1. Thanachat Thanomkulabut
Arrays
Thanachat Thanomkulabut

2. Introduction
If you want to declare variable for store scores of all students in course
If you want to find total of scores
int s1, s2, s3, s4, s5, ... , s700;
int[] s = new int[700];
int total=0;
total = s1 + s2 + s3 + s s700;
int total=0;
for(int i=0; i<700;i++) total += s[i];

3. What is array?
An array is an indexed collection of objects, all of the same type 1 2 3 4 5 6

4. Array We create arrays using new command.
Array elements start with index 0.
Array “a” with 6 elements has a[0], a[1], ... , a[5].
Each element in the array behaves like standard variables.

5. Array Declaration Declare Array Declare variable Syntax Syntax int x;
Name : A
Type : int
Syntax
<type> [] <name>;
Declare variable
Name : x
Type : int
Syntax
<type> <name>;
int x;
int [] A;

6. Array Declaration & Creation
Format I
<type> [] <name>;
<name> = new <type>[<num-elts>];
Format II
<type> [] <name> = new <type>[<num-elts>];
double[] score;
score = new double[5];
double[] score = new double[5];

7. Array Declaration & Creation
double[] score;
score = new double[5];
score id 1 2 3 4

8. Array Declaration & Creation & Initialization
Format I
<type> [] <name> =
new <type>[num-elts] {<value-lists>};
int [] arrayA = new int[3] {3, 6, 9};
Format II
new <type>[] {<value-lists>};
int [] arrayA = new int[] {3, 6, 9};
Format III
<type> [] <name> = {<value-lists>};
int[] arrayA = {3, 6, 9};

9. Array Declaration Example
Declaration only
Declaration with creation
Declaration with initialization
int [] ai;
int [] ai = new int[5];
int [] ai = new int[5] {1, 2, 3, 4, 5};
int [] ai = {1, 2, 3, 4};

10. More examples 1 2 3 4 5 score 1 2 3 score 5 6 7 9
int [] score = new int[6]; 1 2 3 4 5
score
int [] score = new int[4] {5,6,7,9}; 1 2 3
score 5 6 7 9

11. Self test I Declare ,Create and Initialize following array Array I
Name : times
Type : Array of double
Length : 5
Initial value : None
Format I
double[] times;
times = new double[5];
Format II
double[] times = new double[5];

12. Self test II Declare ,Create and Initialize following array Array I
Name : text
Type : Array of char
Length : 3
Initial value : 'c', 'o', 'm'

13. Self test II Format I char[] text; text = new char[3]{'c','o','m'};
Format II
char[] text;
text = new char[]{'c','o','m'};
Format III
char[] text = new char[3]{'c','o','m'};
Format IIII
char[] text = new char[]{'c','o','m'};
Format IIII
char[] text = {'c','o','m'};

14. Accessing Array Elements
Supply an integer index for accessing array elements
indexes are zero-based
int [] score = new int[4]; 1 2 3
score
Run time Error!! -3 4 7
score[0] = -3;
score[2+1] = 7;
Score[3-1] = score[0]+score[3]
Console.WriteLine(score[2]);
score[4] = 5; 4

15. Example1
Initial 0 value for every elements of array (assume length of array is 6)
Show all value in each elements of array (assume length of array is 6)
for(int i=0;i<6;i++)
score[i] = 0;
for(int i=0;i<6;i++)
Console.WriteLine
(“Score{0} = {1}”,i+1,score[i]);

16. How to find Array’s length?
By property Length 5
int[] matrix = new int[5];
Console.WriteLine(matrix.Length);
By method GetLength() 7
int[] matrix = new int[7];
Console.WriteLine(matrix.GetLength(0));

17. Example2
Initial 0 value for every elements of array (assume length of array is 6)
Show all value in each elements of array (assume length of array is 6)
for(int i=0;i<6;i++)
score[i] = 0;
for(int i=0;i<score.Length;i++)
for(int i=0;i<6;i++)
Console.WriteLine
(“Score{0} = {1}”,i+1,score[i]);
for(int i=0;i<score.Length;i++)

