1. CS1061 C Programmuing Lecture 12 Arrays A. O’Riordan, 2004

2. Arrays Grouping of consecutive memory locations with same name and type. To refer to an element, specify the array name and a position number. Format: arrayname[position number] First element always at position 0. n element array named c: c[0], c[1]...c[n – 1] c[0] c[n]

3. Declaring Arrays Declaring arrays is similar to declaring simple variables. Specify size in square brackets, size must be integet constant. int arr[10]; float samples[3284]; Declaring multiple arrays of same type on a single line: int x[100], y[27]; A define directive is often used to specifiy the size. #define MAX_SIZE 100... double readings[MAX_SIZE];

4. Array Example Suppose you want to write a program that will store values for the salaries of employees. You could create a variable for each employee but that would be a lengthy process and there would be no way to access these variables without explicitly referencing them. In C we use arrays. double salaries[NUM_EMPLOYEES]; If no size is given the array can hold no values. To access elements of the vector you use the array-indexing [] operator. The value given to the [] operator is called the subscript. Here are examples: salaries[0] = 42000.00; printf("Jim Smith's salary: %f\n", salaries[0]);

5. Arrays and for Loops It is very common to see arrays and for loops used together. The for loop is ideal for stepping through the elements of an array. For example, you can use a for loop to initialize each value to zero like this: int int_array[100], i; for( i=0; i < 100; i++) int_array[i] = 0; Errors involving the access of non-existent slots, especially out-by-one errors are common. Here is an example of a bounds error: int_array[100] = 5; /* error only 0 to 99 are valid */

6. Array Initialisation The values in an array can be initialised in the declaration. Listing the values in curly braces does this. int digits[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; When initialisations are provided it is legal to omit the array size indicator from the declaration as in int digits[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; If arrays are declared in this way you can calculate how many elements are in the array with the sizeof operator printf(“size is %d",(sizeof digits)/(sizeof digits[0]);

7. Passing Arrays to Functions To pass an array argument to a function, specify the name of the array without any brackets int myArray[24]; modifyArray(myArray, 24); Array size usually passed to function as a separate parameter The size does not need to be given in a function prototype void modifyArray( int b[], int arraySize); Parameter names are also optional in prototype, e.g. int b[] could be written int [] void modifyArray( int [], int);

8. Searching Arrays Search an array for a certain value Linear search- Compare each element of array with key value Useful for small and unsorted arrays Binary search - for sorted arrays Algorithm: Compares middle element with key If equal, match found If key < middle, looks in first half of array If key > middle, looks in last half Repeat Very fast; at most n steps, where 2 n > number of elements e.g. 30 element array takes at most 5 steps

9. Multidimensional Arrays In short, multidimensional arrays are arrays of arrays. They are very useful in scientific applications where you have tables of data such as matrices. int list[10]; /* A 1-dimensional array */ int matrix[10][10]; /* A 2-dimensional array */ matrix can be viewed as a 1-d array of lenght 10 of arrays matrix[10]. You can think of two dimensional arrays as a grid. You have ten rows and ten columns, for a total of 100 elements. Indexing is again from 0.

10. Application: Chess The following declares a 2-dimensional array that represents a chessboard, i.e. 8x8 elements for the 64 squares: char chessBoard[8][8]; In the game of chess one player plays the white pieces and the other plays the black pieces. Let white's pieces be represented in capitals, e.g. white's queen is Q, and let black's pieces be represented in lower-case letters, e.g. black's queen is q. To set up the board, we want to place one of white's rooks (R) in the bottom right square, one of the knight (N) in the square immediately to the right, etc. chessBoard[7][0] = 'R';/* white rook */ chessBoard[7][1] = ‘N';/* white knight */

11. Application: Chess (2) We can also encode the start state of chess game into a single initialiser: char chessBoard[8][8] = { { 'r', 'n', 'b', 'q', 'k', 'b', 'k', 'r' }, { 'p', 'p', 'p', 'p', 'p', 'p', 'p', 'p' }, { ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ' }, { 'P', 'P', 'P', 'P', 'P', 'P', 'P', 'P' }, { 'R', 'N', 'B', 'Q', 'K', 'B', 'N', 'R' } };

12. Eliding Braces The braces in initialisations can be elided except for the outer-most braces, e.g. int matrix[2][2] = {1, 2, 3, 4}; /* instead of {{1, 2}, {3, 4}} */ We know that digits[0] returns the first element in the array digits. But what be the value of the following expression digits This in fact returns the memory address of the first element; so it is in fact equivalent to &digits[0]. The & is the address-of operator (see later).