Lecture 15 Arrays with C++ programming.ppt

Lecture Topics  Array definition  Array declaration  Array assignment  Array Input / Output  Examples

What is an Array?  It’s a collection of variables (the same type) grouped into one name.  More specifically, it’s a group of memory locations for variables of the same type and specified by the same name.  It makes dealing with related variables much easier.

Parts of Arrays Elements  Refers to the number of individual items represented by the array Index (or more formally, Subscript)  Refers to one particular element in the array  The first position in an array is represented by an index, or subscript of 0 (zero). For example, • arrStudentGrades[ 0 ]  The second position is referred to by • arrStudentGrades[ 1 ]

Arrays  An array is an ordered list of values 0 1 2 3 4 5 6 7 8 9 79 87 94 82 67 98 87 81 74 91 An array of size N is indexed from zero to N-1 scores The entire array has a single name Each value has a numeric index This array holds 10 values that are indexed from 0 to 9

Arrays  A particular value in an array is referenced using the array name followed by the index in brackets  For example, the expression scores[2] refers to the value 94 (the 3rd value in the array)  That expression represents a place to store a single integer and can be used wherever an integer variable can be used

Arrays  For example, an array element can be assigned a value, printed, or used in a calculation: scores[2] = 89; scores[first] = scores[first] + 2; mean = (scores[0] + scores[1])/2; printf ("Top = %d”, scores[5]);

So how do we use arrays?  Same concepts as other variables apply  Must declare the array  Must initialize the array  Can use arrays in expressions and functions, setting elements’ values or using their values, similar to the use of ordinary variables

Declaring an array  The declaration is similar to the declaration of other variables (except for the brackets and number of elements): int iMyFirstArray[ 15 ], iMySecondArray[ 20 ];  You can use a #define constant to set the size of the array #define GRID_ROWS_MAX 8 int arrGridRows[ GRID_ROWS_MAX ] ;

Declaring Arrays  The scores array could be declared as follows: int scores[10] ;  The type of the variable scores is an array of integers  The variable scores is set to a new blank array that can hold 10 integers

Declaring Arrays  Some examples of array declarations: double prices[500] ; int factor[12] , age[6]; char codes[30] ;

Initializing an Array  You can initialize an array when you declare it, as you do with other variables  Syntax is slightly different, as you are now initializing more than one element at a time  One way at declaration, using initializers: int iMyFirstArray[ 5 ] = { 0, 0, 0, 0, 0 } ;  Note the braces around the initial zeroes which themselves are separated by commas

Initializing an Array (cont’d)  If you specify fewer initializing values than you have elements, all the rest are initialized to a value of 0. For example: int iMyFirstArray[ 5 ] = { 0 }; would set all elements to 0 int iMyFirstArray[ 5 ] = { 4 }; would set the zeroth element to 4 and the rest to 0!

Initializing an array without specifying size  You can also initialize and set the number of elements with the same step: int iMyFirstArray[ ] = { 0, 0, 0, 0, 0 } ;  Note: there is NO size specified; C automatically makes it 5 elements since you gave five initial values

Initializer Lists  Examples: int units[ ] = {147, 323, 89, 933, 540, 269, 97, 114, 298, 476}; char letterGrades[ ] = {'A', 'B', 'C', 'D', ’F'};  Note that an initializer list can only be used in the array declaration

Initializing array with a for loop  After declaring an array, you can initialize it in the body of your program by using a for loop: int i = 0, iMyFirstArray[ 5 ] ; /* size is 5*/ for ( i = 0 ; i <= 4 ; i++ ) { iMyFirstArray[ i ] = 0 ; } /* end for i */  Note the upper bound is 4, not 5. That is, you loop through 0 to 4 to initialize an array with 5 elements

Bounds Checking  Once an array is created, it has a fixed size  An index used in an array reference must specify a valid element  That is, the index value must be in bounds (0 to N-1)

Bounds Checking  For example, if the array codes can hold 100 values, it can be indexed using only the numbers 0 to 99  If count has the value 100, then the following reference will cause a problem: for (int index=0; index <= 100; index++) codes[index] = index*50 + epsilon; problem

Array Input/ Output  We typically use for loops for any kind of array processing.  To input an array, one by one: for (i=0; i<10 ; i++ ) { printf(“ Enter element %d : “, i ); scanf ( “ %d “, &scores[i] ); }

Array Output  To display an array, one element per line: for (i=0; i<10 ; i++ ) { printf(“ scores [%d] : %dn“, i , scores[i] ); }

#define SIZE 10 int main() { int myFirstArray[SIZE], i; for (i=0; i<=SIZE-1; i++) { myFirstArray[i] = i * 2; printf("myFirstArray with subscript of %d holds the value %dn", i, myFirstArray[i]); } return 0; }

Two-Dimensional Arrays  A one-dimensional array stores a list of elements  A two-dimensional array can be thought of as a table of elements, with rows and columns one dimension two dimensions

Two Dimensional Arrays  Declaration: int arr[5][5];  Traversed using two nested for loops:  for (i = 0; i<=4 ; i++)  for (j = 0; j<=4; j++)  arr[i][j] = i * j;

Multi dimensional arrays  Arrays can be 3 dimensional, 4 dimensional, or of n dimensions.  int arr[3][3][3];

Parallel Arrays  These are independent arrays of the same size, that have a meaningful connection to each other.  For example, one array with a students gpa, and one with his letter grade. GPA grade 3.8 A

Example  Write a program that reads in an array of 10 elements, gets their average, and then display the deviation of each element from that average.

Lecture 15 Arrays with C++ programming.ppt

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Lecture 15 Arrays with C++ programming.ppt