A Digression

1
A Digression

• More about characters
• File I/O (Input/Output)

2
How are characters stored in memory?

• Each character is stored in one or two bytes.
• Everyone agrees on a standard for how these
  characters are stored in memory.
• Two most important standards are
• 1) ASCII 1 byte
• 2) UNICODE 2 bytes
• There are others as well.

3

• ASCII is what we use.
• American Standard Code for Information
  Interchange.
• 1 byte per character.
• A is 01000001 a is 01100001
• B is 01000010 b is 01100010
• C is 01000011 c is 01100011
• With 1 byte 8 bits you have 256 patterns so
  256 possible letters. In fact there are only 128.

4

• 127 characters is plenty for the European
  languages and the characters include
• Upper case
• Lower case
• Digits
• Punctuation
• Non- Printing Characters

5

• Non-printing characters are ASCII characters
  that do things rather the print something.
  (These days they print as smiley faces or do
  something)
• e.g.
• causes carriage to go to start of line.
• causes a move to next line
• causes a bell to ring on the terminal
• end of transmission
• We dont typically use most of them

6

• But if you want other alphabets e.g Chinese,
  Japanese, Greek... etc. 127 characters is not
  enough.
• Go to Unicode 2 bytes
• Used in the Java programming language
• There are 65536 patterns of 16 bits so plenty
  for all character sets.
• We stick to ASCII

7

• Because a byte is also a number each character
  has its associated integer.
• A 01000001 65 blank 32
• B 01000010 66 0 48
• C 01000011 67 1 49
• 2 50
• a 01100001 97 . 46
• b 01100010 98 / 47
• c 01100011 99

8

• We definitely do not need to remember these
  numbers but we can use this fact in c to do
  interesting things with characters.
• consider the following logical expression
• c a This means nothing
  really
• BUT
• Because each byte also has a number value
• c a is true.

9

• So
• z q true
• b d false
• So this can be used to put things in
  alphabetical order and to do other things.

10

• Suppose we read a character and we want to know
  if its a digit.
• char character
• if ( (character 0 ) (character )
• printf ( character is a digit)

11

• 1)Suppose we read a character and we want to know
  if its an upper case letter.
• char character
• if ( (character A ) (character )
• printf ( character is an upper case
  letter)
• 2) Suppose we read a character and we want to
  know if its an lower case letter.
• char character
• if ( (character a ) (character )
• printf ( character is a lower case
  letter)

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• 1)Suppose we read a character and we want to know
  if its a letter.
• char character
• if ( ((character A ) (character ))
• ((character a ) (character z )) )
• printf ( character is a letter)

13

• We can now write two functions which convert
  all letters to upper case or all letters to lower
  case. (Actually it is already in a library).
• char toupper(char letter)
• if ( (letter 'A') (letter
• return(letter)
• else
• return( letter - ('a'-'A'))

14

• A Function to convert to lower case
• char tolower(char letter)
• if ( (letter 'a') (letter
• return(letter)
• else
• return( letter ('a'-'A'))

15

• Actually these functions are already there for
  use.
• Need to include
• toupper( char c)
• tolower(char c)
• isdigit( char c)
• And a whole bunch more.

16

• Exercise Write a function to convert a
  character digit to its equivalent number
• ASCII 0 is 48 we want a value of 0
• ASCII 1 is 49 we want a value of 1
• So subtract 48 or better since you dont have to
  remember subtract a

17

• So a function to convert digits to numbers is
• int ascii_to_int(char digit)
• return( (int)(digit - '0') )

18
FILE I/O

• When we have large amounts of numeric data or
  text its too difficult to type things in at the
  terminal. So put data into a file and have the
  program read from the file.
• Similarly if you have a lot of output have the
  program write to a file.

19

• C allows us to mix and match. We can input
  from different files as well as from the keyboard
  in the same program. The setup is all important.
• Also when reading from files we dont prompt the
  user since there is no-one there to read the
  prompt

20
File Types

• C has two main types
• Binary Files
• - Everything stored as 0s and 1s
• Text Files
• - Usually human readable characters
• but any ASCII characters O.K.
• - Each data line ends with newline char

21
File Storage Access

• Files are stored in auxiliary storage devices,
• Such as the hard drive, CDs, etc.
• Read and Write access is buffered
• A buffer is a storage area
• It is temporary

22
Streams

• In C each file is seen as a sequential stream
  of bytes. Depending on the nature of the file
  these bytes can be seen as characters or binary
  bytes. We will use character streams.For a
  stream of characters the file ends with an EOF
  marker.
• bc fghj qr.,stj tg. /?hyg
  

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How its done

• Somehow the file is tied to or associated with
  your program in the same way the keyboard is tied
  to your program but we dont worry about how.
• We do need to know the precise syntax which are
  the main elements for what has to be done.

24
To Access files there are a series of actions we
need to do.

• We need to include to get all the
  functionality we need.
• We declare the file name as follows
• FILE myfilepointer
• 3) We open the file and check that the file is
  there.
• 4) We can now input from or output to the file.
• 5) We close the file.

25
Opening a File fopen

• Usage
• fopen("filename","mode")
• Returns a pointer to FILE, a Physical file
• Automatically creates buffer

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Examples Opening files

• FILE myfileptr
• myfileptr fopen("TEMPS.DAT","w")
• FILE myfileptr
• myfileptr fopen("A\\TEMPS.DAT","w)
• Returns NULL if there is a problem
• if((myfileptr fopen("T.DAT","w")) NULL)

28
Closing a file fclose

• Usage
• fclose(myfileptr)

29

• There are special functions for reading from
  and writing to files that are similar to ones we
  know.
• Reading characters from files.
• fgetc(fptr) gets a character from file
• fputc(fptr) writes a character to a
  file
• Other read functions
• fscanf( ) we will look at this later
• Fprintf( ) we will look at this later

30

• We now put it all together to read from a file
  and output it to screen.
• First we make a textfile using notepad and
  call it myfile.txt and then we first write a
  program to input the file and output it to the
  screen.

31

• include
• int main(void)
• FILE fptr
• char character
• if( (fptrfopen("d\\junk\\myfile.txt","r"))
  NULL) printf("\n file did not open")
• while ((character fgetc(fptr)) !EOF)
• printf("c",character)
• fclose(fptr)
• return 0

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• The output
• The red fox jumped over the green dog
• What did the green dog do?
• Nothing at all.
• Note that it reproduced the file with the
  correct spacing because it also inputs and
  outputs the \r

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• Note that the statement
• while ((character fgetc(fptr)) !EOF)
• printf("c",character)
• Does the following
• 1.It assigns the input to character.
• 2.It returns the value that is input.
• 3.It evaluates the expression is it equal to EOF

34

• We now modify the code so that it counts the
  lines as well as outputs the file

35

• FILE fptr
• int count 1
• char character
• if( (fptr fopen("d\\junk\\myfile.txt","r"))
  NULL)
• printf("\n file did not open")
• else
• while ((character fgetc(fptr)) !EOF)
• printf("c",character)
• if (character '\n')
  count
• printf("\nnumber of lines is d ",
  count)
• fclose(fptr)