java Array?

Question

Can someone explain these lines of code? This program is for generating temperature of ten locations. I’m learning Array and having difficulty to understand some things.


public class WeatherFan
{
public static void main(String[] args)
{
//Temperature Array
float[][] temperature = new float[10][365];

//Generate Temperatures
for (int i = 0; i < temperature.length; i++)
for (int j = 0; j < temperature[i].length; j++)
temperature[i][j] = (float)(45.0*Math.random() - 10.0);

// Calculate the average per location
for (int i = 0; i < temperature.length; i++)
{
// place to store the average
float average = 0.0f;

for (int j = 0; j < temperature.length; j++)
average += temperature[i][j];
// Output the average temperature of the current location
System.out.println("Average temperature at location +"
+ (i+1) + " = " + average/(float)temperature[i].length);
}
}
}




Explain temperature[i].length and temperature[i][j] in these lines. I’m having difficulty to understand [i] and [i][j] what will these lines do and how will? Please explain these.


for (int j = 0; j < temperature[i].length; j++)
temperature[i][j] = (float)(45.0*Math.random() - 10.0);

Answer 1

The array is:
float temperature[10][365]

The way to think of a two-dimensional array of floats is that it's an "array of (arrays of floats)."

temperature[0] is an array of 365 floats.
temperature[0][0] is the first element of that array.

So, for example this code, with comments

for (int i = 0; i < temperature.length; i++) {
// For each array-of-floats
for (int j = 0; j < temperature[i].length; j++) {
// For each float in the i'th array
temperature[i][j] = (float)(45.0*Math.random() - 10.0);
// Set its value to a random number
}
}

Answer 2

A [4][10] or [i][j] array would probably look something like this:
(where the values 0-9 indicate the j values, 0-3 show the i values)

0 ►| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ▲
1 ►| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |- || temperature[i].length
2 ►| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |- || from 0-3, all 4 slots
3 ►| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ▼ filled by FOR loop
||||||||||◄▬▬▬▬▬▬▬▬▬▬▬►
||||||||||||||||| temperature.length
||||||||||||||||| from 0-9, all 10 slots filled by FOR loop

Before you assign any value, this is probably what your float array looks like.

0 ►| null | null | null | null | null | null | null | null | null | null |
1 ►| null | null | null | null | null | null | null | null | null | null |
2 ►| null | null | null | null | null | null | null | null | null | null |
3 ►| null | null | null | null | null | null | null | null | null | null |

lets say you assign a value to the array
temperature[0][0]=45.6; and another statement
temperature[3][3]=47.9;
the result will be your array looking something like this:

0 ►| 45.6 | null | null | null | null | null | null | null | null | null |
1 ►| null | null | null | null | null | null | null | null | null | null |
2 ►| null | null | null | null | null | null | null | null | null | null |
3 ►| null | null | null | 47.9 | null | null | null | null | null | null |


for (int i = 0; i < temperature.length; i++)
for (int j = 0; j < temperature[i].length; j++)
temperature[i][j] = (float)(45.0*Math.random() - 10.0);

so what this code extract does is assign each and every part of the array with a randomised value.
Likewise you could interchange temperature.length with value 365 and temperature[i].length with value 10 in the for loop. This will still achieve the same effect as the statement before, and help to facilitate a better understanding of how the array works.

if my own explanation wasn't that clear, you can check this link out:
http://www.willamette.edu/~gorr/classes/cs231/lectures/chapter9/arrays2d.htm
Hope this helps.
cheers