© Janice Regan, CMPT 128, January CMPT 128: Introduction to Computing Science for Engineering Students Introduction to Arrays

© Janice Regan, CMPT Simple and Composite Variables  We have studied simple variables  A simple variable describes a single value  A simple variable has an identifier  A simple variable has a type that describes the properties of the value of the variable, the permissible operations for the variable, and the representation of the variable in computer memory  We can also have composite variables  These variables describe a group of values  Arrays: all values in the group have the same type  Structures: different values in the group can have different types

© Janice Regan, CMPT Composite Variables  composite variables describe a group of values  1 dimensional arrays or variables of a particular type (all entries must have the same type)  multi dimensional arrays or variables of a particular type (all entries must have the same type)  Structures containing groups of variables that may be of different types  Composite variables can  Used as arguments of functions  Used as return values of functions

© Janice Regan, CMPT One-Dimensional (1-D) Arrays  An array is an indexed data structure  All variables stored in an array are of the same data type  An element of an array is accessed using the array name and an index or subscript  The name of the array is the address of the first element and the subscript is the offset  In C++, the subscripts always start with 0 and increment by 1

© Janice Regan, CMPT Declaration of a 1-D array  An array is defined using a declaration statement. type array_name[size];  allocates memory for size elements  subscript of first element is 0  subscript of last element is size-1  size must be a constant

© Janice Regan, CMPT Example Array Declaration in C++ int list[10];  allocates memory for 10 integer variables. Ten adjacent locations in memory are allocated  subscript of first element is 0  subscript of last element is 9  C++ does not perform any bounds checking on arrays list[0] list[1] list[2] list[3] list[4] list[5] list[6] list[7] list[9] list[8]

© Janice Regan, CMPT Example Array Declaration in C ++ double v2[6];  allocates memory for 6 double variables. Memory is in a single block  subscript of first element is 0  subscript of last element is 5  C++ does not perform any bounds checking on arrays v2[0] v2[1] v2[2] v2[3] v2[4] v2[5]

© Janice Regan, CMPT Avoid a common problem (1)  C++ does not perform any bounds checking on arrays  This means that you can accidentally change the values of other variables by changing a value you refer to as an element of the array, which is not actually part of the array int myArray[5] = {0}; int count = 6; myArray[5] = 3;

© Janice Regan, CMPT Avoid a common problem (2) int myArray[5] = {0}; int count = 6; After these declarations memory looks like myArray[5] = 3; After the assignment statement above myArray[0] 0 myArray[1] 0 myArray[2] 0 myArray[3] 0 myArray[4] 0 count 6 myArray[0] 0 myArray[1] 0 myArray[2] 0 myArray[3] 0 myArray[4] 0 count 3

© Janice Regan, CMPT Avoid a common problem (3)  C++ does not perform any bounds checking on arrays  By changing an element of the array myArray that is outside the memory area allocated to the array we have changed the value of a completely different variable  It is imperative that you be very careful to avoid using elements of the array that are not within the declared array.

© Janice Regan, CMPT Initializing 1-D Arrays  Arrays can be initialized at the time they are declared.  You can specify individual values for each element of an array // put 0.15 into element 0, 0.25 into element 1 // and 0.3 into element 2 of the array taxrate double taxrate[3] ={0.15, 0.25, 0.3}; // put one character in each element of the array char list[5] = {‘h’,’e’,’l’,’l’,’o’};

© Janice Regan, CMPT Initializing 1-D Arrays  Declare your array without specifying the size  The length of the array will be determined by the number of elements you initialize // the size of array s is 3 // because 3 elements are initialized int s[ ] = {5,0,-5}; l

© Janice Regan, CMPT Initializing 1-D Arrays  You can specify individual values for the first n elements of an array.  In this case the remaining unspecified elements will be zero (or equivalent) // values in array taxrate will be // {0.15, 0.25, 0.3, 0.0, 0.0, 0.0} double taxrate[6] ={0.15, 0.25, 0.3};

