Lesson 2-Array, String

Array is a set of values with same data type.

Array of number

int ar[]={1,3,5,6,78,85,23};

Array of string

String ar[ ]= {"Monday","Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"  };

int array[]=new int[10];

We declare array contain 10 elements. First element is array[0]

Number of element call array size.

int size=array.length;

To print all element in array.

for(int i=0;i<ar.length;i++){           

     System.out.println(ar[i]);

                    

     }   

To print only even number, add if command.

if(ar[i]%2==0)

Run to see result.

Create array ar2 with number from 1-10

int ar2[]={1,2,3,4,5,6,7,8,9,10};

Copy these lines to print.

int sum=0;

for(int i=0;i<ar2.length;i++){

          if(ar2[i]%2==0){

          sum=sum+ar2[i];

          }             

          }   

sum=sum+ar2[i]; mean sum previous value with current value.

You can also write it like this sum+=ar2[i];

If we want to know position of number 85 in ar array, use for like this.

int index=0;

for(int i=0;i<ar.length;i++){

          if(ar[i]==85){

          index=i+1;

          }             

          }   

Value equal 6, plus 1 because i=0 mean first elements.

We want to count how many even number in array ar2.

int count=0;

     for(int i=0;i<ar2.length;i++){

          if(ar2[i]%2==0){

          count=count+1;

          }             

          }   

Declare another array.

int ar3[]={18,34,150,6,78,85,23};

We want to print max number in ar3, copy these lines.

   int max=ar3[0];

     for(int i=0;i<ar3.length;i++){     

          if(ar3[i]>max){

          max=ar3[i];

          }             

          }   

Result is 150.

Next to two dimension array, we declare like this

int array2[][]=new int [2][3];

array2[0][0]=4;

array2[0][1]=8;

array2[0][2]=6;

array2[1][0]=5;

array2[1][1]=9;

array2[1][2]=3;

Now array has 2 rows, 3 column, elements look like this

4 8 6

5 9 3

We can also declare like this

int array3[][]={{4,8,6},{5,9,3}};

Element at row 2, column 3 will be write array3[1][2], it is 3.

To print value in two dimension array, we use for like this.

for(int i=0;i<2;i++){

     for(int j=0;j<3;j++){

     System.out.println(array2[i][j]);

     }             

     }   

To understand double for loop, think like this:

First, i=0, run for below with j from 0 to 2

Next, i=1, run for with j from 0 to 2

That mean we run single for with j in i times. Here i mean number of row, i=0 is first row, i=1 is second row.

Repeat single for mean double for loop, you should understand it in practical.

Make the line System.out.println(array2[i][j]); become comment.

Next to arraylist, we declare like this.

ArrayList  array4=new ArrayList();

Or declare with data type:

ArrayList<String> mang4 = new ArrayList<String>();

To add value to arraylist, use add().

array4.add("Monday");

array4.add("Tuesday");

array4.add("Wednesday");

Arraylist has no fix size, we can add more element in any time.

To get number of element in arraylist, use int size=array4.size();

To get element, use get.

array4.get(0);

To delete an element, use array4.remove(index);

For example, to remove second element,use array4.remove(1);

To delete all, use array4.clear();

Next to string character.

We need to know about split string.

For example, we have a string like this.

String a="This is example text, we will split by comma";

We create an array by split string like this.

String a1[] = a.split(",");

System.out.println(a1[1]);

Array a1 now has two elements, “This is example text” and “we will split by comma"

We can also cut string like this

String cut = a1[0].substring(0, 7);

Result is string “This is”

Blank chacracter is counted as one character.