Transcript Introduction to Computer Science

Introduction to Computer Science
COMP 51 – Fall 2012 – Section 2
File I/O
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2
Business
 Homework 4 – pass it up
 Drop Day – 5pm on Monday, Oct 29th
 Project 4 – Wed, Nov 7th
 Exam 2 – Friday, Nov 9th
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File I/O
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What is a Stream?
 A flow of characters
 Input stream - Flow into program
 Can come from keyboard
 Can come from file
 Output stream - Flow out of program
 Can go to screen
 Can go to file
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Streams Usage
 We’ve used streams already
 cin
 Input stream object connected to keyboard
 cout
 Output stream object connected to screen
 Streams can also connect to files on disk!
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File Basics
 For this class: text files only
 Reading from file
 Program takes input data from file
 Writing to file
 Program sends output data to file
 Files are processed from beginning to end
 Other methods available (i.e. random access)
 We’ll discuss this simple text file access here
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File I/O Libraries
 To allow both file input and output in your
program:
#include <fstream>
using namespace std;
 Library provides two classes:
 ifstream – used for input
 ofstream – used for output
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Declaring Streams
 Streams must be declared like any other
variable:
ifstream myInStream;
ofstream myOutStream;
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Connecting Streams
 Must connect each file to a stream object
myInStream.open("infile.txt");
myOutStream.open("outfile.txt");
 Uses member function open to open file
 Where is the file located?
 Same folder as executable (.exe file)
 Anywhere on the hard drive(s)
 Can specify complete pathname
 "C:\\Users\\khughes\\infile.txt"
 Must use \\ because \ is a special character in C++
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Streams Usage
 Once declared, use normally!
(Just like cin and cout…)
 Example – Input Stream
int oneNumber, anotherNumber;
myInStream >> oneNumber
>> anotherNumber;
 Example – Output Stream
myOutStream <<
<<
<<
<<
"oneNumber = "
oneNumber
" anotherNumber = ”
anotherNumber;
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File Names
 Programs and files
 Files have two labels in our programs
 External file name (e.g. “infile.txt”)
 Sometimes considered “real file name”
 Used only once in program (the open() function)
 Stream name (e.g. “myInStream”)
 Also called “logical file name”
 Program uses this name for all file activity
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Closing Files
 Files should be closed when program is finished
getting input or sending output
 Disconnects stream from file
 Example:
myInStream.close();
myOutStream.close();
 Files automatically close when program ends
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File Flush
 Output is often buffered
 Temporarily stored before written to file
 Written in “groups”
 Occasionally might need to force writing:
outStream.flush();
 Function flush() works for all output streams
 All buffered output is physically written
 Closing file automatically calls flush()
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File I/O Example – Output
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
ofstream output;
output.open("U:\\scores.txt");
output <<
<<
output <<
<<
"John" << " " << "T"
"Smith" << " " << 90
"Eric" << " " << "K"
"Jones" << " " << 85
<<
<<
<<
<<
output.close();
cout << "File written" << endl;
return 0;
}
" "
endl;
" "
endl;
Contents of File
John T Smith 90
Eric K Jones 85
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Appending to a File
 Standard open operation (for writing) begins
with empty file
 Even if file exists, contents are erased!
 Open for append:
ofstream outStream;
outStream.open("important.txt", ios::app);
 If file doesn’t exist, it is created
 If file exists, output is appended to end
 2nd argument is a constant for “input output
stream” (ios)
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Checking File Open Success
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File opens could fail for many reasons – ideas?
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Input – Input file doesn’t exist
Input – No permission to read file
Output – No write permissions to output file
Output – No space left on disk
Other unexpected errors
Member function fail()
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Place call to fail() to check stream operation success
inStream.open("stuff.txt");
if (inStream.fail())
{
cout << "File open failed.\n";
exit(1);
}
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End of File Check – Using eof()
 Use loop to process file until end
 Typical approach
 Two ways to test for end of file
 Member function eof()
myInStream >> next;
while (!myInStream.eof())
{
cout << next;
myInStream >> next;
}
 Reads each character until file ends
 eof() member function returns Boolean
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End of File Check – Using Read
 Second method
 read operation returns Boolean value!
(myInStream >> next)
 Expression returns true if read successful
 Returns false if attempt to read beyond end of file
 Example:
double next, sum = 0;
while (myInStream >> next)
{
sum = sum + next;
}
cout << "the sum is ”
<< sum << endl;
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File I/O Example – Input
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
ifstream input;
string nameFirst, nameMiddle, nameLast;
int score;
// Open file
input.open("U:\\scores.txt");
// Read file until end
while(!input.eof())
{
input >> nameFirst >> nameMiddle >> nameLast >> score;
cout << nameFirst << " " << nameMiddle
<< " " << nameLast << " " << score << endl;
Console Output
}
John T Smith 90
input.close();
return 0;
}
Eric K Jones 85
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File Names as Input
 Stream open operation
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Argument to open() is C-string type
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Can be literal (used so far) or variable
char filename[16];
ifstream inStream;
cout << "Enter file name: ";
cin >> filename;
inStream.open(filename);

Provides more flexibility – Your program doesn’t have to
be hardwired to a single file name
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File Names as Input
 Can also work with C++ strings, but you need to
convert it to an null- terminated C-string first
string filename;
ifstream myInStream;
cout << "Enter file name: ";
cin >> filename;
cout << "Now opening file " << filename << endl;
myInStream.open(filename.c_str());
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Input Example
 Read in a text file containing the names of all US
states and territories in alphabetical order
 One per line
Contents of Input File
Alabama
Alaska
Arizona
...
