Streaming and I0

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Streaming and I/0

• Chapter 14 DD

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Menu

• Reading/writing text file streams
• Reading/writing string streams
• Command line arguments

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Reading/writing text streams

• All programs weve written so far have read input
  from standard input, and written output to
  standard output.
• cin, cout, and cerr are C streams.
• We will now extend this concept to disk files.

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• To access a disk file, we can open a file stream
  variable.
• include ltiostreamgt
• ifstream infile(filename) // input
• ofstream outfile(filename) // output
• fstream inoutfile(filename) // in/output

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• Example (code fragment)
• include ltiostreamgt
• ...
• ifstream input_data
• input_data.open(myfile)
• int n
• double x
• input_data gtgt n gtgt x
• ...

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• Reading strings from streams (code fragment)
• include ltiostreamgt
• ...
• ifstream input_data
• input_data.open(myfile)
• string s
• input_data gtgt s // read word
• getline(input_data,s)
• ...

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• Reading characters from streams (code fragment)
• include ltiostreamgt
• ...
• ifstream input_data
• input_data.open(myfile)
• char ch
• input_data.get(ch) // get one character
• ...

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• Some other member functions of input streams
• input_stream.peek(ch) // look ahead one char
• input_stream.unget() // put the last char back
• Some member functions common to both input and
  output streams
• input_stream.fail() // stream op failed
• input_stream.eof() // end of stream reached
• input_stream.close()
• input_stream.open(filename)

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• Code fragment
• char ch
• ifstream input_stream
• input_stream.open(mydata)
• input_stream.get(ch) // get one character
• if(0lt ch and ch lt 9)
• input_stream.unget()
• int n
• input_stream gtgt n
• input_stream.close()

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• Output file streams (code fragment)
• include ltiostreamgt
• ...
• ofstream output_data(myfile)
• output_data ltlt n ltlt _ ltlt x ltlt \n
• output_data.put(ch) // write a single character
• output_data.close()
• ...

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• Many compilers do not support string parameters
  to the open/close file function.
• ...
• string s myfile
• ofstream output_data
• output_data.open(s) // sometimes supported
• ...
• output_data.open(s.c_str())
• ...
• member function s.c_str() yields standard C
  character array

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• Example paths as strings in Unix
• ...
• string s /home/ugrad/hacker/myfile.dat
• s hacker/myfile.dat // file in home
  directory
• s myfile.dat // file in current directory
• On PC
• string s C\\hacker\\myfile.dat
• // corresponds with file C\hacker\myfile.dat

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File paths

• User could be prompted for a file name
• cout ltlt Type filename
• cin gtgt s
• File name could be computed or concatenated
• string homedir
• s homedir myfile dat

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String Streams

• include ltsstreamgt
• istringstream reads from a string
• ostringstream writes to a string
• Has same interfaces as other stream classes, but
  allows read/write to a string object.

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• Code fragment
• ...
• string input January 23, 1955
• istringstream instr(input) // constructor
• string month,comma
• int day,year
• instr gtgt month gtgt day gtgt comma gtgt year
• ...
• C does the conversion from string to the other
  types.

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• include ltsstreamgt
• int string_to_int(string s)
• istringstream instr(s) // constructor
• int n
• instr gtgt n
• return n

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• include ltsstreamgt
• string double_to_string(double a)
• ostringstream outstr // constructor
• outstr ltlt a
• return outstr.str() // return string equiv.

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• include ltsstreamgt
• string double_to_string(double a)
• ostringstream outstr // constructor
• outstr ltlt setprecision(5) // 5 places after
  decimal
• outstr ltlt a
• return outstr.str() // return string equiv.
• // double_to_string(sqrt(2)) returns 1.41421

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Command Line Arguments

• There are different ways to start a program
• click on icon
• type name of program and return
• the latter is called invoking the program from
  the command line.

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Command Line Arguments

• You can include additional arguments on the
  command line e.g., in Unix
• ls -l
• make main
• g -Wall -ansi main.cpp -o main
• Usually, we interpret symbols preceded by - as
  options.

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Arguments to C main function

• int main(int argc, char argv)
• ...