© Janice Regan, CMPT Initializing 1-D Arrays  You can specify individual values for the first n elements of an array.  In this case the remaining unspecified elements will be zero (or equivalent) // values in array name will be // {‘J’, ‘a’, ‘n’, ‘i’, ‘c’, ‘e’, ‘\0’, ‘\0’, ‘\0’, ‘\0’} // Two ways to initialize char name[10] ={‘J’, ‘a’, ‘n’, ‘i’, ‘c’, ‘e’}; OR char name[10] = “Janice”;

© Janice Regan, CMPT Initializing 1-D Arrays  You can specify that all elements of an array should be 0 // assigns zero to all 100 elements // first element is initialized to 0.0 // remaining elements are initialized to 0. double vector[100] = {0.0};

© Janice Regan, CMPT Initializing 1-D Arrays  Avoid a common error  You cannot specify that all elements of an array should be 1.0 (or any other non-zero value) in a declaration statement, by declaring only the first value // first element is initialized to 1.0 // remaining elements are initialized to 0.0 double vector[100] = {1.0};

© Janice Regan, CMPT Initializing a 1-D array  You can initialize a simple variable either in the declaration statement, or using separate assignment statements following the declaration statements.  You can also initialize the values of the elements of an array using separate assignment statements following the declaration of the array  A for loop (a counting loop since we know how many elements need initializing) is the most natural way to initialize or otherwise manipulate an array

© Janice Regan, CMPT Initializing or using 1-D arrays int myinteger[100]; int index; // Initialize each element of array myinteger to 1 for(index=0; index<100; index++) { myinteger[index] = 1; }

© Janice Regan, CMPT Using a 1-D array for loops are often used to assign values to the elements of an array const int TEN =10.0; double mydouble[TEN] = {0.0}; int index; for(index=0; index<TEN; index++) { list[index] += sqrt( index) * TEN; }

© Janice Regan, CMPT Putting data into a 1-D Array  Another common way of assigning values to the elements of an array is to read data values from a file directly into the array  Each value read from the file is assigned to a single array element (for example motion[6])  A single value stored in location i in the array time is referred to as time[i]  Note that checks to determine the file was opened correctly and that data was read correctly have been omitted from the example, they should not be omitted from your code

© Janice Regan, CMPT Array Input from a data file int k; double time[10]; double motion[10]; ifstream sensor3Stream; sensor3Stream.open(“sensor3.txt”); for(k=0; k<10; k++) { sensor3Stream >> time[k] >> motion[k]; }

© Janice Regan, CMPT Array Input from a data file int k; double time[10]; double motion[10]; FILE *sensor3FilePointer; sensor3FilePointer = fopen(“sensor3.txt”); for(k=0; k<10; k++) { fscanf ( sensor3FilePointer, %lf %lf“, time[k], motion[k] ); }

© Janice Regan, CMPT Arrays as function parameters  Arrays, or parts of arrays, can be passed as arguments to functions.  An element of an array can be used as a simple variable parameter It can be passed by value or by reference  An entire array can be used as a parameter of a function It can only be passed by reference

© Janice Regan, CMPT D Array elements as parameters  Individual elements of an array can be passed by value as arguments that are of simple variable types.In the illustrated function call  Formal parameter par1 of function donothing has the value of element 2 of the array (array[2])  Formal parameter par2 of function donothing has the value of element 4 of the array (array[4]) void donothing( int par1, int par2 ); int main(void) { int array[5] = {1,2,3,4,5}; donothing(array[2], array[4]); return 0; }

© Janice Regan, CMPT D arrays as parameters  Arrays are always passed to a function by reference  The array name is the address of the first element of the array  The maximum size of the array must be specified at the time the array is declared.  The actual number of array elements that are used will vary, so the actual size of the array is usually passed as another argument

© Janice Regan, CMPT Function with array argument double max( double array[ ], int actual_size ) { double max; int index; max = 0; for(index = 0; index < actual_size; index++) { if(max < array[index]) { max = array[index]; } return max; }

© Janice Regan, CMPT Using the function double max( double array[ ], int actual_size ); int main(void) { ifstream exp1; double x[100]; int count=0; exp1.open("inputfile.dat"); while( exp1 >> x[count] && count < 100) { count++; } cout << "Maximum value: “ << max(x, count)<< endl; exp1.close(); return 0; }