CMSC 202
Streams
Warmup

What happens in the following?
vector<string> names;
names.at(0) = “Fred”;
names.at(1) = “Flintstone”;
for (int i = 0; i <= names.size(); ++i)
cout << names.at(i) << “ “;
cout << endl;
What is a stream?
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
Flow of characters (or other data)
Input
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Output
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Characters that come IN to your program
Characters that come OUT of your program
Input/Output (Sources/Sinks)
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Input: Keyboard, File, …
Output: Monitor, File, …
Standard In/Out
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Standard in
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Standard out


cin
cout
Why are these interesting?
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
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Under any OS – these can be “redirect”
You can treat a file as cin
You can treat a file as cout
Error Stream
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cerr

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
Standard error stream
NOT redirected when cout is redirected
Use exactly like cout
Example
if (denominator == 0)
{
cerr << “Fatal error: denominator == 0”
<< endl;
exit(1);
}
File I/O

File streams work EXACTLY like cin/cout

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More setup is required
Open a file
Read from/Write to a file
Close the file
Library

#include <fstream>
File Input

ifstream


Datatype for file input
Opening an input file
ifstream fin(filename);
Or…
ifstream fin;
fin.open(filename);

Reading from an input file (you can read any primitive!)
char a;
fin >> a;

Closing an input file
fin.close();
File Input - Example

Read a list of last names from a file
ifstream fin(“names.txt”);
vector<string> names;
string name;
What’s
while (fin >> name)
{
names.push_back(name);
}
happening
here?
File Output

ofstream


Datatype for file output
Opening an output file
ofstream fout(filename);
Or…
ofstream fout;
fout.open(filename);

Writing a char to an output file
char a = ‘A’;
fout << a;

Closing an output file
fout.close();
File Output - Example

Write a list of names to a file
ofstream fout(“names.txt”);
vector<string> names;
string name;
// Assume vector gets values…
for (int i = 0; i < names.size(); ++i)
{
fout << names[i] << endl;
}
File Streams - #1 Issue

Streams expect a C-string as its parameter

Example
string inFilename = “input.txt”;
ifstream fin( inFilename.c_str() );
string outFilename = “output.txt”;
ofstream fout( outFilename.c_str() );

What does c_str() do?

Look back at the string material!
Practice
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Open a file for input called “phones.txt”
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File has a name (string) followed by a number (int)
Read in the data from the file
Close the input file
Open a file for output called “reverse.txt”
Print the number followed by the name to the file
Close the output file
Input Streams – Pitfall!
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Mixing getline() and >> can be bad!
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>>
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Skips all leading whitespace
Leaves trailing whitespace for next extraction
Leaves the \n character
getline
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Retrieves until end of line character
Removes the \n character, but does not store it
Input Streams – Pitfall!
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Example
int age;
string name;
cout << "Input your age and first name";
cin >> age;
getline( cin, name );
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User types
42
Bob Smith

age == 42
name == “”
Solution?
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cin.ignore();
cin.ignore(1000, ‘\n’);
// discard a single character
// discard up to 1000 chars, stopping at \n
Checking for end-of-file
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End of file (fin) or End of input (cin)
3 strategies
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while (!fin.eof())
while (fin >> variable)
while (fin)
// Best
// Pretty good
// NOT preferred…
.eof()

Returns “true” if EOF character has been seen
Formatting Output
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Setf – set formatting flags

outStream.setf( ios::fixed)
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outStream.setf( ios::showpoint)
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Output is right-justified in output field
outStream.setf( ios::left)
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Floating point values have trailing zeros
outStream.setf( ios::right)
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Floating point values have a decimal point
Output is left-justified in output field
Setf stays in effect until reset
Formatting Output
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outStream.precision( int places )
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Sets the number of places to the right of the
decimal
Stays in effect until next call to precision()
outStream.width( int size )
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Sets minimum number of spaces to use to output
the NEXT item
Only works on ONE item at a time…
HINT: great for aligning tabular output!
Manipulators
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Library
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setprecision( int places )
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Same as outStream.width( int size )
outStream << setw(10) << Name << endl;
fixed
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Same as outStream.precision( int places )
outStream << setprecision(2) << money << endl;
setw( int size )
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
#include <iomanip>
Same as setf( ios::fixed );
outStream << fixed << money << endl;
showpoint
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Same as setf( ios::showpoint );
outStream << showpoint << money << endl;
Formatting Output – Pitfall!
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Recall
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Most manipulators stay “on” until reset
Issue?
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Function that modifies these stream flags…
Solution?
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Save the current state
int savePrecision = outStream.precision();
int saveFlags = outStream.flags();

Reset the current state
outStream.precision( savePrecision );
outStream.flags( saveFlags );
String Streams
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ostringstream

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Format messages and treat like a string
Example
string FormatMessage( int age )
{
ostringstream msg;
msg << "\nThis is my message\n";
msg << "John is " << setw(6) << age
<< " years old\n";
// use the str() function to access the
// C++-string in the msg
return msg.str( );
}
String Streams
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istringstream

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Parse messages based on whitespace
Example
void PrintWords( string stringToPrint)
{
// create and initialize the istringstream
// from the string
istringstream inStream( stringToPrint );
string word;
while ( inStream >> word )
cout << word << endl;
}
Practice
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Use setf, setw, fixed, showpoint, and
setprecision to do the following:
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Ask the user for 2 names and salaries
Print them formatted like this:
Name
--------John Doe
Jane Donner
Salary
-----------$
43523.00
$
3129.97
Challenge
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Use vectors, files and strings to
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Part 1:
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Read in an unknown number of paired values from a
file (Name - string, Phone Number - integer)
Part 2:
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Print the collection to another file
Align the output vertically
Format the phone number using dashes
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HINT: think about using modulus % and integer division!