18. Output
Example3 4
Month4 has 30 days. 1 2 3 4 5 6 7 8 9 10 11
month
This program will reports the number of days in a given month. //assume Feb have 28 days 31 28 31 30 31 30 31 31 30 31 30 31
monthday 4
static void Main(){ }
int[] monthday = {31,28,31,30,31,30, ,31,30,31,30,31};
int month =
int.Parse(Console.ReadLine());
Console.Write(“Month{0} has {1} days.”,
month,monthday[month-1]); 4 3
This program will reports the number of days in a given month. //assume Feb have 28 days

19. Example24 Write the program for calculate sumation of n number Monitor1 2 3
Input n: 4 16 3 5 1 7 3 5 1
Write the program for calculate sumation of n number 7
Sum = 16
static void Main(){
Console.Write("Input n: ");
int n = int.Parse(Console.ReadLine());
int[] num = new int[n];
for(int i=0;i<num.Length;i++)
num[i] = int.Parse(Console.ReadLine());
int sum = 0;
sum = sum+num[i];
Console.WriteLine("Sum = {0}",sum); }

20. Self Test I Write a program for calculating
summation of square (5 numbers, with array)
4* *3 + 7*7 + 2*2 + 5*5
int a, b, c, d, e;
a = int.Parse(Console.ReadLine());
b = int.Parse(Console.ReadLine());
c = int.Parse(Console.ReadLine());
d = int.Parse(Console.ReadLine());
e = int.Parse(Console.ReadLine());
int square_sum = a*a + b*b + c*c + d*d + e*e;
Console.WriteLine("Sum of square is {0}", square_sum);

21. Self test III static void Main(){ int[] num = new int[5];
for(int i=0;i<num.Length;i++)
num[i] = int.Parse(Console.ReadLine());
int square_sum = 0;
squre_sum += num[i]*num[i];
Console.WriteLine("Sum = {0}",square_sum); }

22. String in each elements can read only
Output
String 1 2 3 4 B
name B o s s a o
String is array of chars.
string name = “Bossa”;
Console.WriteLine(name[0]);
Console.WriteLine(name[4-3]);
name[4] = ‘m’; 
String in each elements can read only

23. Looping or Iteration in C#
while
Iteration
for
foreach
do…while

24. foreach
foreach is used to access (read only) all elements in an array and do some action repeatedly
foreach = “for each element in an array ”
Syntax
foreach(var_declaration in array_name)
statement;

25. foreach example 1 string s = Console.ReadLine(); int count = 0;
for (int i = 0; i < s.Length; i++){
if(s[i] == 'A')
count++; }
string s = Console.ReadLine();
int count = 0;
foreach(char c in s){
if(c == 'A')
count++; }

26. foreach example 2 int[] score = new int[5]{4,9,0,8,7}; int count = 0;
foreach(int x in score){
if(x%2 == 0)
count++; }

27. foreach is used to access (read only)
foreach example 3
int[] num = new int[5]{2, 4, 5, 8, 0};
foreach(int n in num){
n += 10; } 
foreach is used to access (read only)

28. Self test IV
Write the Program to recieve n data & find max value of that data.
Program Process
Recieve “n” for check amount of data
Recieve each number and collect it to each element in array
Find max value of all data

29. Self test IV static void Main(){ int n;
n = int.Parse(Console.ReadLine());
double[] data = new double[n];
for(int i=0;i<n;i++)
data[i] = double.Parse(Console.ReadLine());
double max=0;
for(int i=0;i<n;i++){
if(data[i] > max)
max = data[i]; }
Console.WriteLine("max = {0}",max);

30. Arrays Concept id 1 2 3 4 Array is a reference data type.
Reference variables are used to refer to the data, not the data themselves.
score
Create Label
double[] score;
score = new double[5];
Reserve Memory id 1 2 3 4

31. Arrays with Methods :Example1
micky
class AM {
static void Main(){
int[] Arr = {1, 4, 5};
Console.WriteLine
(“1:{0} {1} {2}”,Arr[0],Arr[1],Arr[2]);
Add10(Arr);
(“2:{0} {1} {2}”,Arr[0],Arr[1],Arr[2]); }
static void Add10(int[] micky){
for(int i=0;i<micky.Length;i++)
micky[i] += 10;
Arr id 1 2 5 4 1 15 14 11
Output
1: 1 4 5 2:

