CS 112 Introduction to Programming

1
CS 112 Introduction to Programming

• Lecture 19
• Arrays Examples and Parameter Passing
• Http//zoo.cs.yale.edu/classes/cs112/

2
Outline

• Review
• Examples
• command line array
• strings as arrays
• using array elements as counters
• Parameter passing

3
Review Arrays

• An array stores multiple elements of the same
  type
• Refer to particular element in the array by
  position number
• In C, an array behaves very much like an object
• declaration and instantiation are like objects
• declare an array variable
• create an array using new
• make a variable a reference of an array
• parameter passing is similar to objects
• an array has the Length property
• You can use initializer to create and initialize
  an array

4
Example 1 Command-Line Arguments

• The signature of the Main method indicates that
  it takes an array of string as parameter
• These values come from command-line arguments
  that are provided when the program is invoked
• For example, the following invocation of the
  interpreter passes an array of three string
  objects into Main
• C\gt Calculator 2 3
• These strings are stored at positions 0-2 of the
  parameter
• See Calculator.cs
• To set command line if use command window, see
  above if use VS, Project-gtCalculator
  Properties-gtConfiguration Properties-gtDebugging-gtC
  ommand Line Arguements

5
Example 2 strings as Arrays

• You can use a string as an array
• You can access the length of a string using the
  Length property
• You can access each character of a string using
  , e.g.,string resp Console.ReadLine().ToUpper
  ()for (int i 0 i lt resp.Length i)
  Console.Write( respi )

See StringArray.cs
6
Examples Using the Elements of an Array as
Counters

• Use array elements to keep track of number of
  occurrences of different values
• Create an array with size of the number of
  possible values
• Each element of the array keeps track of the
  number of occurrences of one value
• When a possibility occurs, increase the array
  element by one
• May need to map from value to index

• For example, read a sequence of integers between
  1 to 10 until user inputs 0 keep track of number
  of occurrences of 1 to 10int counters new
  int10int numwhile ( (numInt32.Parse(
  Console.ReadLine() )) ! 0) countersnum-1

7
StudentPoll.cs

• 1 // Fig. 7.7 StudentPoll.cs
• 2 // A student poll program.
• 3
• 4 using System
• 5 using System.Windows.Forms
• 6
• 7 class StudentPoll
• 8
• 9 // main entry point for application
• 10 static void Main( string args )
• 11
• 12 int responses 1, 2, 6, 4, 8, 5,
  9, 7, 8, 10, 1,
• 13 6, 3, 8, 6, 10, 3, 8, 2, 7, 6, 5,
  7, 6, 8, 6, 7,
• 14 5, 6, 6, 5, 6, 7, 5, 6, 4, 8, 6, 8,
  10
• 15
• 16 int frequency new int 11
• 17 string output ""
• 18
• 19 // increment the frequency for each
  response

Declare and initialize integer array responses
Declare and allocate integer array frequency
For every element in responses, increment the
frequency element that corresponds to the answer
Output the number of times each response appeared
8
Example Die Rolling Program

• Roll 12 dice after a user clicks on Roll
• Use an array to keep track of the number of
  occurrences of each face value
• Increment the corresponding array element when a
  face value is rolled

9
RollDie.cs

• 1 // Fig. 7.6 RollDie.cs
• 2 // Rolling 12 dice.
• 3
• 4 using System
• 5 using System.Drawing
• 6 using System.Collections
• 7 using System.ComponentModel
• 8 using System.Windows.Forms
• 9 using System.Data
• 10 using System.IO
• 11
• 12 public class RollDie System.Windows.Forms.F
  orm
• 13
• 14 private System.Windows.Forms.Button
  rollButton
• 15
• 16 private System.Windows.Forms.RichTextBox
  displayTextBox
• 17
• 18 private System.Windows.Forms.Label die1
• 19 private System.Windows.Forms.Label die2

Create a Random object
Declare an integer array frequency and allocate
it enough memory to hold 7 integers
10
RollDie.cs

