1 Arrays In many cases we need a group of nearly identical variables. Example: make one variable for the grade of each student in the class This results in a difficult to maintain and error-prone program. Instead use arrays Array = a named location in memory for many values of the same type, stored sequentially and accessed through an index Compare: variable = a named location in memory for one value.

2 Arrays Think of an array a sequence of boxes, each one holding a value Example: An array named scores that holds the quiz scores of 5 students: element 0element 1element 2element 3element scores

3 Arrays Array declaration syntax element_type array_name [ size ] ; element_type is the type of the elements stored in the array. array_name is the name of the array. Naming rules are the same as with variables. size must be a constant (known at compile time) no variables allowed you cannot change the array size at runtime use a #define directive to define the size.

4 Arrays Declaring and initializing an array Example 1: int scores[5]; the elements of scores have not been initialized. They may be initialized one at a time (usually in a for loop: for (int i = 0; i < 5; i++) { cout << “Enter score: “; cin >> scores[i]; } Example 2: int scores[5] = {11, 10, 12, 9, 11}; the elements are initialized at declaration time. When this is done, the size of the array may be omitted from the declaration.

5 Arrays Accessing the elements of an array. To access the element at index i specify the name of the array and the index enclosed in brackets. Example: scores[3] = 11; CAUTION! Indexing always starts at zero and ends at size-1 int scores[5]; scores[0] = 11; scores[1] = 12; scores[2] = 10; scores[3] = 9; scores[4] = 11; The compiler will NOT catch a wrong index. Typing scores[10] or scores[-1] can corrupt your data.

6 Arrays How exactly do they work? int scores[5]; says: I need a chunk of memory big enough to hold 5 integer variables. That’s a total of 5*4 = 20 bytes. scores itself represents the memory address where this chunk of memory begins. Think of it as a special kind of constant that holds the address of a location in memory. Example: int main () { int arr[2]; cout << arr << endl; return 0; } The output of this program is: 0xbfffb240 (the address of arr in hexadecimal)

7 Arrays How exactly do they work? scores[i] represents the contents in box #i starting at address scores. In other words, it represents the contents at address scores + i * sizeof(int)

8 Arrays & functions Each element of an array acts just like an ordinary variable: Like any ordinary variable, you can pass a single array element to a function as one of its arguments. int main () { int nums[3] = {3, 6, 5}; int sq; sq = square( nums[2] ); cout << nums[2] << “squared is “ << sq << endl; return 0; } int square( int x ) { return x * x; } copy (pass by value) Output : 5 squared is 25

9 Arrays & functions To pass the entire array as an argument: In the function prototype specify that the argument is an array by placing brackets after the name. Do NOT put the size of the array inside the brackets. Example: void printArray ( int arr[ ] ) ; Call the function using ONLY the array name as an argument. Example: int nums[5] = {1,2,3,4,5}; printArray( nums );

10 Arrays & functions int main () { int scores[5] = {11,12, 10, 9, 11}; printArray( scores, 5 ); return 0; } void printArray(int arr[], int size) { for (int i=0; i<size; i++) cout << arr[i] << “ “; cout << endl; } The parameters are again passed by value. However, the value of scores (which is copied onto arr ) is the address where the array begins. As a result, both arr and scores refer to the same location in memory! This allows a function to modify the elements of an array that is passed as an argument.

11 Arrays & functions int main () { int scores[5] = {11,12, 10, 9, 11}; cleanArray( scores, 5); printArray( scores, 5 ); return 0; } void printArray(int arr[], int size) { for (int i=0; i<size; i++) cout << arr[i] << “ “; cout << endl; } void cleanArray(int arr[], int size) { for (int i=0; i<size; i++) arr[i] = 0; } Output : This actually modifies the values stored in scores.

12 Multidimensional arrays 1D array: 2D array: 3D array: 1 row of 5 elements 3x5 array. Think of it as an array of arrays ( = an array of three elements which are arrays of five elements) 3x3x1 array.

13 Multidimensional arrays How are they stored in memory? What are the indices of the elements? row 1 row 2 row 3 3x2 array 0,00,10,2 1,01,11,21,2 0,3 1,3 2x4 array rowcolumn

14 Multidimensional arrays Declaration: similar to 1D, but specify #rows, #columns Example: double stats[10][4]; // a 10x4 array of doubles Initialization: similar to 1D. Use nested for-loops or initialize at declaration. Example 1: Example 2: for (int i = 0; i < 10; i++) for (int j = 0; j < 4; j++) stats[ i ][ j ] = 0.0; int grid[3][2] = { {0, 1}, {1, 1}, {0, 0} }; row 1 row 2 row 3

15 Multidimensional arrays CAUTION! When a multidimensional array is an argument to a function, the size of all but the first subscript MUST be specified. Example: void initArray( int arr[][10], int numrows); // prototype... int main() { int nums[5][10]; initArray(nums, 5); // function call... }