32. Arrays with Methods :Example2
static void Main(){
char[] data;
data = ReadArray(4);
showArray(data); }
static char[] ReadArray(int N){
char[] info = new char[N];
for(int i=0;i<N;i++)
info[i] = char.Parse(Console.ReadLine());
return info;
static void showArray(char[] Nobi){
for(int i=0;i<Nobi.Length;i++)
Console.Write(Nobi[i]);
data
info
Nobi
info id 1 2 3 B e a r
N=4 B e a r B e a r
Monitor

33. Self test V What is the output of this partial program?
static void Main(){
int[] mynum;
mynum = new int[5] {4,0,-1,2,3};
Change(mynum);
for(int i=0; i<mynum.Length; i++)
Console.Write("{0} ",mynum[i]); }
static void Change(int[] arr){
arr[0] = 5;
arr[2] = 7;
arr[3] = -8;

34. Self Test V 5 4 7 -1 -8 2 3 mynum arr id 1 2 3 4 static void Main(){1 2 3 4 5 4 7 -1 -8 2 3
mynum
arr
static void Main(){
int[] mynum;
mynum = new int[5] {4,0,-1,2,3};
Change(mynum);
for(int i=0; i<mynum.Length; i++)
Console.Write("{0} ",mynum[i]); }
static void Change(int[] arr){
arr[0] = 5;
arr[2] = 7;
arr[3] = -8;
Output

35. Example 5 What is the output of this partial program? Output NOBI IBON
static void Main(){
char[] name1,name2;
name1 = new char[4] {'N','O','B','I'};
name2 = reverse(name1);
showArr(name1);
showArr(name2); }
static char[] reverse(char[] arr){
int j=0;
char[] arr_re = new char[arr.Length];
for(int i=arr.Length-1;i>=0;i--){
arr_re[j] = arr[i];
j++;
return arr_re;
static void showArr(char[] arr){
foreach(char x in arr) Console.Write(x);
Console.WriteLine();
What is the output of this partial program?
Output
NOBI
IBON

36. Pass by value VS by reference
We can only change the elements inside the array
Pass by reference
We can change the elements inside the array
We can change the array that variable refers to

37. Pass by Value arr id 1 2 5 4 -1 mynum arr id 1 2 10 13 151 2 5 4 -1
static void Main(){
int[] mynum;
mynum = new int[3] {4,0,-1};
Console.WriteLine
("Before mynum[0] = {0}",mynum[0]);
Change(mynum);
("After mynum[0] = {0}",mynum[0]); }
static void Change(int[] arr){
arr[0] = 5;
arr = new int[3] {10,13,15};
("In Change arr[0] = {0}",arr[0]);
mynum
arr id 1 2 10 13 15
Output
Before mynum[0] = 4
In Change arr[0] = 10
After mynum[0] = 5

38. Pass by Reference mynum arr id 1 2 5 4 -1 mynum arr id 1 2 10 13 151 2 5 4 -1
static void Main(){
int[] mynum;
mynum = new int[3] {4,0,-1};
Console.WriteLine
("Before mynum[0] = {0}",mynum[0]);
Change(ref mynum);
("After mynum[0] = {0}",mynum[0]); }
static void Change(ref int[] arr){
arr[0] = 5;
arr = new int[3] {10,13,15};
("In Change arr[0] = {0}",arr[0]);
mynum
arr id 1 2 10 13 15
Output
Before mynum[0] = 4
In Change arr[0] = 10
After mynum[0] = 10

39. Multi-dimensional Array (introduction)
If you want to keep score of 50 students
If each student have 10 test
double[] score = new double[50];
double[] score0 = new double[50];
double[] score1 = new double[50];
double[] score2 = new double[50];
double[] score9 = new double[50];

40. Multi-dimensional Array (introduction)1 2 3 4 50
score 1 2 3 4 50
score0
score1 1 2
score2 9
score9
double[] score0 = new double[50];
double[] score1 = new double[50];
double[] score2 = new double[50];
double[] score9 = new double[50];

41. Multi-dimensional Array’s declaration
<type> [] <name>;
int [] score;
Multi-dimensional
2 Dimension
<type> [ , ] <name>;
int [ , ] score;
3 Dimension
<type> [ , , ] <name>;
int [ , , ] score;

42. Multi-dimensional Array’s creation
score
1 Dimension
<name> = new <type>[<num-elts>];
score = new int[4];
2 Dimension
<name> = new <type>[<dim1>,<dim2>];
score = new int[4,2];
3 Dimension
<name> = new <type>[<dim1>,<dim2>,<dim3>];
score = new int[4, 2, 3];
score
score

