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CEG 221 Lesson 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”

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