Lecture 9 – Array (Part 2) FTMK, UTeM – Sem /2014

 At the end of this lecture, you should be able to  Differentiate passing array by value and passing array by reference  Pass individual elements from array to a function  Pass an entire array to a function 2

 We can use function to process arrays in large program  Pass by value  The variable ‘s value is passed to a formal parameter  Changing the value of the local parameter inside the function does not effect the value of the variable outside the function  Refer to passing individual elements  Pass by reference  Passing an array means that the starting address of the array is passed to the formal parameter  The parameter inside the function references to the same array that is passed to the function  NO new arrays are created  Refer to passing the whole array 3

By passing individual elements Same as passing any ordinary variable to a function By passing the whole array When we need the function to operate on the whole array 4

 Passing individual element  Array element must matches the function parameter type  When as a value parameter, function cannot change the value of the element in the calling function  E.g : a function, print_square receives an integer and prints its square, using an array, we can loop through the array and pass each element in turn to print_square 5 1-D array : Passing individual elements

6 Passing individual element - Example 1

7 #include using namespace std; void print(int a); // function prototype void main() { int i,j,ary1[4][3]={{1,2,3},{2,4,6},{3,6,9},{3,2,1}}; for(i = 0; i < 4; i++) { for(j = 0; j < 3; j++) print( ary1[i][j] ); cout << endl; } // end for } // end main void print(int a) { cout << a << “\t“; } // end print Passing an individual element - Example 2

 Passing the whole array  When we use large arrays in functions, by passing each value we need an extra memory to do so  E.g : if an array containing 20k elements were passed by value to a function, another 20k elements would have to be allocated in the function and each element would have to be copied from one array to another  Instead of passing the whole array, C++ passes the address of the array 8 1-D array : Passing the whole array

 An array name is the address of the first element in the array  Because of the name of array is in fact its address, passing an array name allows the called function to refer to the array back in the calling function  Two rules associated with passing the whole array  The function must be called by passing only the name of the array  In the function definition, the formal must be an array type, the size of the array doesn’t need to be specified. If provided, it is ignored by the compiler.  A function can change the elements in array by passing the array name without the constant modifier 9 1-D array : Passing the whole array

Passing the whole arrays for updating - Example 1 10 Arrays are passed by reference only The symbol & is not used when declaring an array as a formal parameter C++ does not allow functions to return a value of the type array

#include using namespace std; #define SIZE 7 void reverseOrder(int [], int); // function prototype void main() { int numbers[SIZE]={1,2,3,4,5,6,7}; reverseOrder(numbers, SIZE); } // end main void reverseOrder(int list[], int arraySize) { for (int i = arraySize-1; i >= 0; i--) cout << list[i] << "\t"; } // end reverseOrder 11

 Passing array by reference could lead to errors if your function changed accidentally  Put const keyword before the array parameter to tell the compiler that the array can’t be changed 12

13 Passing the whole array as constants : Example

 Passing an individual element - Pass the individual element by indexing the array name with the row number and the column number  Passing a row  Pass the whole row by indexing the array name with only the row number  Passing the whole array  Use the array name as the actual parameter 14 Passing the two-dimensional array to function

#include using namespace std; void print(int a); void main() { int i,j,ary1[4][3]={{1,2,3},{2,4,6},{3,6,9},{3,2,1}; for(int i=0; i<4; i++){ for(int j=0; j<3; j++) print(ary1[i][j]); cout << endl; } void print(int a) { cout << a << “\t“; } 2D Array: Passing an individual element - Example 15

2D Array : Passing a row - Example 16

2D Array : Passing the whole array – Example 1 17

#define NUMBER_OF_STUDENTS 5 // const int NUMBER_OF_STUDENTS=5; #define NUMBER_OF_QUESTIONS 10 // const int NUMBER_OF_QUESTIONS=10; void checkAnswer(char[], char[][NUMBER_OF_QUESTIONS], int, int, int[]); // function prototype void main() { char key[NUMBER_OF_QUESTIONS] = {'C','C','B','A','D','B','A','B','C','D' }; char answer[NUMBER_OF_STUDENTS][NUMBER_OF_QUESTIONS]= { {'C','D','C','C','B','B','A','B','C','B'}, {'C','C','B','C','D','B','A','B','A','B'}, {'D','D','C','A','D','B','A','B','A','D'}, {'B','C','C','C','B','B','B','B','C','C'}, {'C','C','C','A','D','B','C','B','C','D'} }; int checked[NUMBER_OF_STUDENTS]; checkAnswer(key, answer, NUMBER_OF_STUDENTS, NUMBER_OF_QUESTIONS, checked); // passing array numbers } // end main 18 2D Array : Passing the whole array - Example 2

void checkAnswer(char key[], char answer[][NUMBER_OF_QUESTIONS], int numStud, int numQues, int checked[]) { int i, j; // declaring index for (i = 0; i < numStud; i++) // initializing checked (zeros) checked [i] = 0; for (i = 0; i < numStud; i++) { for (j = 0; j < numQues; j++) { if (answer[i][j] == key[j]) checked[i] += 1; } // inner for cout << "Student " << i << "'s marks : " << checked[i] << "/" << NUMBER_OF_QUESTIONS << "." << endl; } // end for } // end checkAnswer 19 cont. eg. 2D Array : Passing the whole array

 To pass an array to a function, just use the array name: showScores(tests);  To define a function that takes an array parameter, use empty [] for array argument: void showScores(int []); // function prototype void showScores(int tests[]) // function header

 When passing an array to a function, it is common to pass array size so that function knows how many elements to process: showScores(tests, ARRAY_SIZE);  Array size must also be reflected in prototype, header: void showScores(int [], int); // function prototype void showScores(int tests[], int size) // function header 7-21

(Program Continues)

Program 7-14 (Continued)

 Can you describe the difference between passing array by value and reference?  Do you know how to pass single values from an array to a function and pass a whole array to a function?  Do you know the purpose of using constant in argument list of a function when passing an array? 24