Arrays

Declaring Arrays

int counts [26];
//int = data type
//counts = variable name
//[] = indicate were declaring an array
//26 = size - contains 26 integer elements

Accessing a Single Element of an Array

use an index value

counts[11] = counts[11] + 1; 
printf(“%d”,counts[7]);

once we have selected a single element using an index, we can treat that element as we would treat any simple variable of that type

Array Indexes

in C (and Java), the type of value used for an index is an int

Anywhere we would expect an index value, we could have:

• a literal int value,

• an int variable,

• an expression that evaluated to an int,

• a method call that returns an int

counts[x] = 7; 
counts[x+2] = 144; 
z = z + counts[nextElement(4)];

Upper and Lower Bounds

Upper bound (upb): index of the last element

Lower bound (lwb): index of the first element

• upb: 25; lwb: 0 for `counts’ 

In C (and Java), the lwb is always zero

The possible (valid) values for an index lie in the range lwb .. upb

In this example, the index range would be 0 .. 25

Array and for loop

To initialize the counts array, we have a loop as follows: 

for (int i = 0; i < 26; i++ ) 
{
	counts[i] = 0;
}

Bounded Length Queue Using Arrays

Assume queue can have at most MAX_SIZE elements

Can use array of length MAX_SIZE

If enqueue(), when queue size is MAX_SIZE, it fails: “Queue Full”

2-D Arrays

Elements are accessed by 2 indexes, one for the row and one for the column

Creating an Empty 2-D Array in C

int rating[3][4];

Creating an Empty 2-D Array in Java

int[][] rating = new int[3][4];

2-D Array Indices in C

char c[2][3]; 

Generate each pair of indices with a ‘double loop’ or a loop within a loop

Double Loop in C

char c[2][3];

for (int row = 0; row < 2; row++)
{
	for (int col = 0; col < 3; col++)
	{
		print row, col;
	}
}

Setup 2-D Array

#define NUMBER_OF_LANGS 24

int counts[NUMBER_OF_LANGS][NUMBER_OF_LANGS];

24×24 array

Initialising a 2-D Array

void initCounts()
{
	for (int r = 0; r < NUMBER_OF_LANGS; r++)
	{
		for (int c 0; c < NUMBER_OF_LANGS; c++)
		{
			counts[r][c] = 0;
		}
	}
	counts[18][18] = 569;
	counts[18][22] = 32;
	counts[22][18] = 24;
	counts[22][22] = 743;
}

This code uses a “double loop” to ensure that every element of the stats array is set to zero. And we add some dummy data

Examining Elements of 2-D Array

void printCounts()
{
	for (int r = 0; r < NUMBER_OF_LANGS; r++)
	{
		for (int c 0; c < NUMBER_OF_LANGS; c++)
		{
			if (counts[r][c] != 0)
			{
				printf("%d, %d = %d\n", r, c, counts[r][c]);
			}
		}
	}
}

This code also uses a double loop to examine each element of the stats array

If the value of the element is not zero, it prints out the values of the two indices (r and c) and the value of the element

Summing the Total of Results in a 2-D Array

void getTotal()
{
	int total = 0;
	for (int r = 0; r < NUMBER_OF_LANGS; r++)
	{
		for (int c 0; c < NUMBER_OF_LANGS; c++)
		{
			total = total + counts[r][c];
		}
	}
	return total;
}

Summing When the Row and Column are Equal in a 2D Array

int getTotalCorrect()
{
	int correct = 0;
	for (int l = 0, l < NUMBER_OF_LANGS; l++)
	{
		correct = correct + counts[l][l];
	}
	return correct;
}

Pros of Arrays

• We can use a single name to represent multiple data items of the same type

• Random access – very fast

  • We can pick a valid index (at “random”) from the index range of an array and very quickly “access” the value stored at that position within the array

  • As opposed to sequentially going through the array

Cons of Arrays

• Fixed size – cannot be resized

  • If it is possible to dynamically allocate space, we can allocate bigger arrays but there is an associated cost of copying elements over

• Insertions and deletions are costly

  • A similar cost to the one indicated above

#define

• E.g. #define NUMBER_OF_LANGS 24

• Allows us to associate a (meaningful) name or label with a value

• Isn’t a variable - It is a constant

Manifest Constants

• If we use constants correctly, if the number of languages changes then we only have to change one line of code

  #define NUMBER_OF_LANGS 30

• recompile our code and the program should continue to function as expected

• Even if we never have to change the number of languages, using a label or meaningful name increases the readability (and therefore understandability) of our code