CEG 221 Lesson 2: Homogeneous Data Types

CEG 221Lesson 2: Homogeneous Data Types Mr. David Lippa

Overview • Review of Homogeneous Data Types & Their Applications • Arrays • Data structures that we will briefly cover today, and in more detail later in the term: • Lists – an ordinary unordered list • Stacks – a “stack of plates” • Queues – a line of people • Questions

What is a Homogeneous Data Type? • A Homogeneous data type is a data type that is comprised solely of one type of data • Examples: • An array of integers (just a collection) • An (un)ordered list of integers • A stack of struct ProcessType* (an OS environment of running processes) • A queue of struct PersonType (ie. A line in the store or at Disneyland)

Arrays • Arrays are a homogeneous data type that is the most basic collection of members of the same data types in chunks • Statically Allocated – hard-coded number of elements • Example: double pArray[15]; • Dynamically Allocated – variable number of elements, with malloc/free (from stdlib.h) • Example: double *pArray = malloc( 15 * sizeof(double) ); • Both examples are 15 doubles – but you have to allocate memory for everything you are storing at once time; cannot grow when full

Statically Allocated Arrays • The number of members allocated in a statically allocated array is constant, determined at compile time. • Advantages: easy to initialize arrays of basic types to “0”; memory freed when array goes out of scope; results in larger executables • double pArray[15] = {0}; • Disadvantages: fixed number of elements (using a constant); always allocates memory whether it is used or not

Creating Statically Allocated Arrays: Examples • An initialized array of 15 ints: • int pArray[15] = {0}; // each int initialized to 0 • A character string of 15 characters: • char pArray [16] = { ‘\0’ }; • always null terminated with ‘\0’ • An initialized array of 15 people: • struct PersonType pArray [15]; // allocate • for (i = 0; i < 15; i++) { /* initialize values*/ }

Dynamically Allocated Arrays • The number of members allocated in a dynamically allocated array can be determined by any value – a constant or a variable. • Advantages: requires a memset for any variables to initialize values to “0”; results in smaller executables • double *pArray = malloc( 15 * sizeof(double) ); // decl • memset( pArray, 0, 15 * sizeof(double) ); // init • Disadvantages: programmer must free memory when done or results in a memory leak; poor knowledge of pointers results in unstable code

Creating Dynamically Allocated Arrays: Examples • An initialized array of 15 ints: • int *pArray = malloc( 15 * sizeof(int) ); • memset( pArray, 0, 15 * sizeof(int) ); • A character string of 15 characters: • char *pArray = malloc(16); • memset( pArray, 0, 16 ); • An initialized array of 15 people: • struct PersonType *pPerson = malloc( 15 * sizeof(struct PersonType) ); • for (i = 0; i < 15; i++) { /* initialize values*/ } • Remember to free(pArray) when done and checking to see if pArray is NULL (allocation failed) before using it!

Accessing Arrays • To access an array, use the [] operator: • Indices go from 0 to n-1, where n is the size • The ith position starts from 0, not 1 • The value at position 4 is the 5th element in the array • The 5th element of pArray is pArray[4] • Remember when using a dynamically allocated array, check to see if it is NULL

Reading/Writing Statically Allocated Arrays • To write a statically allocated character array to disk: char buf[360] = {‘0’}; … fwrite(&buf[0], sizeof(char), 360, pOutputFile); • Remember statically allocated arrays • are not accessible when out of scope • Are always passed by reference

Reading/Writing Dynamically Allocated Arrays • fwrite(buf, sizeof(char), 360, pOutputFile); • buf is a void* • sizeof(TYPE) • 360 – the number of items being written • pOutputFile – output stream • Remember • that all arrays are POINTERS • to check for ordinary pointers (ie. char*) if they are NULL prior to using them

Deleting Dynamically Allocated Arrays • Dynamically allocated arrays are not deleted for you, as other types are • You must instruct the program to delete it with the command free and then set it to NULL so that no other function can dereference a NULL pointer (results in crash) • Example: free(pPerson); pPerson = NULL;

Putting it all together • Declare an array (& allocate memory if needed) • Initialize its values to 0 (or some empty value) … • Set the elements of the array • Use the elements of the array … • (When done, free memory if needed)

Arrays: Putting it all together { int numElements = 5; int *pIntList = malloc( numElements * sizeof(int) ); double pDoubList[5] = {0}; int i = 0; // populate both lists with needed values // use both arrays free(pIntList); }

Next Time • Advanced Input/Output • Advanced Data Types • Advanced Programming • String Processing using <string.h>

QUESTIONS?

CEG 221 Lesson 2: Homogeneous Data Types