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Saddleback CS 1B - Topic 4 - Arrays

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1 Topic 4 - Arrays Chapter 8 in the shrinkwrap Chapter 9 in Malik Why do we need Arrays? int main () { int item0, item1,item2; int sum; cout << “Enter 3 integers”; cin >> item0 >> item1>> item2; sum = item0 + item1 + item2; cout << “The sum of the numbers = “ << sum <<endl; cout << “the numbers in reverse order are “; cout << item2 <<“ “ << item1 <<“ “<< item0 << endl; return 0; } Topic 4 - ch 8 - Arrays 2 What do we do if we have to store more than 1 piece of information of the same type? We declare different variables.2 What are arrays? A collection of data of the same type ● A special group of variables Arrays can hold ● many pieces of data ● all have the same data type and name, ● but different values.  “Aggregate” data type ● Means “grouping”  Used for lists of like items ● Test scores, temperatures, names, etc. ● Avoids declaring multiple simple variables ● Can manipulate "list" as one entity Topic 4 - ch 8 - Arrays 3 Simple & Composite data types  Simple Data Types ● Data types that store only one piece of information ● What we have been using thus far ● Int, float double, char, long  Structured / Composite Data types ● Each data item is a collection of other data items Topic 4 - ch 8 - Arrays 43 Now on to Arrays Declaring an Array Declaring an array allocates the memory for the array Example int score[5]; // declares an array of 5 integers // named score The number of elements can be… ● a literal (e.g. 5) int score[5] ● Or a named constant const int NUMBER_OF_TESTS = 5; int score[NUMBER_OF_TESTS]; Topic 4 - ch 8 - Arrays 6 Syntax dataType arrayName[number_of_elements];4 Elements and Indexes Each individual item in an array is called an element ● Each element has an index associated with it An index is a number which indicates which value we are referring to Example score[0]  the first element in our array The first element is ALWAYS zero So if we have 5 elements our indexes would be 0,1,2,3,4 or score[0], score[1], score [2], score[3], score[4] Topic 4 - ch 8 - Arrays 7 Note: The brackets specify the size in the declaration and the subscript or index anywhere else Index Memory and Arrays Topic 4 - ch 8 - Arrays 8 int score[5]; score[0]; score[1]; score[5]; 4 bytes First element always has an index of 0 score[4]; score[2]; score[3]; score [5] will not produce a compile-time error It will produce a run-time error. } 0,1,2,3,4 are in bounds all other values are out of bounds5 Indexes An index can be anything that evaluates to an integer ● A literal ◘ e.g. score[4] ● A variable or a constant ◘ e.g. score[i] ● An expression ◘ e.g. score[2 * i - j] Topic 4 - ch 8 - Arrays 9 Initializing Arrays Simple variables can be initialized at declaration: int price = 0; // 0 is initial value of after declaration or equivalently in the code int price; price = 0;  Arrays can be initialized at declaration as well: int item[3] = {2, 12, 1}; or equivalently in the code : int item[3]; item[0] = 2; item[1] = 12; item[2] = 1; Topic 4 - ch 8 - Arrays This is not considered good style  do not do this in this class please6 Initializing Arrays (2) if you have more elements than values in the list then the extras at the end default to 0 int item[5]={2,12,1};  int item [5] = {2,12,1,0,0}; You can also initialize all the elements to 0 using this method int item[5]={0} if you have more values than elements specified then you will get a compiler error int item[5]={2,12,1,2,9,5};  compiler error if you don‟t specify the number of elements it will default to the number of values in the list int item[]={2,12,1,2,9,5};  children will default to 6 elements Topic 4 - ch 8 - Arrays This is not recommended! This is not recommended! This is okay! This is okay! Initializing using a FOR loop #include <iostream.h> int main() { float gpa[5]; // an array holding 5 grade point averages – INP.& OUT. // load the array from the keyboard for(int index = 0; index < 5; index++) { cout << "Enter the gpa for student " << index + 1 << ": "; cin >> gpa[index]; } // output the contents of the array cout << "\n\nStudent Grade Point Averages\n"; for(int ind = 0; ind < 5; ind++) { cout << "\nGPA for student " << ind + 1 <<": " << gpa[ind]; } return 0; } Topic 4 - ch 8 - Arrays 12 NOTE: For Loops are very useful when you need to access every element in an array. This loop outputs the array This loop initializes the array7 int main () { int item[3]; int sum, index; sum=0; for (index = 0; index < 3 ; index++) { cout << “Enter an integer: ”; cin >> item[index]; sum = sum + item[index]; } cout << “The sum of the numbers = “ << sum << endl; cout << “The numbers in reverse are: “; for (index = 2 ; ind > -1 ; ind-- ) { cout << item[ind] << “, ”; } return 0; } Example 13 Do a desk check with Inputs  5, 10, 15 What if we want to have 10 items? What changes will we have to make? index sum ind Output item 0 1 2 Defining a Constant as Array Size  Always use defined/named constant for array size  Example: const int AR_SIZE = 5; int score[AR_SIZE]; ● NOTE: Can‟t do this with a variable  Improves readability  Improves versatility  Improves maintainability Topic 4 - ch 8 - Arrays The number of elements must be known at compile time Note that it must be declared as an integer! Using a constant is considered a best practice8 int main () { const int AR_SIZE =10; int item[AR_SIZE]; int sum, index; sum=0; for (index = 0 ; index < AR_SIZE; index++) { cout << “Enter an integer: ”; cin >> item[index]; sum= sum + item[index]; } cout << “The sum of the numbers = “ << sum <<endl; cout << “The numbers in reverse order are: “; for (index = AR_SIZE-1; ind > -1 ; ind--) { cout << item[ind] << “, ”; } return 0; } Topic 4 - ch 8 - Arrays 15 Instead of all those changes we can use a constant and just change the constant. Note that this is AR_SIZE-1 Works well if they have to enter 10 items  Need to check for out of bounds as well as user controlled LCV … int item[AR_SIZE]; int index; int intInput; index = 0; // load the array from keyboard input cout << "Enter the item (enter -1 when done): "


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