CMSC 202Streams

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? Flow of characters (or other data) Input

Characters that come IN to your program Output

Characters that come OUT of your program Input/Output (Sources/Sinks)

Input: Keyboard, File, … Output: Monitor, File, …

Standard In/Out Standard in

cin Standard out

cout Why are these interesting?

Under any OS – these can be “redirect” You can treat a file as cin You can treat a file as cout

Error Stream cerr

Standard error stream NOT redirected when cout is redirected Use exactly like cout

Exampleif (denominator == 0){

cerr << “Fatal error: denominator == 0” << endl;

exit(1);}

File I/O File streams work EXACTLY like cin/cout

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 filefin.close();

File Input - Example Read a list of last names from a file

ifstream fin(“names.txt”);vector<string> names;string name;

while (fin >> name){

names.push_back(name);}

What’s 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 filechar a = ‘A’;fout << a;

Closing an output filefout.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

Examplestring 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 Open a file for input called “phones.txt”

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! Mixing getline() and >> can be bad!

>> Skips all leading whitespace Leaves trailing whitespace for next extraction Leaves the \n character

getline Retrieves until end of line character Removes the \n character, but does not store it

Input Streams – Pitfall! Example

int age; string name; cout << "Input your age and first name"; cin >> age; getline( cin, name );

User types42Bob Smith

Solution? cin.ignore(); // discard a single character cin.ignore(1000, ‘\n’); // discard up to 1000 chars, stopping at \n

age == 42

name == “”

Checking for end-of-file End of file (fin) or End of input (cin) 3 strategies

while (!fin.eof()) // Best while (fin >> variable) // Pretty good while (fin) // NOT preferred…

.eof() Returns “true” if EOF character has been seen

Formatting Output Setf – set formatting flags

outStream.setf( ios::fixed) Floating point values have a decimal point

outStream.setf( ios::showpoint) Floating point values have trailing zeros

outStream.setf( ios::right) Output is right-justified in output field

outStream.setf( ios::left) Output is left-justified in output field

Setf stays in effect until reset

Formatting Output outStream.precision( int places )

Sets the number of places to the right of the decimal

Stays in effect until next call to precision() outStream.width( int size )

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 Library

#include <iomanip> setprecision( int places )

Same as outStream.precision( int places )outStream << setprecision(2) << money << endl;

setw( int size ) Same as outStream.width( int size )

outStream << setw(10) << Name << endl; fixed

Same as setf( ios::fixed );outStream << fixed << money << endl;

showpoint Same as setf( ios::showpoint );

outStream << showpoint << money << endl;

Formatting Output – Pitfall! Recall

Most manipulators stay “on” until reset Issue?

Function that modifies these stream flags…

Solution? Save the current state

int savePrecision = outStream.precision();

int saveFlags = outStream.flags(); Reset the current state

outStream.precision( savePrecision );

outStream.flags( saveFlags );

String Streams ostringstream

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 istringstream

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 Use setf, setw, fixed, showpoint, and

setprecision to do the following: Ask the user for 2 names and salaries Print them formatted like this:

Name Salary--------- ------------John Doe $ 43523.00Jane Donner $ 3129.97

Challenge Use vectors, files and strings to

Part 1: Read in an unknown number of paired values from a

file (Name - string, Phone Number - integer)

Part 2: Print the collection to another file Align the output vertically Format the phone number using dashes

HINT: think about using modulus % and integer division!