Programming with Standard IO

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Programming with Standard I/O

• CSRU3130, Spring 2008
• Ellen Zhang

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Last Class

• Midterm questions review
• Command line arguments, standard
  input/output/error
• Solutions will be posted soon
• C Programming vs C Programming
• No classes or objects
• Doesnt support function overloading
• Different standard I/O
• Different string operations

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Today

• A script a day
• First C program reverse
• Second C program vis
• Programming with standard I/O

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A script a day

• !/bin/bash
• PATH/bin/usr/bin
• case in
• 0) echo Usage watchfor person 1gt2 exit 1
• esac
• until who egrep 1
• do
• sleep 60
• done

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Change file suffix

• !/bin/bash
• for i in ls .cpp
• do
• old way mv i basename i .cpp.cc
• mv i i/.cpp/.cc
• done

Variable expansion / Substring replacement
var/Pattern/Replacement If suffix of var
matches Pattern, then substitute Replacement for
Pattern
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A script a day overwrite

• To replace UNIX with UNIX(TM) in a file called
  ch2
• How about sed s/UNIX/UNIX(TM)/g ch2 gt ch2
• A general solution ?
• A script overwrite that save standard input to
  a file
• sed s/UNIX/UNIX(TM)/g ch2 overwrite ch2
• sort k 3 n data.txt overwrite data.txt

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A script a day overwrite

• !/bin/bash
• PATH/bin/usr/bin
• case in
• 1)
• ) echo Usage overwrite file 1gt2 exit 2
• esac
• new/tmp/overwr.
• cat gtnew
• cp new 1
• rm f new

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A Real C Program

• reverse accepts one command line argument
• Treats the argument as a string, and reverses the
  letters in the string, prints out the reversed
  string
• For example
• ./reverse apple
• elppa
• ./reverse spring_song
• gons_gnirps
• ./reverse Hello world
• olleH

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Handling strings in C

• A string is an array of characters, terminated
  with null character (\0)
• The array can be statically or dynamically
  allocated
• E.x. char myStr10Hello
• Asking for a statically allocated array of
  characters of size 10, with myStr pointing to
  first element of the array
• Initialize the array with string hello

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Visualize memory

• Memory diagram a way to visualize how variable
  is maintained in RAM
• int num10
• int pNULL
• int pnum
• int nArray20

10
num
\
p

10
num
p

nArray

20


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Handling strings in C

• Now our string
• char myStr10Hello
• myStr0H or equivalently, myStr H
• myStr1e or, (myStr1)e
• myStr2l or, (myStr2)l

myStr
H
e
l
o
l
\0
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Handling strings in C

• char myStr10Hello
• Library functions for operations on string
• strcpy, strlen, strtok, strcat,
• All using the \0 to tell where a string ends
• Examples
• strcat (myStr, !)// append ! To the last
  non-null char
• str_len strlen (myStr)
• str_len is set to 5, the actual length of the
  string, not the size of the array

H
e
l
o
l
\0
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reverse.c (1)

• include ltstdio.hgt / printf /
• include ltstdlib.hgt / malloc, free /
• / MAX is the size of largest string we can
  handle /
• define MAX 1000
• typedef char bigstringMAX

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reverse.c (2)

• / reverses a string in place
• returns a pointer to the string /
• char reverse (char s)
• bigstring buf
• int i, len
• len strlen (s) // fine the length of the
  string
• for (i0iltleni)
• bufi slen-i-1
• bufi \0 // null terminate !
• strcpy (s,buf) // put back to s
• return (s)

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reverse.c (3)

• void main (int argc, char argv)
• if (argclt2)
• printf (Invalid usage must supply a
  string\n0
• exit(1)
• printf (s\n,reverse (argv1))

Make sense of char argv, draw the memory
diagram
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Compiling on Unix

• We can use GNU compiler named gcc
• gcc o reverse reverse.cc

Tells compiler to create executable file with
the name reverse
Tells compiler the name of input file
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Running the program

• ./reverse hidave
• Evadih
• ./reverse This is a long string
• sihT
• ./reverse This is a long string
• Gnirts gnol a si sihT

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Exercise

• Implement our own int mystrlen(char s) function
• Implement our own strcpy
• char mystrcpy (char src, char dest)

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C Standard I/O Libraries

• Standard I/O library a collection of routines
  that provide efficient and portable I/O and
  system services
• getchar, putchar
• printf, scanf, fopen, fread

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Whats inside a file ?

