Streaming and I/0

Chapter 14 D&D

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

Reading/writing text streams

All programs we’ve 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.

To access a disk file, we can open a file stream variable.

#include <iostream> ifstream infile(“filename”); // inputofstream outfile(“filename”); // outputfstream inoutfile(“filename”); // in/output

Example (code fragment):#include <iostream>

...ifstream input_data;input_data.open(“myfile”);int n;double x;input_data >> n >> x;

...

Reading strings from streams (code fragment):#include <iostream>

...ifstream input_data;input_data.open(“myfile”);string s;input_data >> s; // read wordgetline(input_data,s);

...

Reading characters from streams (code fragment):#include <iostream>

...ifstream input_data;input_data.open(“myfile”);char ch;input_data.get(ch); // get one character

...

Some other member functions of input streams:input_stream.peek(ch); // look ahead one charinput_stream.unget(); // put the last char back

Some member functions common to both input and output streams:input_stream.fail(); // stream op failedinput_stream.eof(); // end of stream reachedinput_stream.close(); input_stream.open(“filename”);

Code fragment:char ch;ifstream input_stream;input_stream.open(“mydata”);input_stream.get(ch); // get one characterif(‘0’<= ch and ch <= ‘9’){input_stream.unget();int n;input_stream >> n;}

input_stream.close();

Output file streams (code fragment):#include <iostream>

...ofstream output_data(“myfile”);output_data << n << “_” << x << “\n”;output_data.put(ch); // write a single characteroutput_data.close();

...

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

Example paths as strings in Unix:...string s = “/home/ugrad/hacker/myfile.dat”;s = “~hacker/myfile.dat”; // file in home directorys = “myfile.dat” // file in current directory

On PC:string s = “C:\\hacker\\myfile.dat”;// corresponds with file C:\hacker\myfile.dat

File paths

User could be prompted for a file namecout << “Type filename”;cin >> s;

File name could be computed or concatenated:string homedir = “~”;s = homedir + “myfile” + “dat”;

String Streams

• #include <sstream>

• 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.

Code fragment:...

string input = “January 23, 1955”;istringstream instr(input); // constructorstring month,comma;int day,year;instr >> month >> day >> comma >> year;

...

C++ does the conversion from string to the other types.

#include <sstream>

int string_to_int(string s){

istringstream instr(s); // constructorint n;instr >> n;return n;

}

#include <sstream>

string double_to_string(double a){

ostringstream outstr; // constructoroutstr << a;return outstr.str(); // return string equiv.

}

#include <sstream>string double_to_string(double a){

ostringstream outstr; // constructoroutstr << setprecision(5); // 5 places after decimaloutstr << a;return outstr.str(); // return string equiv.

}// double_to_string(sqrt(2)) returns “1.41421”;

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.

Command Line Arguments• You can include additional arguments on the

command line; e.g., in Unix:

ls -l make maing++ -Wall -ansi main.cpp -o main

Usually, we interpret symbols preceded by - as options.

Arguments to C++ main function

int main(int argc, char *argv[]){

...}

Example command line:myprog -v input.dat

In this caseargc == 3string(argv[0]) == “myprog”string(argv[1]) == “-v”string(argv[2]) == “input.dat”

argv[0] always gives the name of the programargc always >= 1

Number of command line arguments can vary:myprog -v input.datmyprogmyprog -i -v input.dat -o output.dat

argv[0] always gives the name of the programargc always >= 1

What is the value of argc in each of the cases above??

Caesar’s Encryption AlgorithmAlso known as Caesar’s 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

Program arguments

• optional -d flag indicates decryption rather than encryption

• optional encryption key -k<int>• input file name• output file name• Examples:

crypt -k11 input.txt encrypt.txtcrypt -d -k11 encrypt.txt output.txt

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);

Mainif argc < 3 or argc > 5

usage(string(argv[0]));exit;

for all command line argumentsif -d option, then set decryptif -kn option, set key=n, otherwise key=3if not option, input filename or output filename

openfile (either infile or outfile)if(nfile != 2)

usage(string(argv[0]));exit;

if(decrypt) key = -key;encrypt_file(infile,outfile,key);infile.close();outfile.close();

done;

void usage(string program_name)/* PURPOSE: Prints usage instructions RECEIVES: program_name - the name of this program*/{ cout << "Usage: " << program_name << " [-d] [-kn] infile outfile\n"; exit(1);}

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 << "Error opening file " << filename << "\n"; exit(1);}

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));}

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' <= ch && ch <= 'Z') return static_cast<char>('A' + remainder(ch - 'A' + k, NLETTER)); if ('a' <= ch && ch <= 'z') return static_cast<char>('a' + remainder(ch - 'a' + k, NLETTER)); return ch;}

int remainder(int a, int n)/* PURPOSE: Compute correct remainder for negative dividend RECEIVES: a - an integer n - an integer > 0 RETURNS: the mathematically correct remainder r such that a - r is divisible by n and 0 <= r and r < n*/{ if (a >= 0) return a % n; else return n - 1 - (-a - 1) % n;}

int string_to_int(string s)/* PURPOSE: Convert a string to an integer, e.g. "3" -> 3 RECEIVES: s - a string representing an integer RETURNS: the equivalent integer*/ { istringstream instr(s); int n; instr >> n; return n;}

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 < 3 or argc > 5) usage(string(argv[0]));

for (int i = 1; i < argc; i++) { string arg = string(argv[i]); if (arg.length() >= 2 and arg[0] == '-') /* it is a command line option */ { char option = arg[1]; if (option == 'd') decrypt = true; else if (option == 'k') key = string_to_int(arg.substr(2, arg.length() - 2)); }

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); } } }

if(nfile != 2) usage(string(argv[0]));

if (decrypt) key = -key;

encrypt_file(infile, outfile, key); infile.close(); outfile.close(); return 0;} // end of main