Characters

1
Characters

• We have seen the limited use of characters in
  printf to communicate with the user.
• We will now explore the facilities of C to handle
  characters.
• C has the data type char which uses one byte.
• char codes 256 different characters using the
  American Standard Code for Information
  Interchange (ASCII).

2
Character

• char
• ASCII representation of printable and control
  characters.
• 256 codes.
• char keystroke

3
Printing HELLO

• include ltstdio.hgt
• void main()
• char H72, E69, L76, O79
• printf("ccccc", H, E, L, L, O)
• Here the letters are obscure and we have to look
  them up in the ASCII table.
• A to Z 65 to 90, a to z 97 to 122, 0 to 9 48
  to 57

4
Character Assignment

• The compiler can save us this trouble
• include ltstdio.hgt
• void main()
• char H'H', E'E', L'L', O'O'
• printf("ccccc",H, E, L, L, O)
• A single character can be assigned using the
  single quotes

5
White Space

• White space characters are slightly more
  difficult
• c' ' //space
• c'\t' //tab
• c'\n' //newline - start of next line
• c'\r' //return - start of this line
• c'\b' //backspace

6
Non-Printing Characters

• There are a number of non-printing characters
  that are included in the ASCII set, examples
• c'\a' //alert - beep
• c'\0' //null - assign zero to c
• ASCII started as a 128 character set but has been
  expanded to 256 with graphics characters. The
  128 character ASCII set is on page 891 in
  DeitelDeitel.

7
Strings

• Outputing messages one character at a time is
  onerous. C allows us to write several characters
  together in a string
• printf("HELLO")
• Strings are enclosed in double quotes
• include ltstdio.hgt
• void main()
• char hello"HELLO"
• printf("s", hello) //note s

8
How is a String Handled?

• char hello"HELLO"
• The compiler works out how many elements there
  are in hello.
• How does the printf function know when to stop
  taking data from memory and sending it to the
  screen?
• We could save the number of elements in the array
  somewhere OR make the last element in the string
  a null.

9
Printing Strings

• Compiler counts 5 elements 1 for the null and
  sets the last element to null hello5'\0'
• include ltstdio.hgt
• void main()
• int i0 char hello"HELLO"
• while(helloi!'\0')
• printf("c", helloi)
• i

10
String Pointer

• We can do the same job with a pointer to the
  string
• include ltstdio.hgt
• void main()
• int i0 char sptr"HELLO"
• while(sptri!'\0')
• printf("c",sptri)
• i

11
printf

• The first term in printf is defined as a pointer
  to the string which gives the format, consider
• include ltstdio.hgt
• void main()
• char fptr"s", hello"HELLO"
• printf(fptr, hello)

12
Reading Qualifying Lab1

• include ltstdio.hgt
• void main()
• int Lab1
• do
• printf("Input Lab1 marks ")
• scanf("d", Lab1)
• while(Lab1lt0 Lab1gt100)

13
reader

• void reader(char sptr, int ptr)
• do
• printf("Input s marks ", sptr)
• scanf("d", ptr)
• while(ptrlt0 ptrgt100)

14
main

• include ltstdio.hgt
• void reader(char, int)
• void main()
• int Lab1, MT
• char slptr"Lab1", smptr"Midterm"
• reader(slptr, Lab1)
• reader(smptr, MT)

15
Calls to reader
380
Lab1
382
MT
a
L
b
1
'\0'
384
i
M
d
t
r
e
m
'\0'
389
reader(slptr, Lab1) reader(384, 380)
reader(smptr, MT) reader(389, 382)
16
Alternate main

• include ltstdio.hgt
• void reader(char, int)
• void main()
• int Lab1, MT
• reader("Lab1", Lab1)
• reader("Midterm", MT)

17
Character Input

• We should realize that all input from the
  keyboard is character by character
• When we input an integer, say 123, we use three
  keystrokes 1 plus 2 plus 3.
• scanf reads these keystrokes and builds them into
  a number.
• To tell scanf that we are finished, we press the
  ENTER key.
• The keyboard presents its data on the input
  stream stdin.