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• Example command line
• myprog -v input.dat
• In this case
• argc 3
• string(argv0) myprog
• string(argv1) -v
• string(argv2) input.dat
• argv0 always gives the name of the program
• argc always gt 1

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• Number of command line arguments can vary
• myprog -v input.dat
• myprog
• myprog -i -v input.dat -o output.dat
• argv0 always gives the name of the program
• argc always gt 1
• What is the value of argc in each of the cases
  above??

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Caesars Encryption Algorithm

• Also known as Caesars Cipher.
• Scrample message via algorithm
• specify integer key k between 0 and 25
• to encrypt, shift each input character by k
• Example
• k 3, input ABCDEFG
• output DEFGHIJ
• To decrypt message, use key -k

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Program arguments

• optional -d flag indicates decryption rather than
  encryption
• optional encryption key -kltintgt
• input file name
• output file name
• Examples
• crypt -k11 input.txt encrypt.txt
• crypt -d -k11 encrypt.txt output.txt

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functions used

• usage(string program_name)
• open_file_error(string filename)
• remainder(int a, int n)
• encrypt(char ch, int k)
• encrypt_file(ifstream in, ofstream out, int k)
• string_to_int(string s)
• main(int argc, char argv)

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Main if argc lt 3 or argc gt 5 usage(string(argv
0)) exit for all command line
arguments if -d option, then set decrypt if
-kn option, set keyn, otherwise key3 if not
option, input filename or output
filename openfile (either infile or
outfile) if(nfile ! 2) usage(string(argv0))
exit if(decrypt) key -key encrypt_file(in
file,outfile,key) infile.close() outfile.close
() done
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void usage(string program_name) / PURPOSE
Prints usage instructions RECEIVES
program_name - the name of this program /
cout ltlt "Usage " ltlt program_name ltlt " -d
-kn infile outfile\n" exit(1)
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void open_file_error(string filename) / PURPOSE
Prints file opening error message RECEIVES
filename - the name of the file that could
not be opened / cout ltlt "Error opening file "
ltlt filename ltlt "\n" exit(1)
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void encrypt_file(ifstream in, ofstream out,
int k) / PURPOSE Encrypt a file using the
Caesar cipher RECEIVES in - the file to read
from out- the file to write to
k - the encryption key / char ch
while (in.get(ch)) out.put(encrypt(ch,
k))
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char encrypt(char ch, int k) / PURPOSE
Encrypt a character using the Caesar cipher
RECEIVES ch - the character to encrypt
k - the encryption key RETURNS the
encrypted character / const int NLETTER
26 if ('A' lt ch ch lt 'Z') return
static_castltchargt('A' remainder(ch - 'A' k,
NLETTER)) if ('a' lt ch ch lt 'z')
return static_castltchargt('a' remainder(ch - 'a'
k, NLETTER)) return ch
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int remainder(int a, int n) / PURPOSE Compute
correct remainder for negative dividend
RECEIVES a - an integer n - an
integer gt 0 RETURNS the mathematically
correct remainder r such that a - r
is divisible by n and 0 lt r and r lt n / if
(a gt 0) return a n else return
n - 1 - (-a - 1) n
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int string_to_int(string s) / PURPOSE Convert
a string to an integer, e.g. "3" -gt 3
RECEIVES s - a string representing an integer
RETURNS the equivalent integer /
istringstream instr(s) int n instr gtgt n
return n
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int main(int argc, char argv) bool decrypt
false int key 3 int nfile 0 / the
number of files specified / ifstream infile
ofstream outfile if (argc lt 3 or argc gt 5)
usage(string(argv0))
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for (int i 1 i lt argc i) string arg
string(argvi) if (arg.length() gt 2 and
arg0 '-') / it is a command line
option / char option arg1
if (option 'd') decrypt true
else if (option 'k') key
string_to_int(arg.substr(2, arg.length() - 2))

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else nfile if (nfile 1)
infile.open(arg.c_str())
if (infile.fail()) open_file_error(arg)
else if (nfile 2)
outfile.open(arg.c_str()) if
(outfile.fail()) open_file_error(arg)

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if(nfile ! 2) usage(string(argv0)) if
(decrypt) key -key encrypt_file(infile,
outfile, key) infile.close()
outfile.close() return 0 // end of main