• For example, how is the following script saved ?
• !/bin/bash
• for i in ls .cpp
• do
• mv i basename i .cpp.cc
• mv i i/.cpp/.cc
• done

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Octal Dump

• wc command tells us the file has 93 characters
• Octal dump command od
• -c option interpret bytes as characters
• zhang_at_storm Demo od -c changefile
• 0000000 ! / b i n / b a s
  h \n \n f o r
• 0000020 i i n l s
  . c p p
• 0000040 \n d o \n \t m v
  i b a s
• 0000060 e n a m e i .
  c p p . c
• 0000100 c \n \t m v i
  i / . c
• 0000120 p p / . c c \n d o
  n e \n
• 0000135

Positions in the file (base 8)
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everything is binary numbers

• Remember that everything (numbers, characters) in
  computer is a stream of 0s and 1s
• In register, RAM, disk, CD, tape,
• How to represent character in numbers ?
• An encoding scheme is needed
• ASCII (American Standard Code for Information
  Interchange) codes represent text in computers,
  communications equipment, and other devices that
  work with text.

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ASCII code excerpt

• Decimal Octal Hex Binary Value
• ------- ----- --- ------ -------------------------
  -------
• 000 000 000 00000000 NUL (Null
  char.)
• 001 001 001 00000001 SOH (Start of
  Header)
• 007 007 007 00000111 BEL (Bell)
• 008 010 008 00001000 BS
  (Backspace)
• 009 011 009 00001001 HT
  (Horizontal Tab)
• 010 012 00A 00001010 LF (Line
  Feed)
• 011 013 00B 00001011 VT (Vertical
  Tab)
• 012 014 00C 00001100 FF (Form Feed)
• 013 015 00D 00001101 CR (Carriage
  Return)

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ASCII code excerpt

• Decimal Octal Hex Binary Value
• ------- ----- --- ------ -------------------------
  -------
• 062 076 03E 00111110 gt (greater than)
• 063 077 03F 00111111 ? (question mark)
• 064 100 040 01000000 _at_ (AT symbol)
• 065 101 041 01000001 A
• 066 102 042 01000010 B
• 067 103 043 01000011 C
• 068 104 044 01000100 D
• 069 105 045 01000101 E
• 070 106 046 01000110 F

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Octal Dump

• od cb changefile show the bytes as octal (base
  8) number as well
• zhang_at_storm Demo od -cb changefile
• 0000000 ! / b i n
  / b a s h \n \n f o r
• 043 041 057 142 151 156 057 142
  141 163 150 012 012 146 157 162
• 0000020 i i n l s
  . c p p
• 040 151 040 151 156 040 140 154
  163 040 052 056 143 160 160 140
• 0000040 \n d o \n \t m v
  i b a s
• 012 144 157 012 011 043 155 166
  040 044 151 040 140 142 141 163
• 0000060 e n a m e i .
  c p p . c
• 145 156 141 155 145 040 044 151
  040 056 143 160 160 140 056 143
• 0000100 c \n \t m v i
  i / . c
• 143 012 011 155 166 040 044 151
  040 044 173 151 057 045 056 143
• 0000120 p p / . c c \n d o
  n e \n
• 160 160 057 056 143 143 175 012
  144 157 156 145 012
• 0000135

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Our observations

• Unix uses newline (a special character) to
  terminate a line
• In your editor, you hit Enter key to insert a
  newline into the file
• No special character for end of file
• The file just stops
• System simply says there is no more data in the
  file
• e.g. more details later

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C Standard I/O Libraries

• Standard I/O library a collection of routines
  that provide efficient and portable I/O and
  system services
• Today read/write to standard input/output
• getchar, putchar
• printf, scanf

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A simple C program

• vis
• copies its standard input to its standard output
• makes all non-printing characters visible by
  printing them as \nnn, where nnn is the octal
  value of the character.
• Normal file viewing commands
• Do not display non-printing characters
• Note useful for checking if there are some weird
  characters in the file

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Simplest I/O routines

• char getchar()
• Gets/returns the next character from the standard
  input
• Return EOF when it encounters end of file (or
  error)
• putchar (c)
• Puts the character c on the standard output

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vis program

• include ltstdio.hgt
• include ltctype.hgt
• main()
• int c
• while ((cgetchar()) !EOF)
• if (isascii(c) (isprint(c) c\n
  c\t c ))
• putchar (c)
• else
• printf (\\03o,c)
• exit(0)

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Character test macros

• Defined in /usr/include/ctype.h
• isascii(c) whether c is an ascii code
• isalpha(c) alphabetic a-z, A-Z
• isdigit(c) digit 0-9
• ispunct(c)
• isprint(c) printable

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Quick I/O Primer printf

• int printf (const char , )
• Write strings, integers, doubles, etc to standard
  output, perform format conversion
• e.g., write i10 as 10 d10.23 as 10.230
• means variable number of arguments, the first
  argument is required (a string).

