CMSC 202

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CMSC 202

• Streams

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Warmup

• What happens in the following?
• vectorltstringgt names
• names.at(0) Fred
• names.at(1) Flintstone
• for (int i 0 i lt names.size() i)
• cout ltlt names.at(i) ltlt
• cout ltlt endl

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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,

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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

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Error Stream

• cerr
• Standard error stream
• NOT redirected when cout is redirected
• Use exactly like cout
• Example
• if (denominator 0)
• cerr ltlt Fatal error denominator 0 ltlt
  endl
• exit(1)

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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 ltfstreamgt

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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 gtgt a
• Closing an input file
• fin.close()

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File Input - Example

• Read a list of last names from a file
• ifstream fin(names.txt)
• vectorltstringgt names
• string name
• while (fin gtgt name)
• names.push_back(name)

Whats happening here?
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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 ltlt a
• Closing an output file
• fout.close()

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File Output - Example

• Write a list of names to a file
• ofstream fout(names.txt)
• vectorltstringgt names
• string name
• // Assume vector gets values
• for (int i 0 i lt names.size() i)
• fout ltlt namesi ltlt endl

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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!

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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

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Input Streams Pitfall!

• Mixing getline() and gtgt can be bad!
• gtgt
• 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

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Input Streams Pitfall!

• Example
• int age
• string name
• cout ltlt "Input your age and first name"
• cin gtgt age
• getline( cin, name )
• User types
• 42
• Bob Smith
• Solution?
• cin.ignore() // discard a single character
• cin.ignore(1000, \n) // discard up to 1000
  chars, stopping at \n

age 42 name
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Checking for end-of-file

• End of file (fin) or End of input (cin)
• 3 strategies
• while (!fin.eof()) // Best
• while (fin gtgt variable) // Pretty good
• while (fin) // NOT preferred
• .eof()
• Returns true if EOF character has been seen

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Formatting Output

• Setf set formatting flags
• outStream.setf( iosfixed)
• Floating point values have a decimal point
• outStream.setf( iosshowpoint)
• Floating point values have trailing zeros
• outStream.setf( iosright)
• Output is right-justified in output field
• outStream.setf( iosleft)
• Output is left-justified in output field
• Setf stays in effect until reset

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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!

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Manipulators

• Library
• include ltiomanipgt
• setprecision( int places )
• Same as outStream.precision( int places )
• outStream ltlt setprecision(2) ltlt money ltlt endl
• setw( int size )
• Same as outStream.width( int size )
• outStream ltlt setw(10) ltlt Name ltlt endl
• fixed
• Same as setf( iosfixed )
• outStream ltlt fixed ltlt money ltlt endl
• showpoint
• Same as setf( iosshowpoint )
• outStream ltlt showpoint ltlt money ltlt endl

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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 )

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String Streams

• ostringstream
• Format messages and treat like a string
• Example
• string FormatMessage( int age )
• ostringstream msg
• msg ltlt "\nThis is my message\n"
• msg ltlt "John is " ltlt setw(6) ltlt age
• ltlt " years old\n"
• // use the str() function to access the
• // C-string in the msg
• return msg.str( )

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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 gtgt word )
• cout ltlt word ltlt endl

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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 43523.00
• Jane Donner 3129.97

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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!