref Examplepublic static void IncreaseByTwo(ref int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(ref i); Console.WriteLine("Now the value of i is " + i);}
OUTPUTThe value of i is 4The value of x is 6
Wai
ting
here
MEMORY
Main 6i
IBT Rx
By Ref Examplepublic static void IncreaseByTwo(ref int x){ x += 2; Console.WriteLine("The value of x is " + x);}
static void Main(string[] args){ int i; i = 4; Console.WriteLine("The value of i is " + i); IncreaseByTwo(ref i); Console.WriteLine("Now the value of i is " + i);}
OUTPUTThe value of i is 4The value of x is 6Now the value of i is 6
MEMORY
Main 6i
IBT Rx
The Necessity of Ref Parameters• What if a function needs to return a value
when it is complete?– Use the “return” statement and declare the
function of a particular type– Ex: public static int sum(int x, int y) { return x + y; }
• But what if the function should return more than one value?– Create a collection of values to return and return
the collection– Use ref (by reference) parameters to allow
multiple items to be updated/modified
Another By-Ref Example(notice we need to return three values, modifying their original values)
static public void RGBToGrey (ref byte red, ref byte green, ref byte blue)
{ byte average; average = (red + green + blue) / 3;
red = average; green = average; blue = average;}
Out Parameters• We’ve seen parameters that pass information
into a function (via copying the value)• We’ve seen parameters that pass information
into a function and receive information out of the function (via changed values to a reference)
• What about having values come out of a function?– No initial values passed in
Out Parameters
• Think of these similar to ref (by reference) parameters– No initial value passed in– It’s the job of the function to fill in a value
for this type of parameter– When the function ends, the calling
method will retain the value stored in the parameter
Out Parameter Examplepublic static void getUserInfo(out int age, out string name,
out double gpa){ Console.Write(“What is your name? "); name = Console.Read(); Console.Write(“What is your age? "); age = Int32.Parse(Console.Read()); Console.Write(“What is your GPA? "); gpa = Double.Parse(Console.Read());}
static void Main(string[] args){ int user_age; string user_name; double user_gpa;
getUserInfo(out user_age, out user_name, out user_gpa); // more code follows...}
Out Parameter Examplepublic static void getUserInfo(out int age, out string name,
out double gpa){ Console.Write(“What is your name? "); name = Console.Read(); Console.Write(“What is your age? "); age = Int32.Parse(Console.Read()); Console.Write(“What is your GPA? "); age = Double.Parse(Console.Read());}
static void Main(string[] args){ int user_age; string user_name; double user_gpa;
getUserInfo(out user_age, out user_name, out user_gpa); // more code follows...}
Out Parameters “Output” Information
• Notice in the preceding example, the “age”, “name” and “gpa” variables must be filled in with values within the funtion
• After this is done, the calling method (Main) can make use of these values via the “user_age”, “user_name” and “user_gpa” variables
• How is this different from ref (by-reference) parameters?– Notice we couldn’t make use of these parameters (read their
values) until we placed some value into them– So the statementConsole.WriteLine("The user’s age is" + age);at the beginning of the function (first line of the function) would be illegal because this parameter does not have a value assigned to it yet
The Necessity of Out Parameters• Like “ref” parameters, “out” parameters are useful for
returning information out from the function• Unlike “ref” parameters, “out” parameters don’t
receive values from the caller– No values passed in – the parameters are “empty”
• Use “out” parameters when the function is generating the values and returning them to the calling method– Just use the function return type and the “return” statement
if the function returns only one value– Use “out” parameters when more than one value is to be
generated and returned to the calling method