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CSE 232: Moving Data Within a C++ Program

Moving Data Within a C++ Program• Input

– Getting data from the command line (we’ve looked at this)– Getting data from the standard input stream– Getting data from files

• Output– Sending data to standard output (we’ve looked at this)– Sending data to files

• Transfer within the program– Moving data into and out of different types of variables– Moving data into and out of different data structures


Overview of Today’s Session• Using the on-line C++ reference pages (throughout)

• Basic input and output stream features

• Basic file input and output stream features

• Moving data into and out of variables

• Moving character data into and out of strings

• Moving data into and out of a vector container


C++ Reference Link on CSE232 Web Page


Main C++ Reference Page


C++ I/O Page


C++ I/O Examples Page


File I/O Examples

• Exercise: try out examples from C++ I/O reference– Do they work as written?– What files do you need to include to make them work?– What happens if you try to open a file that doesn’t exist?– What other ways can you explore the behaviors of the

features those examples are using/illustrating?


C++ I/O Display All


Review: C++ Input/Output Stream Classes

#include <iostream>using namespace std;int main (int, char*[]){ int i; // cout == std ostream cout << “how many?” << endl; // cin == std istream cin >> i; cout << “You said ” << i << “.” << endl; return 0;}

• <iostream> header file– Use istream for input– Use ostream for output

• Overloaded operators<< ostream insertion operator>> istream extraction operator

• Other methods– ostream: write, put– istream: get, eof, good, clear

• Stream manipulators– ostream: flush, endl, setwidth,

setprecision, hex, boolalpha


Review: C++ File I/O Stream Classes

#include <fstream>using namespace std;int main (){ ifstream ifs; ifs.open (“in.txt”); ofstream ofs (“out.txt”); if (ifs.is_open () && ofs.is_open ()) { int i; ifs >> i; ofs << i; } ifs.close (); ofs.close (); return 0;}

• <fstream> header file– Use ifstream for input– Use ofstream for output

• Other methods– open, is_open, close– getline– seekg, seekp

• File modes– in, out, ate, app, trunc, binary


Redirecting File Output

• Exercise: printing to a file vs. to stdout– Use the standard syntax for main that we used last week– Program always writes out “hello, world!”– If argc > 1 writes to file whose name is given by argv[1]– Otherwise writes to standard output


Review: C++ string Class

#include <iostream>#include <string>using namespace std;int main (int, char*[]) { string s; // empty s = “”; // empty s = “hello”; s += “, ”; s = s + “world!”; cout << s << endl; return 0;}

• <string> header file• Various constructors• Assignment operator• Overloaded operators

+= + < >= == []• The last one is really

useful: indexes stringif (s[0] == ‘h’) …


Review: C++ String Stream Classes

#include <iostream>#include <fstream>#include <sstream>using namespace std;int main (){ ifstream ifs (“in.txt”); if (ifs.is_open ()) { string line_1, word_1; getline (ifs, line_1); istringstream iss (line_1); iss >> word_1; cout << word_1 << endl; } return 0;}

• <sstream> header file– Use istringstream for input– Use ostringstream for output

• Useful for scanning input– Get a line from file into string– Wrap string in a stream– Pull words off the stream

• Useful for formatting output– Use string as format buffer– Wrap string in a stream– Push formatted values into

stream– Output formatted string to file


Using C++ String Stream Classes#include <string>#include <cstring>#include <sstream>using namespace std;int main (int argc, char *argv[]){ if (argc < 3) return 1; ostringstream argsout; argsout << argv[1] << “ ” << argv[2]; istringstream argsin (argsout.str()); float f,g; argsin >> f; argsin >> g; cout << f << “ / ” << g << “ is ” << f/g << endl; return 0;}

• Program gets arguments as C-style strings

• But let’s say we wanted to input floating point values from the command line

• Formatting is tedious and error-prone in C-style strings (sprintf etc.)

• iostream formatting is friendly• Exercise: check whether any

of the strings passed by argv are unsigned decimal integers (leading zeroes still ok)– print their sum if there are any– otherwise print the value 0


In-Memory “(in core)” Formatting

• Exercise: read and translate a file– Read integers and booleans (“true”, “false”) from a file

• Test by writing your own test file with different combinations

– Use a string to get one line of data at a time– Use a string stream to extract space separated tokens into

another string variable– Check whether each token is a boolean (if not treat as int)

• Convert to local variable of that type using another string stream• Printout whether it’s a boolean or an integer, and print out the

value of the local variable, making sure to preserve formatting


A Couple More Things to Try

• Exercise: printing out text from a named file– Open a text file whose name is given in argv– Print out the contents of the file to standard output– Detect the end of the file

• Stop reading text, close the named file, and end the program

• Exercise: typing text into a named file– Read text from the standard input stream– Put the text into a file whose name is given by argv[1]– Detect when the user types in the character sequence q!

• Stop reading text, close the named file, and end the program