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CSIS 113A

CSIS 113A. Lecture 10 Arrays. 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

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CSIS 113A

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

  2. 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

  3. Array Depicted Glenn Stevenson CSIS 113A MSJC

  4. Declaring Arrays • Declare the array  allocates memoryint 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

  5. 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

  6. Access Depicted Glenn Stevenson CSIS 113A MSJC

  7. Arrays & Memory Glenn Stevenson CSIS 113A MSJC

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

  9. A simple program #include <iostream> 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; } Glenn Stevenson CSIS 113A MSJC

  10. for-loops with Arrays • Natural counting loop • Naturally works well ‘counting thru’ elementsof 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

  11. Major Array Pitfall • Array indexes always start with zero! • Zero is ‘first’ number to computerscientists • 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

  12. 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 calledtemperature • 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

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

  14. 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 ONEchange in program! Glenn Stevenson CSIS 113A MSJC

  15. Filling An Array #include <iostream>#include <ctime> 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;} Glenn Stevenson CSIS 113A MSJC

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

  17. 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

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

  19. 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

  20. #include <iostream>#include <ctime> 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; } Glenn Stevenson CSIS 113A MSJC

  21. Depicted Glenn Stevenson CSIS 113A MSJC

  22. 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

  23. #include <iostream>#include <ctime> 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; } Glenn Stevenson CSIS 113A MSJC

  24. 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 void fillArray(int ar[], int size){} Glenn Stevenson CSIS 113A MSJC

  25. #include <iostream>#include <ctime> 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; } Glenn Stevenson CSIS 113A MSJC

  26. 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

  27. 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

  28. #include <iostream>#include <ctime> 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; } Glenn Stevenson CSIS 113A MSJC

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