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Characters

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ASCII representation of printable and control characters. 256 codes. char ... Strings can contain the usual printable characters but a string is terminated by ... – PowerPoint PPT presentation

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Title: Characters

1
Characters
Lect11

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
int main()
char H72, E69, L76, O79
printf("ccccc", H, E, L, L, O)
system("PAUSE") return 0
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
int main()
char H'H', E'E', L'L', O'O'
printf("ccccc",H, E, L, L, O)
system("PAUSE") return 0
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.

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
int main()
char hello"HELLO"
printf("s", hello) //note s
system("PAUSE") return 0

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
int main()
int i0 char hello"HELLO"
while(helloi!'\0')
printf("c", helloi)
i
system("PAUSE") return 0

10
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.

11
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

12
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.

13
Consider

include ltstdio.hgt
int main()
char c
scanf("c",c) printf("d\n",c)
scanf("c",c) printf("d\n",c)
system("PAUSE") return 0
With the input we get
a ENTER 97 ASCII code for 'a'
10 ASCII code for '\n'

14
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'.

15
Reading Over ENTER

include ltstdio.hgt
int main()
char c
do
scanf("c", c)
while(c'\n')
printf("d\n", c)
system("PAUSE") return 0

16
fflush

include ltstdio.hgt
int 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)
system("PAUSE") return 0

17
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

18
Reading Characters Until ENTER

include ltstdio.hgt
int main()
char c
while((cgetchar())!'\n')
printf("d\n", c)
system("PAUSE") return 0
getchar has the same problem as scanf with '\n'.

19
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.

20
Assembling an Integer

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

21
Reading a String

include ltstdio.hgt
int main()
char c20
int i0
scanf("s", c)
printf("s\n", c)
system("PAUSE") return 0

22
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)

23
Searching a String