18
Reading Several Numbers

• To read several numbers from the keyboard, we can
  use one scanf call. The numbers are separated by
  one or more spaces or ENTER
• scanf("d d",i, j)
• Valid input
• 10 SPACE 20 ENTER i10, j20
• 10 ENTER i10
• 20 ENTER j20

19
scanf("c", c)

• scanf("c",c) can be used to read characters
  but as you may have discovered in the laboratory
  there are problems.
• Each scanf requires you to press ENTER.
• The program will not continue past the scanf
  unless you have pressed ENTER and all the
  variables have been satisfied.

20
Consider

• include ltstdio.hgt
• void main()
• char c
• scanf("c",c) printf("d\n",c)
• scanf("c",c) printf("d\n",c)
• With the input we get
• a ENTER 97 ASCII code for 'a'
• 10 ASCII code for '\n'

21
Caution

• When scanf builds any number from the keyboard
  characters, it ignores the SPACE and ENTER
  characters except to use them to define the end
  of a number, but it leaves '\n' in the input
  stream.
• When scanf reads a character, it produces the
  next character from the stream even if the next
  character is the ENTER character used to define
  the end of the last user input.
• In the example, the second scanf gives the user
  the ENTER character '\n'.

22
Reading Over ENTER

• include ltstdio.hgt
• void main()
• char c
• do
• scanf("c", c)
• while(c'\n')
• printf("d\n", c)

23
fflush

• include ltstdio.hgt
• void main()
• char c
• scanf("c",c)
• fflush(stdin) //remove any char left in
• // the stream by scanf
• printf("d\n", c)
• scanf("c", c)
• printf("d\n", c)

24
getchar()

• scanf has its limitations in operation and there
  is a second routine getchar which offers some
  advantages.
• Prototype int getchar(void)
• getchar returns an integer. Assigning an integer
  to a character is possible but the integer allows
  for a negative number to be passed back for
  control
• End of file EOF -1

25
Reading Characters Until ENTER

• include ltstdio.hgt
• void main()
• char c
• while((cgetchar())!'\n')
• printf("d\n", c)
• getchar has the same problem as scanf with '\n'.

26
Reading Characters Until EOF

• while((cgetchar())!EOF)
• printf("d\n",c)
• On the PC we can get the EOF by pressing CTRL Z
• Consider the implications to our area of circle
  program.

27
Assembling an Integer

• include ltstdio.hgt
• void main()
• char c
• int i0
• while((cgetchar())!EOF c!'\n' c!' ')
• ii10c-'0'
• printf("d\n",i)

28
Reading a String

• include ltstdio.hgt
• void main()
• char c20, sptrc0
• int i0
• scanf("s", c)
• printf("s\n", c)
• scanf("s", sptr)
• printf("s\n", sptr)

29
String Contents

• Strings can contain the usual printable
  characters but a string is terminated by a space
  or ENTER on input through scanf. Hence strings
  read with scanf contain no spaces.
• gets will read the spaces
• char c20
• gets(c)
• printf("s\n", c)

30
Searching a String

• include ltstdio.hgt
• include ltstring.hgt
• void main()
• char throws"HHTTHHTHHHTTTTH"
• int i, end, heads0, tails0
• endstrlen(throws)
• for(i0 iltend i)
• if(throwsi'H') heads
• if(throwsi'T') tails
• printf("d throws d heads and d tails\n",
  end, heads, tails)
• 15 throws 8 heads and 7 tails

31
Libraries

• stdio contains the input/output routines viz
• scanf, getchar
• string contains string manipulation routines,
  viz
• strlen, strcpy, strcmp
• ctype contains character manipulation routines
  viz
• toupper, isalpha

32

• stdlib contains string manipulation routines,
  viz
• atof, atoi
• conio (a Borland library) contains character
  routines, viz
• getch