CSIS 113A Lecture 10 Arrays Glenn Stevenson CSIS 113A MSJC

What is an array A contiguous group of homogeneous elements Might be easier to think of it as a series of like data that can be accessed using one name ie. Grades, Scores, temperatures This our first aggregate type –Combine atomic types to make a new type Atomic, int, float, double, etc Glenn Stevenson CSIS 113A MSJC

Array Depicted Glenn Stevenson CSIS 113A MSJC

Declaring Arrays  Declare the array  allocates memory int score[5];  Declares array of 5 integers named ‘score’  Similar to declaring five variables: int score[0], score[1], score[2], score[3], score[4]  Individual parts called many things:  Indexed or subscripted variables  ‘Elements’ of the array  Value in brackets called index or subscript  Numbered from 0 to size - 1 Glenn Stevenson CSIS 113A MSJC

Accessing Arrays  Access using index/subscript  cout << score[3];  Note two uses of brackets:  In declaration, specifies SIZE of array  Anywhere else, specifies a subscript  Size, subscript need not be literal  int score[MAX_SCORES];  score[n+1] = 99;  If n is 2, identical to: score[3] Glenn Stevenson CSIS 113A MSJC

Access Depicted Glenn Stevenson CSIS 113A MSJC

Arrays & Memory Glenn Stevenson CSIS 113A MSJC

Initialize At Declaration Just like any other variables, arrays can be initialized at declaration time. –A little trickier Glenn Stevenson CSIS 113A MSJC

A simple program Glenn Stevenson CSIS 113A MSJC #include using namespace std; int main() { double d[] = {3.44, 2.22, 1.22, 5.55, 1.89, 99.99}; for(int i = 0; i < 6; i++) cout << d[i] << endl; }

for-loops with Arrays  Natural counting loop  Naturally works well ‘counting thru’ elements of an array  Example: for (idx = 0; idx<5; idx++) { cout << score[idx] << “off by “ << max – score[idx] << endl; }  Loop control variable (idx) counts from 0 – 5 Glenn Stevenson CSIS 113A MSJC

Major Array Pitfall  Array indexes always start with zero!  Zero is ‘first’ number to computer scientists  C++ will ‘let’ you go beyond range  Unpredictable results  Compiler will not detect these errors!  Up to programmer to ‘stay in range’ Glenn Stevenson CSIS 113A MSJC

Major Array Pitfall Example  Indexes range from 0 to (array_size – 1)  Example: double temperature[24]; // 24 is array size // Declares array of 24 double values called temperature  They are indexed as: temperature[0], temperature[1] … temperature[23]  Common mistake: temperature[24] = 5;  Index 24 is ‘out of range’!  No warning, possibly disastrous results Glenn Stevenson CSIS 113A MSJC

Defined Constant as Array Size  Should try to use defined/named constant for array size  Example: const int NUMBER_OF_STUDENTS = 5; int score[NUMBER_OF_STUDENTS];  Improves readability  Improves versatility  Improves maintainability Glenn Stevenson CSIS 113A MSJC

Uses of Defined Constant  Use everywhere size of array is needed  In for-loop for traversal: for (idx = 0; idx < NUMBER_OF_STUDENTS; idx++) { // Manipulate array }  In calculations involving size: lastIndex = (NUMBER_OF_STUDENTS – 1);  When passing array to functions (later)  If size changes  requires only ONE change in program! Glenn Stevenson CSIS 113A MSJC

Filling An Array Glenn Stevenson CSIS 113A MSJC #include #include using namespace std; int main() { int array[20]; srand(time(0)); // Fill the array!! for(int i = 0; i < 20; i++) array[i] = rand() % 101; // print the array for(int i = 0; i < 20; i+=2) cout << array[i] << "\t" << array[i+1] << endl; return 0; }

Indexed Variables as Arguments  Indexed variable handled same as simple variable of array base type  Given this function declaration: void myFunction(double par1);  And these declarations: int i; double n, a[10];  Can make these function calls: myFunction(i);// i is converted to double myFunction(a[3]);// a[3] is double myFunction(n);// n is double Glenn Stevenson CSIS 113A MSJC

Subtlety of Indexing  Consider: myFunction(a[i]);  Value of i is determined first  It determines which indexed variable is sent myFunction(a[i*5]);  Perfectly legal, from compiler’s view  Programmer responsible for staying ‘in-bounds’ of array Glenn Stevenson CSIS 113A MSJC

Entire Arrays as Arguments  Formal parameter can be entire array  Argument then passed in function call is array name  Called ‘array parameter’  Formal parameter entire array is placed Glenn Stevenson CSIS 113A MSJC void foo(int ar[]) { } Int main() { int myArray[5]; foo(myArray); return 0; }

Arguments To Functions Arrays get passed to function by reference. – In actuality, it is a copy of the base address (Address of Element 0) of the array that gets passed. Glenn Stevenson CSIS 113A MSJC

#include #include using namespace std; void fillArray(int ar[], int size); void printArray(int ar[], int size); int main() { int array[20]; fillArray(array, 20); printArray(array, 20); return 0; } void fillArray(int ar[], int size) { srand(time(0)); // Fill the array!! for(int i = 0; i < size; i++) ar[i] = rand() % 101; } void printArray(int ar[], int size) { for(int i = 0; i < size; i+=2) cout << ar[i] << "\t" << ar[i+1] << endl; }

Depicted Glenn Stevenson CSIS 113A MSJC

Passing Array Pitfall It is possible to write pass the end of the array –BAD things may happen Compiler will issure NO warnings or errors –Could cause your program to crash unexpectedly –Best to see example Glenn Stevenson CSIS 113A MSJC

#include #include using namespace std; void fillArray(int ar[], int size); int main() { int array[20]; fillArray(array); return 0; } void fillArray(int ar[]) { srand(time(0)); // Fill the array!! for(int i = 0; i < 21; i++) ar[i] = rand() % 101; }

Pass the Size In C++ when you pass an array, you should also pass the size so the function knows how many it can act on. –Makes function generic Glenn Stevenson CSIS 113A MSJC void fillArray(int ar[], int size) { }

Glenn Stevenson CSIS 113A MSJC #include #include using namespace std; void fillArray(int ar[], int size); void printArray(int ar[], int size); int main() { int array[20]; fillArray(array, 20); printArray(array, 20); return 0; } void fillArray(int ar[], int size) { srand(time(0)); // Fill the array!! for(int i = 0; i < size; i++) ar[i] = rand() % 101; } void printArray(int ar[], int size) { for(int i = 0; i < size; i+=2) cout << ar[i] << "\t" << ar[i+1] << endl; }

Returning Arrays As it stands right now arrays cannot be returned from a function –The are created on the stack and get destroyed when function returns We will learn how to do it later on Glenn Stevenson CSIS 113A MSJC

Read Only Arrays Passing arrays can be dangerous because the function they are passed to can always modify them. In the previous program, the printArray function only needs the ability to read the variables. –Since it is only printing the array, it shouldn't be given permission to modify it. – In these instances, you want to use the const keyword. Glenn Stevenson CSIS 113A MSJC

#include #include using namespace std; void fillArray(int ar[], int size); void printArray(const int ar[], int size); int main() { int array[20]; fillArray(array, 20); printArray(array, 20); return 0; } void fillArray(int ar[], int size) { srand(time(0)); // Fill the array!! for(int i = 0; i < size; i++) ar[i] = rand() % 101; } void printArray(const int ar[], int size) { for(int i = 0; i < size; i+=2) cout << ar[i] << "\t" << ar[i+1] << endl; }