More On File IO

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More On File IO
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What is a File

• A file is a package of information with a name
  attached to it.
• Files are used for various purposes
• Files can record data, such as text or numbers.
• Some files record ways to perform various
  processing procedures on data. These are referred
  to as programs or commands.
• Conceptually, a file is a sequence of characters,
  which resides somewhere on a disk.

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Access Files

• To access a file
• Open
• Read / Write
• Close

Read/write
A data file
Your c code
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Methods

• FILE - structure for holding file
• fopen
• Read - getc
• Write - putc
• fclose

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More reading - fscanf

• Read like scanf does, just from a file
• Takes an additional argument which is the file to
  read from
• fscanf(inFilePtr, f, fv)
• Returns number of arguments read and assigned or
  EOF if end of file is reached before anything is
  assigned

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More writing - fprintf

• Writes like printf does, just to a file
• Takes an additional argument which is the file to
  print to
• fprintf(outFilePtr, This is how its done\n)

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Reading lines- fgets

• fgets(buffer, n, filePtr)
• buffer is where the line is stored
• n is the max number of characters to be stored in
  buffer
• filePtr is where to read from
• Reads characters from file and stores them in
  buffer
• Stops when \n is reached or when n-1 characters
  have been read
• Returns NULL on failure and buffer on success

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Writing strings - fputs

• fputs(buffer, filePtr)
• Writes the contents of buffer to filePtr
• Writes each character until the \0 is reached
• Does not write \0 to the file

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Constant Files

• Three files are automatically opened in every c
  program
• Identified by constant file pointers defined in
  ltstdio.hgt
• stdin standard input (console input from user)
• scanf
• stdout output to the terminal
• printf
• stderr normally outputs to terminal but
  different file

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Constant Files (cont)

• scanf() fscanf(stdin, )
• printf() fprintf(stdout, )
• getc(stdin) getchar()
• putc(stdout) putchar()
• fgets(buffer, n, stdin) gets(buffer)
• fgets includes \n at the end of buffer
• fputs(buffer, stdout) puts(buffer)
• fputs does not append \n to end of buffer
• puts appends new line to end of buffer

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Arrays of Structures
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Structures

• Structures in C allows us to represent a
  collection of variables (possibly of different
  types) under a single name.
• It allows us to create record style data with
  various fields.
• More generally, it allows us to model real-world
  entities.
• An entity is something that has a distinct,
  seperate existence.
• An entity has a set of attributes that describes
  it.
• Any object in the real-world can be modeled as an
  entity.
• A book is an entity that can be distinguished
  from say a car.

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Structure Declaring

• A collection of values (members) type
  declaration
• struct struct_name
• type1 data_member1
• type2 data_member2
• typeN data_memberN
• Declare a structure variable instance
  declaration
• struct struct_name instance_name

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Declarations

• Three ways

struct date int day char month10 int
year struct date today
typedef struct int day char month10 int
year date date today
struct int day char month10 int year
today
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Initialization

• struct
• int day
• char month10
• int year
• today 15, June, 2007

typedef struct int day char month10 int
year date date today 15, June, 2007
struct date int day char month10 int
year struct date today 15, June, 2007
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How to use
.day

• To access the members in the structure
• specify the variable name, followed by a period
  and the member name
• today.day 15
• today.year 2007
• today.month0J
• today.month1u
• today.month2n
• today.month3e
• today.month4\0
• OR
• today.day 15
• today.year 2007
• today.monthJune

15
J
u
n
e
\0





2007
.month
today
.year
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How to use structure cont.

• Structure variable can be passed as a parameter
  to a function.
• Structure variable can be returned from a
  function
• Can have arrays of structures, and structures
  containing arrays or other structures.
• struct date list_of_days10
• struct journalEntry
• int location
• struct date when
• char entry1000

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Arrays of Structures

• Just like integer arrays and character arrays, we
  can also have an array of structures.
• An integer array allows us to associate a list of
  numbers with a single variable name.
• Similarly, an array of structures is a way of
  associating a a number of structures (entities)
  with a single variable name.
• A class of students

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Define the structure first

• Before we can create an array of book structures,
  we need to declare a structure to represent a
  book
• typedef struct
• char title50
• char author30
• float price
• book
• Usually placed at beginning of program (after
  include statements)

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Arrays of Structures

• To represent a collection of books, we can create
  an array of book structures.
• The declaration is identical to how we would
  declare an integer array.
• For example, if we want to represent a set of 10
  numbers we would declare
• int numbers10
• Similarly, to declare an array that can hold 10
  book structures
• book library10
• Here library is the variable that we use to refer
  to the entire array.
• Each element in the array is of type book.
• Notice that book is a structure type that we
  declared earlier using the typedef struct syntax.

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Accessing elements

• Recall accessing elements in an array of ints
• numbers0 ? first number in the array
• numbers7 ?eighth number in the array
• can manipulate numbers i just like a regular
  int for a valid index i
• Access it, assign to it, multiply by it, add to
  it, etc
• Same is true for an array of structs
• library0 ? first book in the array
• library5 ? sixth book in the array
• Then you can just add . to the end and whatever
  field you want to access
• libray2.author gets the author of the third
  book in the array

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Accessing elements (cnt)

• Individual elements in the array can be accessed
  using the subscript operator .
• Consider the code snippet that initializes the
  first book element, located at index position 0
  in the array
• strcpy (b0.title, "Harry Potter and the
  Sorcerer's Stone")
• strcpy (b0.author, "J.K.Rowling")
• b0.price 14.99
• Similary, we can initalize the remaining book
  elements b1, b2, b3 and so on till the
  last book element at b9.

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Iterating over the array

• Assume we have initialized all the book elements
  for the array b. Our goal is to now print the
  details of each book element in the array.
• This can be done by using a loop
• int i 0
• while (i lt 10)
• // display book information
• printf ("Title s \n", bi.title)
• printf ("Author s \n", bi.author)
• printf ("Price f \n", bi.price)

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Passing Structures to a Function

• A structure variable or structure array has
  similar properties like an integer variable or
  integer array.
• A function that receives an integer array along
  with the number of elements in the array will be
  declared as
• void function (int numbers, int sz)
• Similarly, a function that receives a book array
  along with the number of elements in the array
  can be declared as
• void function (book library, int sz)