Fesq, 3/12/01 1 16.070Composite Data Types3/12/01 Lecture #14 16.070• Many programs operate on large amounts of data• Useful to store/manipulate data in logical manner• Two techniques for representing collection of related data types! Arrays -- to handle collection of data of same data type! Structures -- to handle collection of data of different data typesFesq, 3/12/01 2 16.070Arrays• An array is a collection of identically typed variables storedcontiguously in memory• An array is made up of a fixed number of elements, which arereferenced by subscripts• The basic purpose of an array is to store large amounts of related datathat share the same data type! Example: Analyze temperature fluctuations over the course of the year− Store average temperature for each day− Requires 365 memory locations− Declare 365 variables, each with a unique name?− In linear algebra, use a one-dimensional matrix (vector): t(1), t(2), …, t(365)Fesq, 3/12/01 3 16.070Array Declaration in C• Array Declaration defines a template that describes the number ofelements, and the type of the elements:Format: <type> <array-name>[<num-of-elements>];! Example: int daily_temp[365];Memory now is allocated – storage space for 365 integer values for Arraydaily_temp! Example: float sc_torque[3];Memory now is allocated – storage space for 3 float values for Arraysc_torque• Memory size is determined by array type and number of elements• Array Declaration defines a variable object, and allocates memory ofthe specified range and of the specified type for the elementsFesq, 3/12/01 4 16.070Array Subscripts• Array elements are referenced by subscript, also known as indexsc_torque[0] – the first element in array sc_torquesc_torque[1] – the secondsc_torque[2] – the third• Subscripts must be discrete (integers) and within the range defined inthe Array Declaration Statement! Example: sc_torque[1] = 1.1;Sets the second element of sc_torque equal to 1.1! Example: x = sc_torque[0] + 1.0;Reads the value stored in first element of sc_torque, adds 1 to it, and storesresult into variable x• Subscripts can be any legal C expression of integer typesc_torque[y+1] = sc_torque[y] + 2.0;! Remember: C provides zero-based subscripting! First subscript is 0, not 1.Fesq, 3/12/01 5 16.070Initializing Arrays• Arrays, just like variables, should be initialized• Without initialization, array elements contain random data - garbage! Initialization can be defined at the time of declaration, and will beperformed at compile time:float sc_torque[3] = {0.0, 0.0, 0.0};− Initializing more values than elements will cause compiler error− Initializing fewer values than elements, remaining elements are initializedto zero− If no array size is specified, compiler defines array size based on numberof initial values! Initialization can also be done at run time, e.g., with a loop:for (j = 0; j < 3; j++)sc_torque[j] = 0.0;• Consequences for not initializing arrays?Fesq, 3/12/01 6 16.070ElementAddress(in hex)Contents8 bytessc_torque[0]sc_torque[2]sc_torque[1]1001081101181.01.5?How Arrays are Stored in Memory• Consider the 3-element float array sc_torqueExample 1:float sc_torque[3] = {1.0, 1.5};Example 2:float sc_torque[3];sc_torque[0] = 1.0;sc_torque[1] = 1.5;Fesq, 3/12/01 7 16.070Array Attributes• C defines attribute function sizeof that can be used to determine thebounds of an array! sizeof (<array-name>) returns the size of the array in bytessizeof (sc_torque) evaluates to 24 - three 8-byte floats! sizeof (<array-name> [<element number>]) returns the size of theelement in bytessizeof (sc_torque[0]) evaluates to 8! Can you use sizeof to determine the number of elements in an array?num_elements = (sizeof (sc_torque)) / (sizeof (float))Fesq, 3/12/01 8 16.070Operations on Arrays• Operations performed on arrays are ones that can be performed on anyvariable whose type is the same as the array/* print # of days for each month for non-leap years */#include <stdio.h>int main (void){const int months = 12;int days[months] = {31,28,31,30,31,30,31,31,30,31,30,31};int index;for (index = 0; index < months; index++) printf (Month %d has %d days.\n", index+1, days[index]);/* end for */return 0;}Fesq, 3/12/01 9 16.070Example of Array Operations• Add two vectors, represented by arrays, by adding correspondingelements from each array to calculate the sum#define Vector_size 6int main (void){int i;int j;int VectorA[Vector_size];int VectorB[Vector_size];int VectorSum[Vector_size];…… /* input Vectors A and B */…for (i = 0; i < Vector_size; i++)Vector_Sum[i] = VectorA[i] + VectorB[i];}…Fesq, 3/12/01 10 16.070Exceeding Array Boundaries• !It is the responsibility of the programmer to make sure that arraysubscripts are within bounds!• Exceeding array subscript bounds will overwrite other memorylocations, often producing garbage output -- a difficult error to debug! Compiler does no checking to determine if a subscript is within the boundsof an array! Exception: Compiler will report an error if number of values in a compile-time initialization statement exceeds the number of elements in the arrayfloat sc_torque[3] = {1.0, 1.5, 2.0, 2.5};/*compiler-detected error*/! Example of error not detected by compilerint sc_torque[3];for (j = 1; j <= 3; j++)sc_torque[j] = 0.0;Fesq, 3/12/01 11 16.070Structures• A structure is a collection of related data values that can be of differenttypes• A structure is made up of components or members, which can be ofdifferent types• Components have names instead of subscript values• A structure is a programmer-defined data type (unlike arrays); theprogrammer defines names of components, types of components, andorder of components! Example: Characterize reaction wheel behavior− Determine power state: 0 = off, 1 = on− Identify torque for each axis: roll, pitch, yaw− Read speed command: 0 = 0 speed, 255 = full speed (e.g., 2000rpm)Fesq, 3/12/01 12 16.070Declaring Structure Data Types• Declaring a structure defines a template that describes the format of thestructure, and the name and format of each component:Format: struct <struct_name>{<type component_name1><type component_name2>…};Example: struct rw_struct{int power;float roll_torque;float pitch_torque;float yaw_torque;int cmd;};• A structure declaration is the master plan that describes how thestructure is put togetherFesq, 3/12/01 13 16.070 Declaring Structure Variables• Structure