43. Multi-dimensional Array’s Initialization
int[] score = new int[3] {6, 1, 3};
2 Dimension
int [,] score = new int[2, 3] { {1, 8, 4} , {3, 6, 9} };
int [,] score = { {1, 8, 4} , {3, 6, 9} };
int [,] score = { {1, 8, 4} , {3, 6, 9} };
score 6 1 3
score 1 8 4 3 6 9

44. Index of Multi-dimensional Array
int[,] score = { {5,3,1},
{9,2,4} }; 1 2
score 5 3 3 7 3 1 3 1 9 2 4
score[0,1] = 7;
score[2-1 , 1+1] = 0;
Console.Write(score[0,0]);
for(int i = 0; i<=2 ; i++) score[0,i] = 3;
Console.Write(score.Length); 5 6

45. Selftest VI Declare, create and initialize array name Matrix Matrix
Format I
‘v’
‘a’
‘p’
‘y’
‘q’
‘s’
Matrix
‘z’
‘b’
char[,] Matrix;
Matrix = new char[4,2]{
{'v','a'},
{'y','q'},
{'p','z'},
{'s','b'} };

46. Array.GetLength() Get numbers of ROW in ARRAY
score 1 2 5 3 9 4
Array.GetLength()
Get numbers of ROW in ARRAY
arrayname.GetLength(0);
score.GetLength(0);
Get numbers of Column in ARRAY
arrayname.GetLength(1);
score.GetLength(1);
Get numbers of all elements in ARRAY
arrayname.Length;
score.Length; 2 3 6

47. Example 6
Student 1
Write the program to get score from 4 students (each student has 2 test)
score1= 3
score2= 8
Student 2
score1= 6
score2= 7
Student 3
score1= 8
score2= 10
Student 4
score1= 9
score2= 7

48. Student 1
score1=
score2=
Student 2
Student 3
Student 4 3 8 6 7 10 9
Example 6
Write the program to get score from 4 students (each student has 2 test)
double[,] score = new double[4,2];
for(int i=0;i<score.GetLength(0);i++){
Console.WriteLine("Student {0}",i+1);
for(int j=0;j<score.GetLength(1);j++) {
Console.Write("score{0} = ",j+1);
score[i,j] =
double.Parse(Console.ReadLine()); }
Console.WriteLine();
score 1 2 3 3 8 6 7 8 10 9 7

49. Example 6 with Method static void Main(){ double [,] arr;
arr = ReadArray2(2,4); }
static double[,] ReadArray2(int row, int col){
double[,] score = new double[4,2];
for(int i=0;i<score.GetLength(0);i++){
Console.WriteLine("Student {0}",i+1);
for(int j=0;j<score.GetLength(1);j++){
Console.Write("score{0} = ",j+1);
score[i,j] = double.Parse(Console.ReadLine()); }
Console.WriteLine();
return score;

50. score 1 2 3 8 6 7 10 9
Self test VI
From example 6, write the partial program to find sumation of all scores of all students
double sum = 0;
for(int i=0;i<score.GetLength(0);i++){
for(int j=0; j<score.GetLength(1); j++){
sum += score[i,j]; }
Console.WriteLine("Sumation = {0}",sum);

51. score 1 2 3 8 6 7 10 9
Self test VII
From example 6, write the partial program to find average of scores of all students in first test
double sum = 0;
for(int i=0;i<score.GetLength(0);i++){
sum += score[i,0]; }
double avg = sum/score.GetLength(0);
Console.WriteLine ("Average of First test = {0}",avg);

52. score 1 2 3 8 6 7 10 9
Self test VIII
From example 6, write the partial program to find average scores of all test of last student
double sum = 0;
for(int j=0;j<score.GetLength(1);j++){
sum += score[3,j]; }
double avg = sum/score.GetLength(1);
Console.WriteLine ("Average of last student = {0}",avg);

53. How to find Array’s #dimension?
By property Rank
int [] matrix = new int[5];
Console.WriteLine(matrix.Rank); 1
int [,] matrix = new int[5,2];
Console.WriteLine(matrix.Rank); 2
int [,,] matrix = new int[5,2,3];
Console.WriteLine(matrix.Rank); 3

54. Any question?