 Print each state on console
 One per line
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#include <iostream>
#include <fstream>
using namespace std;
int main()
{
ifstream input;
input.open("states.txt");
if (input.fail())
{
cout << "Unable to open input file" << endl;
return -1;
}
char state[32];
while(!input.eof())
{
input >> state;
cout << state << endl;
}
input.close();
return 0;
}
Initial Solution
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Initial Solution Output
...
Montana
Nebraska
Nevada
New
Hampshire
New
Jersey
New
Mexico
New
York
North
Carolina
North
Dakota
Ohio
...
This is a familiar problem…
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getline Example
 Read in a text file containing the names of all US
states and territories in alphabetical order
 One per line
 How do we handle states like this?
 New Hampshire
 New Jersey
 New Mexico
 New York
 North Carolina
 North Dakota
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#include <iostream>
#include <fstream>
using namespace std;
Revised Solution
int main()
{
ifstream input;
input.open("states.txt");
if (input.fail())
{
cout << "Unable to open input file" << endl;
return -1;
}
char state[32];
while(!input.eof())
{
input.getline(state, 32, '\n');
cout << state << endl;
}
input.close();
return 0;
}
What is happening?
getline() will read from the stream
“input” and save results to the array
“state” until it sees the newline (\n)
character, which is discarded. It will
read up to 31 characters. The C-string
will be null-terminated.
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#include <iostream>
#include <fstream>
#include <string>
using namespace std;
Revised Solution
using C++ strings
int main()
{
ifstream input;
input.open("states.txt");
if (input.fail())
{
cout << "Unable to open input file" << endl;
return -1;
}
string state;
while(!input.eof())
{
getline(input, state, '\n');
cout << state << endl;
}
input.close();
return 0;
}
What is happening?
getline() will read from the stream
“input” and save results to the string
“state” until it sees the newline (\n)
character, which is discarded.
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Revised Solution Output
 Properly handles multi-word state names
...
Montana
Nebraska
Nevada
New Hampshire
New Jersey
New Mexico
New York
North Carolina
North Dakota
Ohio
...
 getline is flexible – the delimiter can be any
character!
 Newline: \n
 Tab: \t
 Any symbol you want: #$%&!(@)
 Implication: getline can capture multiple
lines if the delimiter is NOT the newline
character
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>> and getline
 If you use both >> and getline on the same input
stream, they may get “out of sync” on delimiters
 On Lab 6, using cin >> letter followed by
getline(array,16) would result in a null Cstring being read immediately for the array
 This is because >> stops reading at the newline
 To handle this, use cin.ignore(100,’\n’) to
flush the remaining input
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 Project goals – Gain
experience with:
 File I/O
 Strings of characters
 Arrays of characters
 Arrays of other data
types
Hang ‘Em High, © United Artists, 1968
Project 4 – Hangman
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Hangman
 The goal of the game is to guess a word letter by letter
before you run out guesses (and body parts)
 Your program needs to
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Randomly pick a string of characters from a file
Track which letters have been guessed
Find all letters that match a guess
Recognize when a letter is and is not matched
Update the display
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Program Operation
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Read the entire file once to determine how many lines there are
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Tip: Use getline()
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Pick a capital at random from the file
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Display the gallows and the word
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Substitute an underscore ‘_’ for each letter
Tip: for strings, use the .size() function to get the size of the
string (i.e., int size = myString.size(); )
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Prompt the user for a letter
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Determine how many characters in the capital match the letter
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If any, need to display them from now on
If none, the user gets a miss
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ASCII Characters
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ASCII Character Arithmetic
 You can do simple arithmetic to change an ASCII
character by adjusting its “number”
char letter = 'A';
letter = letter + 3;
cout << letter;
// output is ‘D’
 You might not need it for this project, but it’s a
good thing to know.
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Program Operation
 Questions you need to answer for yourself
 How do you know which letters to display, and
which to not display?
 How do you decide which body parts need to be
displayed?
 How do you keep track of which letters the user has
used?
 How do you tell when the user has won?
 And how do you test your solutions to these
questions?
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Testcase Development
 Why are we requiring test cases?
 Common scenario: “Sam” writes 1000 lines of code,
compiles and runs it, and finds out the program doesn’t
work right
 Where should Sam start looking for the error?
 Should never have gone this far without testing!
 Many people tried to add test cases after writing and
testing the full program
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This is totally pointless!
 Also, a test case isn’t just making a copy of the function
and renaming it
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Testcase Development
 Write your twin functions as you are developing
the rest of the program
 As you write the function, think about how you
can test what it does
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Questions?
?