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Simple printf

• Given a simple string, printf just prints the
  string (to standard output).
• printf (Hi I am a string\n)
• printf (I\thave\ttabs\n)
• char s100
• strcpy(s, printf is fun!\a\n)
• printf (s)

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Escape sequence in C

• \' Single quote
• \" Double quote
• \\ Backslash
• \nnn Octal number (nnn), \xnnn Hexadecimal
  number (nnn)
• \0 Null character (really just the octal
  number zero)
• \a Audible bell
• \b Backspace
• \f Formfeed
• \n Newline
• \r Carriage return
• \t Horizontal tab

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Doing more with printf

• You can tell printf to embed some variables
  values in the string
• Example
• printf (here is an integer d\n, i)
• printf(\\03o,c)
• o means prints the parameter, c, as octal
  numbers

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More integer arguments

• printf (d d d\n, x, y, xy)
• for (j99jgt0j--)
• printf (d bottles of beer on the wall\n,j)
• printf (d is my favorite number\n, 17)

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printf is dumb

• d is replaced by the value of the parameter when
  treated as an integer, even if the parameter is
  not an integer variable
• printf (print an int d\n, Hi Dave)
• print an int 134513980
• printf ("print an int d\n",12.3)
• print an int -1717986918
• printf ("print chars as unsigned int u u
  u\n",'a','b','c')
• print chars as unsigned int 97 98 99

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Other formatting tags

• printf formatting tag
• flagswidth.precisionlengthspecifier
• Specifiers
• d treat the corresponding parameter as a signed
  integer
• u means unsigned integer
• x means print as hexadecimal
• s means treat it as a string
• c is for character (char)
• f is for floating point numbers

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Controlling the output

• printf formatting tag
• flagswidth.precisionlengthspecifier
• Width
• A number specifying the width of the output (left
  padding with space if needed)
• Flags
• 0 means left padding the output with 0 default
  is space
• Precision
• the number of digits after decimal points
• Example
• printf (square root of 10 is 20.15f\n,sqrt(10)
  
• square root of 10 is 3.162277660168380

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Fun with printf

• printf(\\03o,gt)
• print gt as octal number of 3 character long,
  left padding with 0
• 076
• char s Hi Dave
• printf (the string \s\ is d characters
  long\n, s, strlen(s))
• printf (The square root of 10 is f\n,sqrt
  (10))

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Lining things up

• int i
• for (i0 ilt5 i)
• printf (2d f 20.15f\n,I,
  sqrt(i),sqrt(i))
• 1 1.000000 1.000000000000000
• 2 1.414214 1.414213562376095
• ..

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Before we move on

• There are more formats and format options for
  printf
• The man page for printf includes a complete
  description
• man 3 printf

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Reading from standard Input - scanf

• scanf provides input from standard input
• scanf use pointers, or addresses
• Every C variable is stored in memory
• Every memory location has an address
• In C, you can use variables called pointers to
  refer to variables by their address in memory
• int x
• int p
• px

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scanf

• int scanf (const char format, )
• Remember means variable number of arguments
• Uses format string to determine what kind of
  variable(s) it should read
• The arguments are the addresses of the variables,
  i.e., pointers pointing to variables
• Example scanf (d d,x,y)
• x,y should be two integer variables

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A simple example of scanf

• float x
• printf (Enter a number\n)
• scanf (f,x)
• f stands for floating point (similar to
  printf)
• printf (Square root of f is f\n,x,sqrt(x))

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scanf and strings

• Using s in a scanf tells scanf to read the next
  word from input not a line of input
• char s100
• printf (Type in your name\n)
• scanf (s,s)
• printf (Your name is s\n,s)

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Reading a line

• You can use function fgets to read an entire line
• char fgets (char s, int size, FILE stream)
• size is the maximum of chars
• FILE is a file handle, for now, remember
• stdin (a constant) standard input
• Read a line (i.e., read character until newline
  is met or until reach maximum ) from specified
  file, and save to the string pointed to by s

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Example of fgets

• char s101
• printf (Type in your name\n)
• fgets (s,100,stdin)
• printf (Your name is s\n,s)

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Exercise

• Write a C program
• That asks user to input two integer values
• Prints out the sum of the two values

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Next class

• More on scanf
• File access library calls
• GCC compiler chain
• Introduction to gdb