• 36 public RollDie()
• 37
• 38 InitializeComponent()
• 39
• 40
• 41 // Visual Studio .NET generated code
• 42
• 43 STAThread
• 44 static void Main()
• 45
• 46 Application.Run( new RollDie() )
• 47
• 48
• 49 private void rollButton_Click(
• 50 object sender, System.EventArgs e )
• 51
• 52 // pass the labels to a method that
  will
• 53 // randomly assign a face to each die
• 54 RollADie( die1 )

11
RollDie.cs

• 71
• 72 displayTextBox.Text
  "Face\tFrequency\tPercent\n"
• 73
• 74 // output frequency values
• 75 for ( int x 1 x lt frequency.Length
  x )
• 76
• 77 displayTextBox.Text x "\t"
• 78 frequency x "\t\t"
  String.Format( "0N",
• 79 frequency x / total 100 )
  "\n"
• 80
• 81
• 82 // end Main
• 83
• 84 // simulates roll, display proper
• 85 // image and increment frequency
• 86 public void RollADie( Label die )
• 87
• 88 int face randomNumber.Next( 1, 7 )
• 89

Get a random number from 1 to 6
Display die image corresponding to the number
rolls
12
Outline

• Admin. and review
• Examples
• command line array
• strings as arrays
• using array elements as counters
• Parameter passing

13
Recall Two Types of Variables

• A variable represents a cell in memory
• Value type
• int, char, byte, float, double, stringA value
  type variable stores a value of the type of the
  variable in the memoryint x 45double y
  45.12
• Reference type
• A variable that stores object or array
  actually stores a reference to an object or
  array, e.g.,
• A reference is a location in computers memory
  where the object or array itself is storedTime3
  t1t1 new Time3(11, 45, 59)

x
45
y
45.12
t1
14
Implications of the Two Types of Variables
Assignment

• An assignment of one value variable toanother
  value variable copies the value, e.g.,int x
  45double y 45.12int zz x
• An assignment of one reference variable
  toanother reference variable copies the
  reference, e.g.,Time3 t1t1 new Time3(11, 45,
  59)Time3 t2t2 t1

x
45
y
45.12
t1
15
Implications of the Two Types of Variables Change

• Change the value of one value variablewill not
  change the otherint x 45double y
  45.12int zz xx 23
• Change the content (state) by one reference may
  affect another reference variableTime3 t1t1
  new Time3(11, 45, 59)Time3 t2t2
  t1t2.SetTime(22, 22, 22)

x
45
23
y
45.12
t1
16
Passing Arguments by Value and by Reference

• Passing a value type argument to methods
• The value of an actual argument is copied to the
  formal argument
• The formal argument has its own memory location
• Any changes to the formal argument in the method
  do not affect the actual argument
• Passing a reference type argument to methods
• A copy of the reference to the object/array is
  made to the formal argument
• Any changes to the contents of the object/array
  in the method, do affect the object/array outside
  the method
• Because both the formal argument and the actual
  argument are reference to the same object/array

17
Calling a Method

• Each time a method is called, the actual
  arguments in the invocation are copied into the
  formal arguments
• If a value type, it is the value that is copied
• If a reference type, it is the reference that is
  copied

int num SquareSum (2, 3)
2
num1
5
num2
3
18
Calling a Method Value Type
2
n1
int n1 2
n2
int n2 3
3
int num SquareSum (n1, n2)
2
num1
5
num2
3
19
Calling a Method Value Type

• Even if formal arguments and actual arguments
  have the same name, modifications to formal
  arguments in a method will not affect actual
  arguments

2
num1
int num1 2
num2
int num2 3
3
int num SquareSum (num1, num2)
2
num1
5
num2
3
20
Calling a Method Reference

• Each time a method is called, the actual
  arguments in the invocation are copied into the
  formal arguments
• If a value type, it is the value
• If a reference type, it is the reference

int array 1, 2, 3
array
int num SquareSum (array)
array
21
Calling a Method Reference

• Each time a method is called, the actual
  arguments in the invocation are copied into the
  formal arguments
• If a value type, it is the value
• If a reference type, it is the reference

int array 1, 2, 3
array
DoubleArray (array)
array
22
Arrays as Parameters Summary

• A reference to a array is passed, making the
  formal and actual parameters reference of the
  same array
• Changing an array element in the method changes
  the original
• An array element can be passed to a method as
  well, and follow the parameter passing rules of
  that element's type