Difference between revisions of "Java/C2/Introduction-to-Array/English"

Welcome

Learning Outcomes

• to create arrays and
• access elements in arrays.

Tools Used

• Ubuntu 11.10
• JDK 1.6 and
• Eclipse 3.7.0

Prerequisites

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Arrays

Highlight index column as you explain.

For example, a list of marks, a list of names, a list of temperatures or as shown on the slide, a list of numbers.

Each item has an index based on its position.

The index of the first element is 0.

The second element has index 1 and so on.

Let us now see how to store this data.

So switch to Eclipse.

Eclipse should contain the following code

public class ArraysDemo{

public static void main(String[] args){

}

}

Within the main method, let us add the rainfall data.

int rainfall[] = {25, 31, 29, 13, 27, 35, 12};

int rainfall open close square brackets equal to within curly brackets 25, 31, 29, 13, 27, 35, 12 semicolon.

Note the square braces after the variable name rainfall.

This declares rainfall as an array of integers.

The braces are used to specify the elements of the array.

Now let us access data.

System.out.println(rainfall[2]);

Highlight as you explain.

System dot out dot println within brackets rainfall then within square brackets 2 semicolon.

We are printing the element with index 2.

In other words, the third element in the array.

Let us now run the code.

Highlight the ouput

Now let us try a different index.

save and run. Point to output

As we can see, the output is the first value.

Now let us modify a value.

rainfall[0] = 11;

save and run. Point to output

Type rainfall[0] = 11;

Now let us see its value.

As we can see, the value has been changed to 11.

Let us see how to create such array.

int squares[] = new int[10]; This statement creates an array of integers having 10 elements. The name of the array is squares. Now let us add some values.

squares[0] = 1;

squares[1] = 4;

squares[2] = 9;

squares[3] = 16;

squares[0] = 1;

squares[1] = 4;

squares[2] = 9;

squares[3] = 16;

We have entered the squares of first four numbers. Now what about the other elements of the array. Let us see what they contain.

System.out.println(squares[5]);

Type System.out.println(squares[5]);

It would be a long process if we have to type each value into the array. Instead, let us use a for loop.

int n, x;

for(x = 4, x < 10, x = x + 1){

n = x + 1;

squares[x] = n * n

}

int n, x;

for(x = 4, x < 10, x = x + 1){

n = x + 1;

squares[x] = n * n

}

We iterate over numbers from 4 to 9 and set the corresponding element in the array.

Now let us see the output.

In fact now we can set all the values inside the for loop.

squares[0]... to squares[3]...

Change x = 4 to x = 0

Change println(squares[5]) to

println(squares[2])

save and run. Point to output

This way all the elements from index 0 to index 9 are set to the corresponding squares.

We shall now see the value of the third element.

change 5 to 2

Save and run

As we can see, the value of third element has been set in the loop and it is 9.

Slide 7

Summary

In this tutorial we have learnt how to use for loop in java.

Given an array of integers, find the sum of all the elements in the array.

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